Sunday 6 October 2013

XPS vs. EPS - Which holds up better in a below grade environment?

To reduce thermal bridging through your slab on ground, it is generally best practice to insulate below your slab.  This becomes even more important when you have hydronic heating pipes running through the concrete slab, as they increases the temperature differential between the slab and soil below increase the rate of heat flow out of the dwelling down into the ground (how much insulation to place below your slab is also under debate and will be discussed on future postings).

EPS rigid foam insulation is commonly used below slabs due to its relatively low cost compared to XPS, however, many in the building science community recommend XPS for its water resistance properties and ability to retain R Value.  Based on ASTM C272 tests, XPS has generally exhibited more resistance to moisture abortion when compared to EPS, but the EPS industry feels the ASTM tests are too short (24 hours/48 hours/and 30 days) and that EPS is actually better are resisting water take-up over XPS long term.

The EPS industry bases their claims on a singe case study performed by a EPS foam company (ACH Foam Case Study) that showed that after 15 years the EPS foam was dryer and retained more of its thermal resistance than XPS in the same environment.  The problem with this case study is that it was performed by a party with invested interest in the outcome, and as a result has very little credibility within the building envelope community.

SENWiEco will try to provide conclusive non-biased results as to whether EPS or XPS is the better choice for below grade installations based only on water absorption and thermal resistance properties (we will not discuss cost or embodied energy of the products). We will also included foam glass in the testing, as it is starting to receive attention on high performance homes with a desire to reduce embodied energy of the insulation products.

To this end, we today started a test that involves monitoring samples of each material in the following conditions:

  • Buried below grade in a location that will see regular ground water
  • Submerged in a water bath
  • Sealed in an airtight zip-lock bag
  • Stored in indoor conditioned space on a shelf.
At the end of approx 6 months, the samples will be re weighted to determine the volume of moisture absorbed and also sent to BCIT where they will be tested (ASTM C518 conductivity test) to determine their thermal resistance properties after aging the samples in the various conditions listed above.

Proof the selected site will definitely see ground water.  This was the result after a heavy rain approx 3 days prior.

Samples cut and place in bottom of hole (approx 4ft depth

Gravel added to ensure water can flow around samples and to identify location when dug out in spring.

Samples tucked away for the winter.
Samples to be held under water

Extra pieces of foam to push test samples under water

Lid of test chamber strapped on to keep samples submerged.  Have also now taped seams to prevent evaporation (not shown in photo).
Samples stored in air tight zip-lock bags.

Samples stored on office shelf

We would like to express our gratitude to the following sponsors of this testing:

PlastiFab (EPS)
Pittsburgh Corning (Foam Glass)
Home Depot (Discounted XPS)


Unknown said...

What are your thoughts about using Perlite under the slab? I'm getting some pricing & availability. It seems like a reasonable material, especially given its ability to last indefinitely underground.

I know they use a lot of energy to pop the stuff into the glass beads. But can it be much worse than FoamGlas? I don't know much about FoamGlas.

SENWiEco Designs said...

I have never heard of using it in this application so Googled and came up with This looks like a very poor system that would have significant thermal bridging. (Not to mention the photos showing plastic below the insulation and sand above which are both very poor practice). I would also imagine that the cost is quite high, but have no data to support. FoamGlass is an option (recommended by some for a perception of a lower embodied energy - I have not yet confirmed) but it is $$$ and harder to detail than the sheet products like EPS/XPS.

Unknown said...

I've seen the pdf. I've contacted the Perlite Institute. Perlite is commonly used in industry as an insulator. You don't need to use the plastic bags, you can just lay it out like gravel. And you don't need to use a sand layer. It just needs to have something on top to level it out and to distribute the weight of the installation crew so the glass doesn't get too crushed. It can be granular (maybe with a filter cloth just to keep the perlite from mushing around into the granular layer).

Basically you just build a few inches of Perlite into your granular under-slab layers. It's volcanic rock, so it's impervious to pretty much anything. Should last forever. Only thing is it doesn't have a super-great R-value. Somewhere in the low 3's. Which is similar to FoamGlas. So 4" would get you around R-12.

I don't have pricing yet, but I'm gonna bet it's significantly cheaper than FoamGlas. I'll let you know when I get pricing.

