Air Leakage of Old House - Does it matter?

*** Update - Upon further reflection of this topic, I do feel there is one additional reason to perform energy upgrades and discuss it in my project journal http://www.theenclosure.ca/windy-house ***
 
This blog entry will analyze the air leakage of a 1954 house and relate that to the energy use and durability of the house.

Before I started tearing down my 1954 single storey 1500 sq ft bungalow to make way for theEnclosure.ca, I decided to have the house tested for air leakage by Michael and Deborah from H&H small home solutions inc (hhsss at shaw dot ca).  H&H typically perform leakage testing to determine the EnerGuide rating for new construction, so this was going to be a new experience for them (and their blower door).

They came by in late March of 2014 after I had moved out but before I had completely emptied the house and before any deconstruction had been performed. The house was prepped by sealing the wood burning fireplace insert, range hood, and the through-wall mailbox before setting up the blower door.

We first ran the door in a B configuration and were unable to get up to the final test pressure of -50 Pa. But even in this configuration we got some scary results.  I had not really made any great attempts to air seal this house over the years.  There was weather stripping on 1 of the 3 doors, and the cedar siding had each coarse sealed to the next and the siding sealed to most of the window and door frames. But no attempts had been made to seal the interior interface with the attic plane and 1 of the doors had a cat door in it and the main door itself had very large gaps around it.

Prior to starting the test I had predicted an air leakage around 8-10 ACH (Air Changes per hour) @ -50 Pa, but early in the test we could tell it was going to be well above this. Deborah could tell just from the sound the fan was making (there previous worst house they tested was 11 ACH @ -50 Pa which was scary as it was new construction).

Right off the bat we reached 28 air exchanges at only -15 Pa!  To give you an idea, 15 Pa relates to a wind speed of only 11 MPH.  In the fall, I regularly recorded winds speeds on the property at this speed or higher so during those events all of that CONDITIONED air in the house was potentially changing over 28 times every hour. That is a lot of extra load on the heating plant and also explained the drafts my wife typically was feeling.


Over the next hour or so we tested at various configurations until we ran the fan at full open configuration (no restrictor plates). Only then could we get enough air volume to allow us to reach the target test pressure of -50 Pa.  AT the full test pressure we recorded an air leakage of 21.74 ACH -50Pa.

Now wait, some of you may have noticed that that is less air leakage than what we observed at only -15 Pa.  How is this possible you say?

Well, it is actually quite common.  As more and more negative pressure is placed on a dwelling, the two surfaces on each side of an air path (leak) can start to come together.  Eventually they can close up tight and stop that leak.  This why I have a bit of an issue (actually quite a bit), of testing dwellings at -50 Pa.  This relates to a wind speed of 20 Mph which is much higher than some locations would experience on a regular basis and much lower than other locations average wind speeds.  As such, it is my opinion that their should be a standard test pressure PER geographical region.  This could be a table much like the climate zone charts, but would be based on the average yearly mean wind speeds for a specific area.  Homes in higher wind speed regions should be tested at higher pressures than homes in lower wind speed regions.

(Side bar - the reason the wind speed is important is that it is this force that will effect the pressures on a dwelling.  Yes a house can depressurize under mechanical ventilation, but these are usually infrequent where wind depressurization or pressurization of the dwelling could occur for months at a time in windy locations).

So - these tests showed that the house was VERY leaky.  What does that mean in terms of heating bills, comfort, and building durability.

Heating Bills

The truth is, this air leakage did not really make a huge difference in energy costs.  My heating bills (for space and domestic hot water) were typically well under $2000 a year (We are under $2000 for both gas AND electrical use).  As I am a heavy bath user, it is safe to say aprox 40% of this was domestic hot water use.  This leaves an estimated $1000 in annual space heating costs.  That works out to less than $100 a month, or well less than the cost of a weekend dinner out.

We typically had the heat set for 72-73F in the wintertime and basically did not tough the thermostat all year.  It was not uncommon for the heat to come on during cold late spring and early fall evenings.  We did however use a programmable thermostat that was set to come on at 7 AM, step down to about 65F at 8:30 AM, come back to temp at 4 PM, and step back down to about 68F at 11:30 PM.  This was only partially for energy savings.  The night time set backs were used because we had hydronic heating through large built-in wall registers (1-2 per room) via a 1980's gas boiler.  The pipes went through and rubbed on the wood sub-floor assembly, so if the heat came on during the night the 'clicking' would wake me up.  So we partially closed the bedroom door (so cats could still get in and out and not cause another source of nighttime wake-ups) and used an electric oil heater to maintain a comfortable temp in the bedroom.

While air tightness is important, it will not make a huge difference to your pocket book unless you have a very large and leaky house.

