Thermal Bridging and Dew Points

Anyone who has heard me speak for more than 10 minutes has probably heard me talking about dew-point potential.  It is at the heart of good building science principles.  Move the dew-point to somewhere outside of your assemblies, and your assemblies will have a built-in safety regardless of any air movement through them.

But for many, this dew-point conversation is esoteric and abstract, they can not usually visualize a dew-point occurring (that they are aware of, the condensation on the outside of a glass or can of cold liquid is a demonstration of dew-points) and so cannot appreciate the importance of reducing its potential in building enclosure construction.

Well by happenstance, I had an excellent example present itself to me at my home this afternoon.  I was coming back from my neighbour’s house after ‘supervising’ the dismantling of some scaffolding, when I observed condensation on the inside of the window near my front door.  Now, I generally have single pane heavy aluminum frame windows throughout my home, so condensation around the frame margins of the windows is not an unusual occurrence.  However this window had a wood frame (site built) and did not usually suffer from condensation.

Why the difference?  Insulation and air films!

Condensation spotted on the inside of the window.

Inside of window is condensation free except behind some mail propped up against window.

Condensation is definitely on the inside face of the window.

General window surface temperature 56.5° F

Surface temperature drops to below 51°F behind the mail.

Temperature in nearby region is 68.1°F

Temperature behind mail 56.4°F with an RH of 64%

So, what does this all mean? 

I have to admit, that this took a lot longer than it should have to figure out.  On the first round of measuring, I had RH levels well below 50% and temps of 66.3°F ‘near’ the window (my office is at 44% and 72°F on the other side of the house and benefiting from all the heat being pumped out from the computer equipment). At these windows temps and RH, the dew-point should have been around 47°F which was clearly well below my initial recorded window surface temp of 53°F behind the mail. 

So why the condensation? 

It then occurred to me that this was a delicate microclimate and by removing the mail for too long to do the testing and photos,  I had inadvertently changed the local conditions and disturbed the air film that would have been against the surface of the window.  So, I put the mail back to let the conditions come to equilibrium again and reran the measurements to get the figures in the photos above.  As you can see, the temperature within a couple of ‘ft’ to the window was now over 68F and the humidity that was building up behind the mail was over 64%.  This works out to a dew-point around 56°F and with a window surface temp of just under 51°F, I was well below the dew-point and had condensation.

The mail was creating enough of a barrier to heat loss (acting as insulation), that it was lowering window surface temperatures over 5F and at the same time was creating a micro-climate where the humidity was ‘trapped’ by still air and building up well above the rest of the house.  

This demonstration highlights how delicate the balance can actually be and why heat-bridging in assemblies is so critical.  Whenever you have isolated conditions that allow a small area of your assembly to cool in relation to the assembly as a whole, or that allow the build-up of humidity, you have the recipe for  liquid water formation and when hidden inside an assembly, that can and often does lead to disaster!

Thank-you Readers!

You are all awesome!

I am now over 650 hits per month on this blog and I cannot thank you enough.

First, it shows a strong interest in the subject matter. Second, it keeps me very motivated to continue posting, and obviously I need to actually keep making progress on the project if I am to have something to post, so you are helping keep the project moving along.

Finally, it shows potential sponsors of the project, that there really will be great exposure for any products that I showcase in the blog and official building site (to be launched early spring).  And any sponsorship received, will ensure an even better and educational website and building lab.

Click to enlarge

Double Vapour Retarders are NOT Fine by Me!

Far too often, a poster on LinkedIn makes comments that defy good building science.  As I am often busy, I try to bite my tongue and just move on, but often the posts push too many of my buttons and I find myself in the position where I MUST comment or commit hari kari!

A resent post titled "Double Vapor Retarders are Fine by Me!" was one such post that pushed me to reply:

I find it interesting that often when I see what (in my view) is bad building science, the individual in the conversation is almost always in the business of selling or pushing foam. The facts are that Physics has not changed – EVER- and the rules are not being ‘smashed’, only disregarded – often to the building owners peril.

Re OP - I believe this is propagating bad language and bad science and (wish) to review my view of the basics.

VB or vapour control layers are to address vapour movement by diffusion. The requirement for the ‘tightness’ of this control layer is based on the vapour gradient across the assembly and location is based on the direction of the pressure. The VB should always be on the high pressure side, so generally inside in heating climates, outside in most cooling climates, and carefully designed and managed in the rest of the climates. Diffusion is a very small vapour driver, and while it cannot be ignored, having a pretty good VB is more than adequate. You do not need to sweat the details on a VB and do not need to worry about holes and untapped seams. A 90% effective or even 80% effective VB is going to be just fine.

An AB on the other hand is critical, and even small holes in AB’s can move large amounts of moisture by means of convection. The AB can pretty much be anywhere in the assembly and many (myself included) like to detail this layer on the exterior of the sheathing, where the number of penetrations are reduced and much easier to detail. When on the exterior, the one issue to address from a thermal performance point of view, is to reduce convection currents within the stud bays by use of a denser insulation.