I don't understand the thermal bridging comments. Is it because the bags pictured in the pdf make an irregular surface? I suppose one could use bags, and then fill the voids with loose perlite if one was concerned with bridging. Or just do it all loose. Again, I don't see how you can suggest it's a "very poor system" with "significant thermal bridging". Of course anyone can make a very poor system if the installation details are poor - no matter what products are used. Am I missing something?

SENWiEco Designs said...

Re Thermal bridging - yes the bags represent a huge numbers of seams and gaps. You would have to reduce your effective R value by quite a bit if installed in this manner. I am concerned about FOamGlass tiles for the same reason. You end up with a lot of seams and the edges of the foamglas erode easily which would lead to voids. Even when using sheet products, best practice is to install in two layers and stagger the seams.

The 'gravel' installation method would be better, but as you point out a bit more difficult to detail and probably stil very $ compared to even XPS. There is also FoamGlass gravel that can be used but again $$$$

Unknown said...

I wonder about thermal transmission of a slab. I've seen suggestions that you need a "thermal break", and that would go a long way to making sure the heat goes up (assuming hydronic heating) instead of down. And a thermal break can be like R-5.

But I know radiating heat goes in all directions. So that would suggest that R-value is important, and one would want more than just a nominal break. I wonder what the ideal balance between R-value under-slab and cost/pain-in-the-rear factor is. How much "R" are you targeting?

That's also fascinating about the FoamGlas gravel. Which sounds a lot like perlite. Now I can't wait till tomorrow so I can get a quote on the supply of perlite.

I just don't trust foam products to do what they claim. Maybe your torture test will help with that.

SENWiEco Designs said...

Your right - you do not need a lot to achieve a thermal break. We should also note that if not hydronic slab heating and you are in a hot dominated climate, you may actually be better off not having insulation below your slab or footings to allow them to become heat sinks to get rid of interior heat in the summer.

I plan on 2" of EPS or XPS under the slab thickening to 4" for the last 48" around the outside perimeter. I will also place 1.5" around the edge of the floor slab between the slab and foundation. If I was putting hydronic heating in the slab (mine will be in the ceilings), then I would go with a full 4" under the whole slab.

I am definitely not a 'foam' guy. I would never use it above grade walls or even on the exterior of the foundation. But it makes the most sense below the slab from a practicality and cost standpoint. It has a proven track record and even if it does loose some R value, you can afford to put extra down to make up for what ever target you have. The non foam alternatives are just too expensive in my view for the limited benefit they may provide.

Unknown said...

You say you'll provide an additional 1.5" break at the foundation wall. I assume that means vertically, so there will be a little chunk of insulation between the wall and the floor slab.

But if you are using Durisol, and if all the insulation is on the exterior of that wall, then why do you need another break from the Durisol wall?

I could see if you wanted a little something extra between the footing and the floor slab (however, that would mean making the bottom of your slab 4" higher than the top of the footing). But I don't understand why/(if) you are isolating from the wall.

Unknown said...

Oh, and you say that FoamGlas is $$$$. Can you quantify that? I looked on their site, and they don't even show Canadian distribution. So I wonder how one would even get the stuff.

SENWiEco Designs said...

Here is my detail for your info

Because I do not plan to wrap the footing in insulation, I want to reduce slab edge losses as much as possible (they are stronger than thermal flow downwards). There is only a thin panel of Durisol between the concrete core and slab edge, so by wrapping the slab, I significantly reduce the possibility of a thermal bridge at this location. I do not have the funds to pay someone to perform a 3D THERM analysis to determine optimal design, so the extra 1.5" of insulation is cheap insurance.

SENWiEco Designs said...

Re FoamGlas - I have not gone for quote - I am basing my comments on discussions at multiple seminars and courses I have attended. I believe 'STEELS' carries the product in Canada. The samples I received came directly from the factory in the USA.

Unknown said...

Ok, I got pricing for the Perlite. It's $11.05 for a 3.5 cuft bag, from my local Co-op farm supply.

It works out to $1.42/sqft for my planned 5.4" thickness, for R-16.9

That's 8.5 cents per square foot per "R".

I got (quick) pricing from the local Home Hardware, and they want $27.43 for a 2'x8' sheet of 2" thick XPS. So double thickness is $54.86, and that covers 16 square feet. So it's $3.43 per square foot. And it's (supposedly) R-20. So it's 17.1 cents per sqft per R.

Not sure if the building science guys would price stuff in $ per R per sqft, but I don't know what else to use to make it comparable.