Comfort

The air leakage did however make a big difference in occupant comfort and should, in my opinion, be the biggest (and probably only) reason to upgrade an older home.  The house was uncomfortable to sit in near any exterior wall in the winter months due to the drafts present.  I was quite surprised when I saw how leaky the fixed, but home made, windows in the living room were.  The builder had just placed the single pane of window glass against a wood surface and clamped it with a second wood component.  At -15Pa, the wind just whistled through these locations.  There is no question, that making the house more air tight would have made the house more comfortable.

Durability

Normally when one discusses the reasons for making a dwelling air tight, it is in the context of a 'modern' home with current levels of code required insulation.  With modern levels of insulation, it is critical to ensure that air leakage does not occur, in order to prevent interior air leaking into the wall or roof assembly and condensing on cold sheathing.  Left unchecked, this will often lead to mold and rot within the assembly.

The key here is the qty and location of the insulation.  As soon as enough insulation is placed inside of the sheathing to allow the sheathing to cool down below the dew point of the interior air, you now have an assembly with a very high liability should any appreciable amount of air leak into that assembly from the conditioned interior. This is because air currents are the #1 mover of moisture next to bulk water leaks caused by plumbing leaks or incorrectly detailed cladding or roofs that permit bulk rain water entry into the assembly.

But in older houses like the one I took down (which had ZERO insulation in the walls), there is not enough insulation present to block the heat loss from the house enough to allow the sheathing to get to the dangerous dew point conditions.  If you never reach the dew point, you can have huge amounts of moisture moving into the wall via air leakage and never have to worry about it because it stays in vapour form and just moves on through either to the outside of the dwelling or back into the inside. There is never liquid water that results from this air leakage. This is the reason why older homes have performed so well over many decades without the presence of air barriers, vapour barriers, or even effective water shedding surfaces.  The heat loss has always been enough to 'cook' any accumulated moisture out of the assembly.

Conclusion

We have identified in this article that there is not a huge financial penalty for a leaky house.  In my case, the costs per month for space heating were under $100/month in what is considered a cold-heating-dominated climate.  This $1200 annual investment would not get very far in paying for a deep energy retrofit which typically would cost 10's of thousands of dollars.  Lets say you could reduce the heating load even as much as 75% (purely speculative and most likely could not meet), this would represent $900 annual contribution to renovation costs.

A REALLY cheap stud level renovation for my home (including new windows and doors) would have been at least $60K (going to need to rip out parts of bathrooms and kitchens so most likely will totally renovate those rooms - my budget of $60 assumes very low end cabinets for these rooms).

A very intensive attic floor plane sealing regime would have been at least $15K (not going to do this process without bringing attic up to current insulation levels when done).

At a highly inflated $900 annual savings, these two projects would have a 66 and 17 year payback respectively. The attic plane sealing payback would most likely be much longer as only sealing this plane would probably represent only 50-70% of all air leakage present and therefore there would be reduced energy savings.

And my house did not represent an unusual annual energy bill. This US Energy Summary shows that for the West, the average annual winter heating bill per household varies between $1300 and $800 depending on year.

In the end, due to our really low energy costs, and the likely hood that they will not appreciably escalate for many decades due to Government interference, it makes very little sense to upgrade an existing homes energy performance for personal financial savings.  Therefore the type of renovation needed to reduce air leakage or increase thermal performance, only makes sense if the home is being renovated anyway for cosmetic or occupant comfort reasons.

On a separate track - this logic also holds true when analyzing extreme new construction programs like Passive House.  The costs to reach passive house levels of energy reduction will not be paid back over the lifespan of the dwelling in most cases. The added detriment of these programs is that the embodied energy of the insulation products built into these dwellings also do not have a pay back within the lifespan of the dwelling.  Instead for new construction, it makes more sense to build a "Pretty Good House" (coined by Joe Lstiburek) and then use the excess capital available to either contribute to distributed or on-site energy generation.

It is however critical that air leakage be reduced down to a minimum (experts do not agree how little is adequate - but the number is somewhere between 1ACH+/-50 and 3ACH+/-50) for new construction or energy retrofits IF, you have built an assembly with enough insulation inboard of the sheathing to cause the sheathing to cool down to the dew point potential of any leaking interior air.   If you build a safer assembly with the insulation outboard of the sheathing (or enough outboard to maintain the sheathing above the dew point potential), then while air leakage is still important to address from an energy loss standpoint (the costs to get it right during construction are minimal and will be paid back by reduced energy usage), it usually will not cause a durability concern for the assembly.  This of course is all from the perspective of a heating dominated climate.  The direction of flow and order of layers for the assembly are different in a cooling dominated or mixed climate.

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.