Can you build a wall assembly with two VB’s? Yes in theory IF you have a perfect AB always. But in practice this is foolish, in my view, and will come back to bite you a significant number of times, because AB’s are almost never perfect in the field and or do not stay so for long even if they start out perfect. So conventional wisdom, based on decades of experience by many, pretty much always recommend that the assembly is generally vapour open from the VB control layer towards the low pressure side of the assembly. Careful design using modeling may show that you get away with a barrier on one side high side and a retarder on the low pressure side, but (in) my opinion, this is only going to consistently work in dryer regions where the load is low anyway. 

The reason a freezer works is because the two metal shells on each side of the foam are perfectly sealed as air barriers and the manufactures go to great lengths to ensure this during assembly with both sealants and gaskets.

The OP mentions “any mistakes are easily corrected by the air leaks”. Really? Are you promoting an ineffective AB? Are you promoting air movement that can move HUGE volumes of moisture into and through a wall??

The only time a dew-point does not exist, is when the temp and humidity of the air on both sides of an assembly never reach each other’s dew-point. Obviously, this is extremely rare. You can however negate a dew-point in various fashions. A) You can remove all of the air in and through an assembly so that there is no moisture to condense. (easy to do in a manufactured fridge – hard in a site built structure) B) You can design your assembly so that your thermal control layer is mostly/all on the low pressure side of the VB and WRB control layers (so that the condensing surface never reaches the dew-point temperature).

The spray foam crowd claim that filling stud bays with foam meets strategy 1. But I have yet to see a spray foam stud cavity where the foam has not pulled away to some degree from the studs (ore probably has been compressed away from the studs as the studs expand and contract into a material with no elasticity). SO, in these circumstances, the ‘air tightness’ has been lost and you better have a Plan B.


In my Linked-IN Post I also commented:

• Both OSB and Plywood are a Vapour retader in dry conditions, but only plywood opens up to over 6 perms in a wet-cup environment, that for instance all sheathing sees in my region. 
• My vapour diffusion holes question was a bit of a trap. Only way they work is when you have air movement through them, which is obviously something you do not want happening. I did a lot or research before posting on this subject http://goo.gl/0SAiSJ

Of course, I should have also added a third method to my post for negating a dew-point, and that is to ensure that all materials down stream of the high pressure side and VB control layer are vapour open enough to allow drying to the low vapour pressure side. 

It really frustrates me, that in order to sell foam (spray or rigid) to the building industry, manufacturers, vendors, and installers all try to twist the science to meet their objectives.  While this will help them sell their product or service, it will often leave the building owners with assemblies containing much higher risk for condensation, rot, and mould. 

I feel it is up to the rest of us to cry foul when we come across these inaccuracies and try to provide some protection to potential victims of bad science.

Determining Lifespan - Updated to 50+ yrs

Back in June (previous post), I wrote about the need to determine the probable lifespan of a building in order to determine the backpack for energy efficiency upgrades and determining the embodied energy of the dwelling.

I wrote that unfortunately, I suspected that the life span for my new dwelling could be as little as 20 years and that I would use 25 years as my expected horizon.

Well I am pleased to advise that I have now significantly extended the time line based on conversations I have had with the outgoing and new District planners for my area.  The following factors weighed in on the discusion:

• The neighbourhood has limited vehicle access with two 'exits' for approximately 700 homes and as such would not be suitable for densification.
• The current owners in the neighborhood are VERY active and vocal and would not support the addition of multifamily into the neighbourhood.
• District is considering allowing for Lane Way and Carriage houses for our neighbourhood instead.
• The real estate values in our neighbourhood are just too high (lots start at $800K and quickly climb.  My lot, at just over 10K sqft, is assessed at over $1M for just the dirt).

So it appears I may have underestimated the 'bastion' mentality and reality of my neighbourhood, and it is going to stay pretty much as it is now for some time to come. I actually welcome this news, as it was a bit disheartening thinking the dwelling may be torn down in aas little as 20 years.

Is this revised time line going to change the way I build? Probably not, the reality is that determining the sweet spot for say insulation levels, requires modelling that I just do not have the experience, time, or money to do right now.  I am instead, going to go with my gut based on research I have done over the last 1-5 years, and on recommendations done by bodies like Building Science Corp.  Once the house is built, and I am able to determine energy use by actual consumption, and learn to use various modelling programs, I will then crunch the number and report back at how close I got to the sweet spot (the sweet spot for me is when the cost of adding site energy generation is cheaper than further reducing energy use).

Stay tuned!

We have an engineer!

As regular readers will know (previous discussion on the topic), I have struggled to find the right structural engineer for my project for a very long time now.  