So Perlite: 8.5 cents
XPS (Dow): 17.1 cents

I'm assuming installation details and labour will be approximately the same. There will be no cutting and fitting with the Perlite. But there will be extra care to protect its top-surface.

I tried to get pricing on FoamGlas. The best I could do was find a website that says it's "about 2-and-a-half times that of XPS". So that would make it around 42.75 cents per sqft per R. Which is definitely $$$$, as you suggested above.

Maybe you should rig up a loose Perlite addition to your testing somehow. Given that it appears to be the cheapest insulation by a long shot. Again, I could be missing something, so I await your critique!

Unknown said...

I see your foundation detail. You plan to perch the floor slab up high - way up off the footing. So yes, I see how you could lose a bit less from the slab edge through the wall and then through the footing. Definitely cheap insurance. ALthough you're going to go through a lot more gravel (assuming a level excavation).

I don't have that kind of headroom. My slab will be way, way lower than that. So I will be losing heat at the edges. Maybe I should detail something right at the footing as well. I was just going to live with the losses through the footing. And I was going to run hydronic tubing in the walls as well, so there was never going to be much hope for the footing-to-wall joint.

I did read up on this at the FoamBuildingAdvisor - ahem, I mean the GBA. And the story there was that yes, there is some loss in this area. But the "delta-T" was reasonably low 4-6' underground, and that without heroic efforts under the footing, you may just have to bite the bullet and lose some heat.

SENWiEco Designs said...

Re slab location - Sorry Zenon - I do not understand your comment "you plan to perch your floor slab up high - way off the footing". Floor slabs typically get placed just above the top of the footing (you do not want resting right on footing as settlement of the fill under the slab can lead to cracking of the slab if it hangs up on the footing). Insulated slabs sandwich the insulation between the footing and slab in typical construction, but they typically only use 2". So the only key difference with mine is that I am adding another 2" around the perimeter.

Re gravel - pretty common hear (and best practice) to have 4-6" of granular fill (not crushed rock)under the slab. Less common to put granular fill under footings on lower end homes but common on HP homes. I will have a level excavation except where the footings go, this will be cut down a little further to allow for the granular fill. SO not really using much more rock than normal.

You write, "my slab will be way lower and that you do not have any head room". This does not make sense. Your slab must be above the footing and you make the depth of your footing and slab what ever you want/need them to be. My excavation will be 10'9" below grade. This allows me to have a 8'6" ceiling in the basement and still have the top surface of my first floor level with the outside grade (I do not want thresholds).

You mention hydronic tubing in walls - bad idea - this will increase the thermal flow through the walls as it raises the delta T. Keep the tubing in the slab or ceiling. Otherwise you will need to significantly increase insulation on foundation wall.

I agree re the limited benefit of insulating a footing. That is why I am not going to do so but instead concentrate on thermal bridges through the slab edge (in some hot climates you actual want this thermal drain to bleed off heat from inside the house during the summer).

At a recent seminar, they discussed slab edge losses. They were talking about above grade so a larger delta T but a slab edge representing 1-3% of the opaque wall area lowered the over all wall area thermal value by 40-60% and unless you took care of the thermal bridge, you could never insulate the rest of the wall high enough to overcome the thermal losses at the bridge. Horizontal thermal bridges matter more than just about any other details in a building envelope from a heat flow perspective.

Re Perlite - interesting info. Do you pan to use it bagged (I would not) or loose? How do you plan to install it so it is 'protected' during construction. This is no small matter to address. Cutting and fitting sheet material is fast and would be a lot less effort than somehow protecting the perlite. Does it have the strength to support the slab. You need at least 30 PSI strength to be suitable for below slab installation and much higher for below footing installation. I was unable to find any compression specification for Perlite other than rigid insulation for industrial uses. What happens when it gets wet (it will)?

Also note - Home Depot pricing does not realistically represent what you would pay a wholesaler, so your factors would be off a bit.

Perlite sounds interesting, but I would need a lot more information before considering it for a below slab application.

Unknown said...

I was debating lowering the floor to be either even with, or lower than the footing. And my footing will probably be 10" thick. So by comparison, your floor was appearing much higher. I don't want to go very deep with the excavation, as I'll run into water problems. Keeping it high will be better. So I end up with height considerations.

I was thinking of laying the perlite loose. I'm going to buy a bag and experiment a little with what kind of topping may keep the upper-most glass bubbles from breaking. I don't have compressive strength numbers either. Good point. Although it's been used under many slabs, so I presume it's fine. I will look into that further.