Visualizing Wind Patterns

I remember multiple lectures during the BCIT BLDC 3050 & 3060 Building envelope courses that discussed the strange patterns wind can take when it hits a building.  Well some smart individual figured out a way to visual it in real time.

Check out this cool video

Why do we continue to drill holes into our buildings?

I was recently driving by yet another building with vapour ‘diffusion’ ports (VDP’s) and it got me thinking:

Why do we continue to let our buildings be drilled full of holes?

I have always believed that the ports did not work based on previous information I had reviewed, and they are certainly not a recommendation made in the Best Practices Guide published by the Home Protection Office (HPO) in collaboration with some of the best engineering firms in the North America, firms like RDH, RJC, and Morrison Hershfield.

So why do a limited number of buildings still incorporate these ports in their designs?

First of all, we need to try and understand the intent of vapour diffusion ports. As I understand it, the intent is to help walls ‘dry out’ by allowing the wall assembly to ‘breathe’ more easily.  The ports are intended to increase the breathability of the assembly by means of diffusion, over a solid sheathing base line. By ‘breathing’, we are referring to the wall’s ability to ‘lose’ moisture in a vapour form, by allowing that moisture to go ‘through’ a wall’s materials and evaporate into the outside air (low vapour pressure side). We say walls can ‘breathe’ if all of the materials on the low vapour pressure side (which can change direction depending on conditions) are vapour-open, or that they have a high permeability.  In simple terms, this means that the pores of the material are large enough to allow a water molecule in vapour form to pass through the material.  The mechanism of passing through the assembly is usually termed ‘diffusion’, but a review of Chapter 8 of Building Science for Building Enclosures by Straube/Burnett details that the actual mechanisms involved with moisture movement through a porous material are extremely complex and can involve surface diffusion, capillary action, evaporation, convection, absorbed moisture transport, etc.  Even today, the flow of moisture through materials is not fully quantifiable by scientists or fully predictable. What is a provable fact is that in order for vapour diffusion to take place there needs to be a difference in vapour pressure across the two sides of the material (or assemblies) in question.

So are these ports working as intended?   How wide a range does each port have? Are there other mechanisms at work?  Is this an effective drying strategy?

In researching this topic I came across two related studies performed in the late 1990’s after the leaky condo crisis.  The first study titled The Envelope Drying Rates Analysis Study looked at the abilities of various wall assemblies to dry out (in lab conditions) while changing up components like sheathing materials, capillary break depth, and sheathing membrane materials.  What I found most surprising was that none of the panels were dry to safe levels (below 20%) even at the end of the 2000 hour study.

The second study titled Evaluation of Vapour Diffusion Ports on Drying of Wood-Frame Walls Under Controlled Conditions utilized the same panels, but drilled out vapour diffusion ports on some of the panels and then re-ran parts of the experiment to look at the effectiveness of the vapour diffusion ports specifically.  What was again remarkable was that, although the VDP’s did provide a minor increase in the initial drying rate of the OSB panels (no change in the rate of drying in the plywood panels which was already higher than OSB with or without the VDP’s ), all of the panels (OSB & Plywood) again contained areas with dangerous levels of wood moisture at the end of the study (which repeated the first studies results).

After reviewing these studies and Building Science for Building Enclosures,  I now have a better grasp on the true mechanisms at work with vapour ‘diffusion’ ports, and only a little of the process actually involves diffusion.  Where the ports have been effective to any measurable level, some form of convection must take place in conjunction with lots of capillary movement. Let’s look at these mechanisms in a bit more detail.

Convection (or air movement) can occur because of air leakage through an assembly from the high to low pressure side.  It can also occur by means of the convective currents that can develop within each stud bay if a low density batt and fill insulation (i.e. fibreglass) is used.  Neither of these mechanisms is desirable from an enclosure performance standpoint. Convection forces within the wall assembly can deliver moisture-rich air derived from the ‘wettest’ parts of the wall assembly (studs, plates, & insulation) and deposit that moisture onto the back side of the sheathing (which can allow a generalized increased drying rate from the sheathing by means of diffusion to the outside air). Where that moisture is close enough to a port, there can also be a localized marginal increase in permeability over the base-line sheathing.  This can help dry an assembly if no additional moisture is introduced through the convection mechanism, but air leakage through an assembly can often bring with it high levels of moisture into the assembly from the interior of the dwelling.  And once in the assembly, that moisture can then condense on any of the wall elements with surface temperatures that are below the dew point of the leaking air. (See my July/2012 and March/2012 blog entries for more info on dew points and moisture in wall assemblies).  Stud bay convective currents often create ‘air pumps’ that can lead to cold interior walls and thermal bridging, causing additional heat loss (you are bypassing the insulation by delivering heat to the outside sheathing and taking it from the inside wall board).