The process started last March when I chose someone who had a structural issue with 2x4 framing and wanted to put an 8" concrete core inside an ICF.  On the first rear day of activity, he thankfully advised this was not the job for him, something I was in total agreement with.  But this left me in a real pickle, as I initially had planned on starting construction this last spring.  After frantic calls to other engineers showed that no one would be available on short notice, my wife and I discussed and decided that putting the project off a year made the most sense (I did not want to start any later than May 1 in order to get the roof on before the October rains).  And in hindsight, I was no where near ready to start this year anyway and had a lot of technical challenges that still needed to be worked through. So all in all, the delay has been for the best.

Because, I had been 'full tilt' for several months up to the March debacle, I used the decision to delay a year as an excuse to 'take a break'.  The problem is that a break becomes far too comfortable and weeks very soon turn into months.  The last 'break' I had been on took a year!  Fortunately I was a bit more disciplined this time and started the design engine up again in late May.  I received a list of ICF friendly engineers through my good buddy Murray Frank, and started contacting each of them to see if they had the time to fit in my project.  I was finding that between people that did not do ICF anymore, were totally out of business, did not do residential, were not interested, or just did not have the time, my options were limited.

I settled on fellow recommended by people on my first list, who promised a 2-3 week turnaround when I met in his office, but after 6 weeks, not only had we not started, but he had never returned a call or email.  I thought, if we start out this way, how long is it going to take to finish the task and decided to cut my losses before I wasted any more time.

I then contacted some of the people that were previously too busy and some new names I had been given.  I was left with 3 or 4 people willing to take on the work, but based on their own terms.  This generally meant they wanted to take over complete control of the design and move all structure out of Part 9 of the BC Building Code (A prescriptive path to construction) and into Part 4 (An engineered path for all structure).  I just needed assistance on items I could not meet prescriptively like beam sizing and engineered floor and roof trusses, and of course the ICF foundations (and only those because I am a bit higher than the prescriptive code allows for).  The all encompassing engineers wanted to do up pages of detail drawings and in some cases even choose the products I was to use.  And they wanted to charge me $15K+ for the privileged! (my original engineer from March quoted $2500).  This was my design, I had already drawn it up in both 2D AND 3D.  I had already drawn up many of the details I wanted to figure out before building to ensure they worked and were buildable.  I knew what I wanted to build and knew how to build it.

The problem with Part 4 is also that it was going to cost me a lot more money to build. For instance, the Part 9 prescriptive approach requires very little if any manufactured anchors for braced wall panels.  As long as you have the right volume of panels per floor, you can use conventional framing with plywood and everyday nails to build these panels, whereas the Part 4 system often make exclusive use of the Simpson StongTie anchors and rods.  These can add thousands to a typical build.   I had already designed the dwelling to the Part 9 Seismic requirements and did not need any assistance in this regard.

I DID NOT NEED THIS PREMIUM SERVICE and in fact most of this effort would have just been wasted!  I could also tell, that preserving the integrity of my design and my ideas for thermal bridge reduction was going to be difficult with several of the individuals.

So at the end of August I threw a 'Hail Mary' and contacted a name I had received from Durisol (ICF block manufacturer) back in March.  I had originally dismissed the name because they worked out of Guelph Ontario and I thought how is this ever going to work?.  But I was desperate and so contacted Nathan Proper of Tacoma Engineers and was thrilled in his responses.  He advised that he had a BC stamp and that we could arrange any Building Official required inspections with a local engineer at a very reasonable cost.

He further advised "We would be happy to be involved with your house and to help you out by designing the components which need our design. The approach we normally take with these items is to design only the specific items which the owner asks us to --- these are commonly the items which are not covered by Part 9 of the building code.  This is more cost-effective for the owner than checking every little item." 

I thought I had died on gone to heaven, and my neighbour came out to ask why I was running around the front yard hooping and hollering.  The news literally brought me a few tears as I was so relieved after the conversations I had had with others over the last 6 months.  I had hit the jackpot!  There apears to be a dramatic difference in how the design professionals here operate compared to back east (I have often seen this with other construction related items as well).  The best part - they would charge $5K for the basic design package!

Nathan and his team have been responsive, approachable, and co-operative with my goals and ideals.  They approach the tasks in a straightforward, logical, AND practical matter.  They are also sensitive to my budget constraints and have already made suggestions where I can provide input (drawing) instead of utilizing staff in their office.  We only started the real work on  Tuesday, but I feel we have already made great progress.  I came up with a concept for supporting my sun shade assemblies, and a lot of people I am sure would just ignored my suggestions and done their own thing (often at my increased cost).  But they ran with it and advised it should work and that they had done something similar previously.  I can now be a constructive part of the team instead of a bystander, which is what I had always been looking for.

So, it goes to show, trust your gut.  If something does not feel right, it probably isn't and should be fixed or past over. It took a long time, but I KNOW I have found the right person for the project and will be enhanced by their involvement.

And is that not what you are looking for when you are hiring someone to help you build your house?  

AutoCad 2D model of a 3 level single family home.

Ever wonder what a completed model of a 2D three level home prepared in AutoCAD 2002 with all 43 layers turned on at once looked like?

Thought so!

Finished 2D model ready to send to the engineer.  Kind of frightening knowing each line had to be created manually.