When it gets wet, the water will run in and around the little bubbles. And then it will drain away. Like gravel. Only with better insulation value. The little beads I believe are unaffected by water. It's just rock. So it's not going to rot or mold. They don't expand or contract, or "absorb" water. At least that's what I'm learning so far.

I'm hoping that there won't ever be bulk water inside the perlite layer. It will be several inches above the drainage plain. But if there is, it shouldn't be affected.

SENWiEco Designs said...

Need to keep this fast as I need to get some work done. You cannot make your floor lower than your footing. Doing so would interrupt the bearing surface of your fitting which is not allowed. There is also no reason to do so, just make your footing deeper if you want your floor deeper. Just deal with the water issues, it is easy to do so. Use a fully adhered membrane on the outside of the ICF and install proper perimeter drain. Besides, how do you 'know' where the water table is throughout the season. What is the sense of limiting your excavation to say 7ft if the water table can come as high as 4ft. Just deal with the water. Stick with common and best practices for your construction to save yourself a lot of aggravation.

DO not presume anything. There are many many examples of people building in a certain way to later on find that that may not have been the best way to build. For instance in my region, we built literally 100's of thousands of homes with faced sealed stucco to later find out that was a very poor idea and that we really need to build with a rain screen assembly in an area that has the volume of rain we do. Just because someone who is trying to sell you a product shows a site where their product is being used as below slab insulation does not mean it is appropriate. Don't forget their are many areas even in the USA, where building codes are either not present or not enforced. Just logically, Perlite sounds fishy, if it needs protection during installation so it is not crashed, what do you think is going to happen when concrete is poured on top of it. My advise is to stick to materials that have a tried and true track record.

My point about getting wet, is what happens to the R Value when the field of Perlite becomes saturated during high water table events that overwhelm your drainage plane. I would suspect it drops significantly. This is not a problem with sheet stock as there is no where for the water to go except very minute layers at seams and between layers.

Unknown said...

You are expecting to have events that overwhelm your drainage plane? Does that mean you expect your drainage system as a whole to be overwhelmed (I'm thinking they are one and the same)? Because that sounds like a flooded basement to me. Please explain.

I'm not suggesting undermining footings - that would be ridiculous, and never pass inspection. I'm just considering all the options when it comes to where to place the slab. I only meant below the top of the footing. Like they do in old houses when they "lower the floor" without underpinning. Again, this isn't decided yet. This is just exploring options and gathering information.

My excavation limit is self-imposed so that I can have gravity/daylight drainage and not rely on sump pump(s). Our site has unique features, so it's more complicated than just digging deeper. And we have a very good idea of where the water table is through the year.

I will investigate perlite in more detail, as I was going to do anyway. And it does have a tried and true track record in many applications. Just because it's not common to see it underslab doesn't make it a poor choice. Especially if it can perform as well or better than XPS at half the cost.

A "drainage plane overwhelm event" would be a much bigger problem than a loss in R-value because the perlite got wet. But yes, of course, if it was underwater, it would have negligible R value. But R value isn't everything.

SENWiEco Designs said...

It is expected that there will be times where water will come up to the perimeter drainage pipe and build up some head in the pipe. This will typically be after a heavy rain but can also be when underground springs are present or your pump fails. Point is you need to design for failure. If you can convince your engineer to allow the pipe to be buried in the granular layer below the footing, all the better.

I am very aware of houses that lower their slabs (dig down) without lowering the foundation and footings. I see this in my home inspection practice many times on teh older 100yr old specials. It is a very POOR practice that leads to water ingress as now your slab will be below the perimeter drain pipe. These eventually all need to be fixed by lower external drain or adding an internal drain. The slab belongs above the footing and their is no reason to build any other way as it is easy to lower footing to right height.

Water tables change all the time. All it takes is construction nearby to dramatically change the table on your site. It is much easier to just pump the storm water than to restrict your design to a perceived water table depth. Pumps are cheap, easily provided with back up power, and alarms. A very high percentage of homes have pumps in my region, and as long as some basic due diligence (backup/alarms/regular service), there have been few problems with these installations.

My advise is to make the depth what you need/want and pump the perimeter storm. You should be separately piping the roof storm runoff anyway and this can be gravity fed. Then you only have to pump ground water which hopefully is limited and seasonal.