Now lets look at capillary and diffusion forces in connection with these ports - as areas close to the ports dry out via diffusion through the holes in the sheathing (which have a higher permeability because the moisture has to only diffuse through the sheathing membrane and not though the membrane AND the sheathing), or by air leakage through the holes (because the sheathing membrane has been damaged or is not sealed as an air control layer), that moisture is replaced by wetter neighbouring regions.  The process continues until, over time, the moisture levels within the assembly equalize.  The assembly as a whole is also trying to equalize with its lower vapour pressure neighbours (so in this geographical region, usually to the outside).  The entire process continues until all regions have the same vapour pressure (something that in practice rarely occurs as there is always changing interior and exterior humidity and temperatures which lead to changing vapour pressures).

The problem however, with relying on the diffusion and capillary forces alone is that, although capillary forces are faster than diffusion, both forces will not dry a wall assembly out fast enough to be an effective deterrent to decay and fungal growth.   This was confirmed in both studies, where portions of every test panel still had wood moisture levels in the danger zone for promoting (not just maintaining) rot, even after close to three months, regardless if VDP’s were installed or not!  These studies only had one wetting period - What happens when you have repeated introductions of moisture (interior air leakage, exterior leaks, or plumbing leaks)?

What does all of this mean? Are these results important?

Let’s look at where moisture can originate and how effective VDP’s are at addressing that moisture source.  There are five causes that come to mind:

• Initial wood moisture elevated during construction due to the rain forest we are building in, 
• Bulk water entry (after construction) from the exterior around poorly detailed penetrations and even through vapour diffusion ports, 
• Bulk water entry from water pipe and drain leaks within the assembly,
• Air leakage through the assembly causing condensation,
• Vapour diffusion into the assembly (usually caused by excessive RH% levels in the dwelling’s interior and/or inadequate vapour control layer)

The easiest way to address the first cause is to leave the drywall, poly, and insulation off the interior of the structure until the assembly dries out.  The problem is that construction schedules are so tight; we typically do not allow adequate drying time.  If we are unable to allow sufficient time for a wall to naturally dry out before boarding, then we need accelerate the process by adding heat and air movement to the mix (and in some extreme cases, de-humidification). If we rely on the VDP’s alone to allow the wall to diffuse the moisture out and close up the assembly in a wet state, we could be waiting many months for safe moisture levels within the assembly to occur, and by that time a healthy crop of fungi would most likely have already taken root leaving us with a sick wall assembly. 

VDP’s Score: F

The second and third mechanism only has one solution – stop the bulk water entry!  There is no effective strategy to manage bulk water entry once it has occurred. Even BC’s Rain-Screen requirements will not solve repeated bulk water entry into a wall

assembly.  You must stop it at the source. 

VDP’s Score: F

The fourth mechanism of moisture entry is also somewhat simple to address. Stop the air flow through the assembly by means of an effective air control layer.  Unless you stop the air flow and the resulting condensation, the rate of moisture input will again often overwhelm the slow drying rate of a wall assembly by means of diffusion only, with or without VDP’s.

VDP’s Score: C- down to F (depending on the incoming moisture levels and type of sheathing installed). 

The final mechanism also has an easy solution in most dwellings.  Control the interior humidity by means of mechanical ventilation and/or install an effective vapour control layer in the assembly.

VDP’s Score: C down to F (depending on incoming moisture levels and type of sheathing installed).

Let’s summarize this information.  If a VDP is sealed against airflow, we are generally relying on diffusion and capillary forces and possibly stud bay convection currents to help dry out a wall assembly.   However these processes are slow, and the VDP’s only account for a limited improvement of drying potential over base lines.  As a result the assembly would typically not dry out in time to prevent decay regardless of the presence of the VDP’s.  If we allow air movement to occur through the ports, the air movement that would occur through the assembly can often bring with it levels of moisture that are easily able to overcome the limited additional drying capabilities represented by the VPD’s.

Are there downsides to including VDP’s “just in case”?

Well, it turns out these holes are usually conveniently placed just below windows and other enclosure penetrations where any incorrectly detailed flashing or membrane (like the holes drilled through all the water-resistant layers in the photo - below left) could direct water along the surface of the sheathing and into the wall assembly through these strategically placed inward water highways!  Is this really a wise practice?

We need to stop this insanity.  I put a challenge out to all of the engineering firms that know better.  Please stop allowing your buildings to be drilled full of holes. Start taking the architects, building officials, and contractors to task to stop this poor building practice. Start educating the teams you work with and let’s start building smarter!

WHAT WERE THEY THINKING?

Figure 1 Vapour ‘Diffusion’ ports (holes in sheathing that act as inward water highways) on buildings under construction in North Vancouver, BC