Air Tight

Airtightness is the key to a truly energy efficient home. After all, it doesn’t matter if your home is well insulated if it is leaking cold air (in winter) or hot and humid air (in summer). For this reason we put a lot of effort into making this home as airtight as reasonably possible. This post outlines the steps we took and the results we achieved. At the beginning of the LEED Certification process we attempted a blower door, and to nobody’s surprise the home was so leaky that we couldn’t even get a reading. The home was leaking everywhere; air was coming in around windows, all through the attic, and even straight through the brick! Because we wanted to dramatically improve the energy efficiency of the home we decided to go all in with a full set of new windows (casement, not double hung) that would be more airtight and make sense with the new layer of insulation on the exterior of the home. Therefore, the story if making this home airtight is largely the story of window installation. But first… the brick.

At the time I purchased the home it was painted grey, and looked pretty good! But the latex exterior paint was getting a little old and we were about to cover the brick permanently, so we wanted to make sure that we were doing the job right. Therefore I used an orbital sander to scuff and remove any loose paint and powerwashed the exterior. Then I applied a special paint / sealer:

Sherwin Williams “Sherlastic” Elastomeric paint. This paint is airtight but vapor open, which means that even though air cannot penetrate it, water vapor can. Therefore if in the future the brick somehow got wet, it would be able to dry out through the Sherlastic. This was my first step toward air tightness — to make sure air couldn’t pass straight through the walls. I put 2 coats on the entire house.

Now that the brick walls were airtight, the next step was to ensure that the new windows were installed in airtight fashion. Since the windows were installed as “outies” they were in line with the new exterior siding. Since the new windows would now be sticking out 6 inches beyond the exterior brick wall, we used a window buck strategy.

Above we can see the original window. The white paint is the Sherlastic, and along the sides, in a light blue, I have a caulked a line of Henry Air-Bloc Liquid Flashing.

Next we installed a window buck made of plywood. It is just the four sides you can see in this picture. We sized it to fit the window perfectly, and any difference between the buck and the rough opening was filled with great stuff or fiberglass. As you can see we used Henry Blueskin to seal the buck to the brick. The Blueskin is airtight but vapor open so that in the event there is moisture it can dry. The buck sticks 4.5 inches away from the wall so that in the end the windows will be flush with the exterior siding.

Next we took 2×6’s and attached them on the flat to the brick wall. This was to support the part of the buck that was sticking out.

Next a 1.5″ layer of rigid Polyiso insulation (the silver stuff — it is foil faced) was placed between the 2×4’s and 2×6’s.

Next a continual layer of 2″ rigid Polyiso Insulation was placed over everything. Note that the outside of the plywood window buck and the Polyiso came out flush.

Next 1×4″ furring strips (the wood surrounding the window opening above) where attached with 3″ screws that went through the 2″ layer of continual Polyiso Insulation and into the 2×6’s underneath. This created the structural integrity needed to support each window.

Next the windows were installed to be flush with the exterior (aka “outies”) in the usual fashion.

Here is the window now fully installed with the exterior siding in place. We just abutted the exterior siding to the window frame without additional trim which I think made for a great look. Note that above each window I caulked additional fluid applied flashing (see the light blue goop) to make sure no water coming from above would seep behind the window flashing tape. This is the strategy we used to install the windows in as airtight a fashion as we could.

Now that the brick and the windows were airtight the next major area to consider was the roof. The roof was done after the exterior walls, so we applied Henry Blueskin to the exterior brick wall and left a flap of it tucked up under the shingles so that it could connect to the new roof later. Basically the goal was to wrap the entire roof in Blueskin to create an airtight / vapor open layer that was continuous with the Sherlastic paint on the brick.

Here you can see the Henry Blueskin stuck to the brick at the bottom, going up over a piece of framing (hidden by the Blueskin in this picture) and then tucked up underneath the shingles.

A layer of 2″rigid Polyiso insulation was placed over the Blueskin / Framing, and the 1×4″ furring strips after that. Now the wall is all set for the new roof to be applied.

Here we can see the basic strategy with the roof. The old roof has been removed down to the boards, and a layer of Blueskin has been applied directly to those boards, forming an airtight layer over the entire roof. Note that we are not depending on this as a water barrier. Two layers of 2″ rigid Polyiso Insulation were then applied, with OSB on top of those. Long screws were used to attach the OSB to the boards underneath. Then a watertight roof wrap was applied over the OSB.

Remember the Blueskin attached to the exterior wall? Here (in the lower left corner of the picture above) you can see how it has been applied to the roof boards, then the Blueskin on the roof layered over top of it. This is how we maintained an airtight layer at the intersection of the roof and exterior wall.

Here is the roof now fully wrapped, with a 2×4 awaiting a gutter.

We went with a standing seam white metal roof.

The completed metal roof with metal flashing over the exterior siding.

And the completed roof with box gutter.

So now we have a continual airtight layer of either Sherlastic paint or Blueskin covering the entire home (and the paint and Blueskin sealed to each other), with windows installed in an airtight fashion. This left a few things that needed to be considered to make home truly airtight.

One was the existing doors, which were not replaced. I replaced all of the airsealing strips (“Kerf” strips) and otherwise adjusted them as best I could. Some penetrations I had to just seal with Great Stuff spray foam.

When possible I used gaskets such as the one above from Tescon, which creates a great seal.

In the end we pretty much went as far as we could go in term of making this home more airtight. So how did we do? Many of you reading this will be familiar with a blower door test, in which a fan is placed in a doorway to depressurize the home and measure how much air is leaking through the structure of the house. This test gives a number called ACH50 (measured Air changes per hour when the home is depressurized to 50 Pascals). Really bad is in the range of 10 – 15 and incredibly good is down below 1. The initial blower door test found that the home was so leaky that it couldn’t even be measured, probably somewhere in the ballpark of 15. Our final blower door test revealed this:

For those who know what they’re looking at the number on the left (52.0) means the home has been depressurized by 52 Pascals, and the number on number on the right means that 782 cfm (cubic feet per minute) are leaking through the home. Plugging these numbers into the equation for home airtightness gave us an ACH50 result of 2.1, which is not Passivhaus level, but is a drastic improvement. After a year of renovating, boy was it a relief to see this number!

I will note that the goal for the project for LEED Certification was an ACH50 of 4, so we essentially did twice as good as that. With the home being this airtight, now use of an ERV (Energy Recovery Ventilator) makes sense. Having lived in the completed home for approx a year now, I can definitely say that it is drastically more comfortable now — there are no drafts in winter and the home stays dehumidified much better in the summer. And, of course, the home’s energy use has decreased drastically as well!

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