Unknown said...

You know the perimeter insulation you are proposing - to reduce slab edge loss? That stuff could be ComfortBoard, right? There is no weight on it. And if one wanted to have the floor topping come right up close to the wall, one could cut the ComfortBoard on a 45 degree angle and the slab edge could at least appear to go right to the wall - with minimal heat loss. Correct?

In other news, I found a local sustainable building group that apparently buys lots of Perlite from the local co-op for use as insulation. So I've got a call in to them to see what their experience is with the product, and what they use it for. And also questions about compressive strenght, and protecting from crushing during a concrete placement (if they even use concrete - these guys are straw-bale types), etc, etc. I'll let you know what I learn.

SENWiEco Designs said...

I will use rigid foam for this because I want as high a r value per inch as possible.

Chamfering this insulation is a really bad idea. You will loose a majority of your thermal break. There is also no reason to do so as your services wall inside of the foundation will cover the gap. The only issue is when you have areas of the ICF exposed without a service wall (this is limited because of the need to run electrical outlets). In these areas, I plan on either having flooring above the concrete slab which would easily bridge the gap or figuring out a S.S. cover plate. The other problem with chamfering is that the concrete in that area will be weak and crack anyway if any load is applied.

Goo luck with the perlite. Hope it works out but suspect the PSI will not be high enough and then by the time you figure out a way to protect it, you will be at the cost of EPS (common choice).

Be careful of the straw heads. There is a lot of these designs that do not meet best building science practices. Construction method only appropriate in dry regions. You also NEVER want a dirt floor. You want the concrete slab.

Unknown said...

By "services wall", it sounds like you plan to frame a stud wall inside the Durisol wall. Is that correct? Why would you go through that extra effort and expense when electrical can be run inside the Durisol (or routed into the face)? Part of the appeal of Durisol is that it's done as soon as the concrete is poured. At least that's what I was thinking - at least for utility space. The other appeal is that if it's finished with plaster, then what you see is what you get in terms of the foundation wall. If there ever is any leaking, it will be readily apparent right on the wall. Why cover that up with another wall?

I could see using XPS for the vertical break. Even with 1", you get R-5, and then something like R-2.5 with the Durisol material, so a total R-7.5. Also easily boosted to R-12.5 with 2" of xps.

SENWiEco Designs said...

There are many reasons for a service wall:

- Stud wall is much faster and cheaper to frame compared to routing out ICF or worse incorporating conduit into the ICF. Both of these approaches also mean you have to perfectly plan your needs and will be reducing your future flexibility.
- While I plan on some utility areas (HVAC room) to 'showcase' the exposed Durisol finish, I definitely will cover the rest with drywall. An exposed Durisol wall will suck the light out of the room and will build up dust and be hard to keep clean.
- But most importantly, you still need SOME form of VB on the interior of the wall space. You do not want poly, but you want some resistance to outward vapour flow. Best approach is VB paint so would need drywall anyway.

Don’t forget, my basement will be ‘living area’ and not just storage and utility. I do not believe I would go through the cost of putting in a basement if only for storage and utility. You can stick utility into a closet on the main floor and old always build an accessory building for storage. As far as leak detection, without poly in the wall, any significant moisture coming in the foundation will be easily identified on the floor.

Re thermal break – yes when dealing with such a small gap, you want the biggest bang per inch. 2” becomes harder to cover with wall and still have wall sitting on enough of the slab for support. An important note is that this service wall in my dwelling will not be ‘bearing’. My floor joists will be attached by hangers to a ledger that is attached by anchors to the ICF. This allows me to put more edge foam in because the service wall just has to hold itself up.

Unknown said...

The hangers - are you using Simpson ICF hangers? The ICFVL?

I'm planning on coating the walls with plaster. Either natural hydraulic lime, or American Clay. And maybe paint. Or maybe just leave it natural. Still have to figure that out.

Another full studded wall inside the Durisol sounds like a waste of trees and drywall to me. And you're probably not going to want paper-faced drywall in a basement, so now you are also into expensive fiberglass based material. A little anti-sustainable.

I don't see how a full studded wall, with full drywall coverage, could be cheaper than passing a router over the face of the Durisol, and then plastering it over. I find that baffling.

And what's wrong with incorporating conduit inside the cavity (other than future accessibility - I can understand that)??

SENWiEco Designs said...

Got to keep this short as I really need to get some work done.

Planning on using standard drywall. I am managing my moisture by using a fully adhered membrane on the exterior of the ICF and VB paint. This services raceway will also not be air tight anyway.

The Clay is not a durable product based on my research, it cannot handle any humidity without becoming soft and rubbing off. Plus is more $ than drywall and paint. It also is not a qualified VB.

The studs will probably be reused from the house I am tearing down.

Routering out the ICF may be close to the cost of framing a wall when using conventional labour. Hove you done it? It is not straightforward. You typically have to mount straightedges to the ICF to run the router against for each run. Mounting the boxes is also a pain. You then have to foam up the channels so you are no loosing R value. PITA

As far as running conduit - major PITA. Have you placed either conduit or ICF before. When combining, you have long pieces of conduit you have to thread the ICF blocks over (on vertical run conduits). At close to 50 pounds a block, this is no easy matter. And have you ever run cable through conduit? At least some of the runs are going to fight you and have a reversed lab exactly where you cannot get at it. I have spent hours trying to run cable the last 5ft through a conduit. Again - a PITA.

I am all about reducing carbon footprint, but not to the point of reducing the durability and flexibility of a dwelling. I tend to go with the best product for each use taking into account many factors including carbon footprint.

I do not make low embodied energy the driving force in my decision and instead concentrate on much larger carbon footprint reductions based on how I live. I drive a vehicle that burns WVO and I typically go on vacations within a 3 hour drive. I am building a house that will significantly reduce our energy load and will be built with many materials that are low VOC. I do not get hung up on the minutia of 'green' products such as programs like LEED. Many of these 'green' products I have found are not durable and therefore not suitable for purpose.

It does not matter how 'green' a product is if it does not stand up to its intended use and must later be replaced or repaired.

Looks like we may have different priorities. Just make sure you understand the implications of your choices and try speaking with people that have had both good and bad experiences with those products.

I have just signed up my structural engineer so will need to concentrate on design for the next couple of weeks.

Unknown said...

You say that running hydronic tubing inside a wall is a much worse idea than putting it in a floor. Because it makes the wall hotter than it would otherwise be, and increases the delta T. And that one would have to insulate the wall much more to compensate. I understand that.

But the thing I don't understand is how the floor is automatically a better choice. There will be much less insulation under the floor. Maybe R-15. However, on the wall, I can easily put many layers of relatively cheap Drainboard/Rockboard/Comfortboard on the lowest part of the wall. Even without extra layers of drainboard, I was always going to get higher R-values in the wall than in the floor. At the lowest part of the wall, the delta T between the earth under the slab and the earth adjacent to the lower part of the wall will be similar.

For clarity, I would only put the tubing in the lower half of the wall anyways - as recommended by the local contractor who builds with Durisol. He says it's a nice heat, and you can choose between the walls and the floor, or both. And it radiates its way up the wall.

Yes, it also radiates its way down to the footings where it can/will thermally bridge its way out. A matter that can more easily be controlled within the floor.

You said that the presentation you saw about slab edge losses was talking about slabs-on-grade, where the delta T would be much higher.

I guess ultimately the question is: "would I rather have a higher delta T in a (large) area with lower R-value (the floor), or would I rather have a higher delta T in a (smaller) area with much higher R-value, but with a built-in thermal bridge (in an area with similar delta-T)"??

Maybe it's smart to rough in both, and at least have the options for the future?

And a follow-up thought - the only experience I have with in-floor heat is in our bathroom, with an electric system. We keep it at 74. That makes the floor warmer than the rest of our house and is a perfect temperature, if you ask me. So I'm expecting the slab in the basement will also be pleasantly comfortable at 74, or even a bit lower. Maybe targetting 72. If the earth is 55-60, then I'm only trying to keep the slab (or walls) somewhere between 12 and 19 degrees warmer. Which is vastly different than keeping an interior at 72 when the above-grade temps can be -20 to -40F (delta T of up to 112 degrees).

I understand that a hydronic system uses water off a boiler or water heater tank. And the lowest output would probably be around 120F if the system was combined with domestic hot water supply (although our current tankless system is set to only about 108). So I'd be pumping 120 degree water through a slab, trying to get the slab to maintain 72 degrees. Does that mean that the walls/floor will temporarily exceed 72, until they reach equilibrium - or until an in-floor sensor placed between tubes will hit 72? So my delta T will actually be a mix of 120F areas and 72F areas?

I wonder if I can modify the system to output 72 (or 80, or whatever) degree water, and to keep the circulation running longer. I wonder if the energy consumption is mostly the heating, and not so much the pumping. I assume the pumping can be very efficient. Maybe even buy a "cheap" hot water tank, and keep it at the minimum temp for the hydronic system. And then have another tank do the domestic hot water. Would that decrease my delta-T problems??? Maybe it's extra smart because I could eliminate heat-exchangers, and just have the hydronic be completely isolated. Maybe even change the fluid to be something even better than water. Just thinking out loud here.

SENWiEco Designs said...

Zenon - you are right re wall. You can compensate. My reaction was more instinctual because heat likes to go sideways more than it likes to go down, so the conduction forces are larger going through a foundation than a slab.

Any reason you are not considering the ceiling (where I will put mine). Heat lost will just end up in the main floor where it can be used anyway, and now you are not increasing thermal conductance through the slab or foundation. (Main reason I am using this method throughout the home is that it is fast to react which is important in a HP home where solar gain is encouraged and will be used to lower energy needs - stay tuned as I will blog about this topic in the future if you want to discuss further).

Let me be clear - slab edge losses are huge regardless of where the slab edge is. Above grade, grade, or below grade - does not matter.

Heated slabs will be a lot warmer than 72 or 73 on average. You will typically be running 110-120F water through them (even hotter for wall or ceiling panels)and their surface will be in at least in the 80's. I suggest you buy "modern hydronic heating" by John Siegenthaler as a primer on hydronic heating. It is an excellent book that I have pretty much read cover to cover. I am also a certified Residential Hydronic Designer having completed the local TECA week long course, but I got a lot more out of reading the book. It will answer all of the questions you have (sorry, I am rusty and would have to research the answers which I do not have time to do right now, I will look at the subject in the spring when I design my hydronic ceiling panels and will blog about that activity then). HWT's used for space heat are hardly ever a good idea.

Unknown said...

I guess I don't understand the physics behind the assertion that "heat likes to go sideways more than down". I know that heat moves through conduction, convection and radiation. I'm pretty sure that radiation emanates outward in all directions equally (like the sun). And that conduction chooses the most conductive route & exploits it (and if there are no conductive routes, then the movement via conduction is negligible, by definition). And then convection is blowing air around, and the "hot air rises" thing.

So I don't see how any of the methods automatically mean that "sideways" is the path of choice for heat. Yes, if the side has an obvious conductive bridge, then it will have the greatest amount of energy moving through it. But otherwise, I don't understand. Where did you get this principle?

I will look into the book - thanks for the tip.

I'm not discounting the ceiling, but I find it hard to understand why I would want to heat a ceiling that's 9' away from my feet (which are the coldest part of me). And then the heat will convectively stay up there, so the floor will be the coldest part of the assembly. Which is exactly what makes basements around here suck so much. Because their floors are freezing cold (and usually uninsulated in tract homes).

It defeats the point of radiant heating, IMO. I may as well just stick with forced air in the basement, because a heated ceiling is almost the same thing. Again, I must be missing something, because the stuff you are saying is just way, way different than what I'm reading or expecting. So something is amiss. I wonder if it's an east vs west/climate thing. Maybe your winters are just so much milder, and maybe your underground temperatures are just so much warmer. I didn't think it was that big a difference, because cold feet are cold feet no matter where you live. But I'm obviously mistaken.

SENWiEco Designs said...

Zenon - I am afraid that the responses needed to address your questions are getting more involved than I really have time for based on my need to finish my design and get my permits. The train of this discussion has also veered way off the original topic.

Please continue posting, but try to post on similar topics as the content I am posting and contacting me privately for other stuff. I will answer as much as I can when I have time (on or off line) or at least point you in the right direction.

Re your last post, you really need to read the Modern Hydronic Heating book I recommended. You comments indicate a lack of knowledge as to how radiant heating performs as it is a totally different animal than convective heating. Radiant heating heats objects not air and as long as the object can 'see' the heat source it will be warmed (even the floor from a ceiling panel will be warmed). Think how the sun works for an understanding of radiant heating. This is not an east coast/west coast thing.

Also, note the zone that is most important for human comfort is around our head not our feet. And in fact our feet do not like to be that warm, just not cold. So ceiling heat is closer to the intended target and what I am utilizing on all floors of my house.

Re the direction of heat - this is just physics. Not sure I have the technical information to provide you. It is just taught. For instance, the air film that attaches to all surfaces has completely different values depending on the direction of interest (see for some technical info).

This is why slab edge around a house with a slab @ grade is so much more important to address than sub slab insulation. It is also why insulation levels prescribe by code throughout the world have the highest levels in the ceiling and the lowest levels below grade, with the walls being in between. So - strongest drive Up, next strongest drive out to the side, and last is down (believe conductive air currents and stack effect also are part of this equation).

By the way - hot air does not 'rise' per say. It floats on top of cold air sinking.

Unknown said...

I ordered the (expensive!) book, so I will read up on it - especially the ceiling option.

If one thinks of radiant heat transmission as "the warming of things", then it makes sense that slab edges would transfer a lot of heat, because they have a big "thing" right next to them. Especially if you also think of heat as a bit of a fluid that finds the paths of least resistance and exploits them with flow.

In the meantime, I still haven't done any experimenting with Perlite (which was pertinent to the topic) as an under-slab insulation alternative. I'll let you know when I get a chance to play around with it & talk to the local tree-huggers who have used it. I may mock up a little section of floor, complete with concrete, and see more clearly what I'm dealing with.

All the best with your permits! Great news about your engineer!

SENWiEco Designs said...

The book is worth every penny and should be required reading for anyone in the hydronic heating industry (and quite frankly the forced air as well)

Unknown said...

I got the book. Didn't realize it was a "textbook". But now I understand why it could be so expensive. Reminds me of university.

Anyways, just wanted to thank you for pointing me to the book. It's very good. I've learned a lot just in the first chapter and a half. I believe I will build a better house, with a better HVAC system because of it.

SENWiEco Designs said...

You are very welcome - I knew you would appreciate it. He is very knowledgeable and is considered the guru of hydronic heating.

SENWiEco Designs said...

Well I dug up the samples today and delivered to BCIT for testing. I first weighed them and it is very clear that EPS absorbs a LOT more water than XPS. The samples were continually wetted for the last 8 months. The XPS samples all were 130 grams when buried and came out at 158/173/180 grams. The EPS went in at 139/133/133 grams and came out at 511/447/494 grams. So proof is in, XPS is more appropriate for below grade. I will post the results of the official testing from BCIT when available.

Also an update on the density of the below slab insulation. My structural engineer did not really have any concerns and basically indicated what ever I wanted to use was OK as even the lower densities would have more than enough resistance.

Unknown said...

Hi. I leave in Cyprus. The climiate here is quite hot during summer reaching up to 40 degrees celcius. During winter the temperatures could fall down to 5 celcius but this is just for a few days. The rest of the winter temp is around 15 -25 celcius. We also have a lot of days during the year with quite high humidity.I read so manu information about xps vs eps and i cannot decide which one i should install for external insulation of the walls of my house. What do you suggest?

SENWiEco Designs said...

Hi Demetris - Thanks for visiting my blog.

I am a firm believer that a wall should be highly vapour open to the low pressure side of the assembly. This is why I would not utilize any type of rigid or spray foams for above grade assemblies in my designs. This is regardless of how good the air barrier is. I instead prefer a mineral wool insulation on the cold side of the assembly.

I am comfortable with my climate zone (heating dominated) but not cooling dominated zones, so really cannot comment definitively on your conditions. In heating dominated zones you want the majority of insulation on the exterior side of the sheathing but in your zone, it may make more sense to do a 50/50 split to ensure neither side of the sheathing will ever reach the dew point.

SENWiEco Designs said...

Demetris - For info specifically on your climate zone and for assemblies that use rigid foam - see John and Joe are masters at building science and you can be rest assured that the assemblies listed will work for you. I personally just feel they are not a sustainable option and still prefer the safety of a more permeable wall.

gravelld said...

Can you tell us more about the EPS/XPS used?

E.g. density, any added fillers (graphite EPS etc - assume not as it appears white)?

Unknown said...

Hi GravelID,

Thanks for visiting.

The XPS was Foamular C-300 from Owens Corning.

The EPS was PlastiSpan 30 from Plasti-Fab

Both of these actually exceed the compression strength of the 20 lb or less material typically placed under slabs.

I am just about finished installing a full time lab that will test below grade insulation solutions.

Let me know if you need any further info

Anonymous said...

Are the results of this anywhere?

SENWiEco Designs said...

Yes the test results are available here: