Building Science-Passing a Blower Door Test

In my last blog, I talked about vapor diffusion and the perm rating.  Sometimes it is better to eliminate the polyethylene sheeting most builders in Minnesota are using for both the air and vapor barrier.  The problem then becomes passing the blower door test.
The builders and insulating contractors in Minnesota have become very good at using polyethylene as an air barrier.  I have only tested one new home that did not pass a blower door test whereas other areas of the country, builders struggle to achieve 3ACH50.  I have also only tested one home that did not use either polyethylene or closed cell spray foam as the main air/vapor barrier measure.  That home passed the blower door test with 1.78ACH50.  (3 ACH50 is the minimum code requirement).  It can be done!

The changes that need to be made.  When using poly, most builders don’t have to give much thought about air sealing.  Usually the insulating contractor takes care of the details.  Without the poly, we really need to think about how air moves though a building assembly.  I believe the air sealing needs to start outside the building envelope, during the framing process.  Many builders believe the water resistive barrier, or WRB, such as Tyvek, is acting as an air barrier.  It does, but only in a limited capacity.  The air sealing should start inside the WRB with the sheathing and framing connections.  How the wood framing is sealed to a concrete foundation and how the wall framing is sealed at the roof connection are two of the most important details to get right. These areas have the highest natural air leakage in the home due to the stack effect.  The windows and mid-wall sheeting have less natural air leakage, but still are important areas to air seal correctly.  Make sure the connection of the windows and doors to the exterior air barrier and WRB and detailed correctly! There are many new products on the market designed to reduce air leakage. Products like Zip Sheathing, which bakes a water-resistive barrier directly onto a sheet of OSB along with Zip Tapes that produce a very impressive bond across joints, or fluid applied WRB’s used to water proof and air seal difficult framing transitions are designed to keep water and air out of the building assemblies.  There are even peal and stick WRB’s which make a great air barrier.  There are many more air sealing and weatherization products that I will be talking about in the coming months.  All these air sealing methods and materials decisions need to be made during the planning stages, not after framing has begun.  Because there isn’t enough to worry about, I’ll throw one more thing to consider in this paragraph.  We also need to think about the perm ratings of the exterior assemblies.  Read my last blog posting here.

Interior air sealing measures.  In my opinion, all air sealing needs to start on the exterior, but should continue to the interior.  The air tight drywall method will help further reduce air movements.  By using an acoustical caulking or other fluid applied gaskets and sealants, drywall can be sealed to the home’s framing.  One challenge with this method is the interior air sealing measures have moved from the insulator to the drywall installer.  The drywall installer now has to care about the blower door test!  Eliminating recessed can lighting will also reduce air movement.  All new recessed lighting fixtures installed in the building ‘s thermal envelope are required to pass an air infiltration test of 2CFM@75PA.  This is a test conducted in a laboratory.  Let’s say 20 recessed cans are installed in a home and they all leak air at 1.5 CFM for a total of 30 CFM.  This leakage could result in a failed blower door test.  Another good idea is to keep the ductwork inside the conditioned space.  This will not only reduce the blower door test results but also reduce the costs of building the home. Building code requires that all ducts that leave the building envelope be tested for duct leakage (many houses fail this test) and be insulated.  The one new home I have tested that did not pass the blower door test failed because the ductwork was located in the attic space.

Closed cell spray foam.  Closed cell spray foam makes an excellent air barrier and 2 1/2 inches will achieve the required perm rating.  Most closed cell foam has an R-7 per inch insulation value, one of the highest R-values per inch of all the different common insulation types.  The drawback is the cost.  It is also one of the most expensive insulations.  I typically recommend using closed cell foam on the difficult to insulate areas, such as rim joists or roof to wall connections when roof ventilation is being used.

Other air sealing suggestions.  Train the sub-contractors!  The best air sealing methods can all be erased by a sub-contractor that doesn’t understand the building envelope.  I remember working as an electrician back in the late 1990’s and early 2000’s.  Many times, we were cutting big holes to feed wires into attics or cutting the polyethylene sheeting to modify some electrical circuit.  Some of these changes in a buildings envelope, especially in a newer, high performance home could cause significant damage to the buildings structure.  The builder and all the sub-contractors need to have an understanding on who addresses any changes made to the air barrier during the construction process.

A hole cut in the air barrier by an HVAC contractor.

A new air sealing process.  I was recently at a new home under construction in the Minneapolis area.  A fairly large structure with a finished basement plus two additional stories, around 4,000 square feet.  The home’s utilities were completed, there was polyethylene sheeting on the ceiling (a ventilated attic space) and minor air sealing done on the building’s shell, mainly caulking between joints of the framing lumber.  The rim joist was spray foamed, but no other insulation had been installed.  The exterior OSB sheeting was the main air barrier. The house tested at 3.5ACH50, just half an air change from passing the blower door test.  If the sheetrock were to be installed using an air tight installation method, this home would easily pass a blower door test.  This house was a demonstration home for AeroBarrier

Aerobarrier air sealing application equipment.

and  being tested by Minnesota Center for Energy and Environment (MNCEE).  After the AeroBarrier application, the house tested under .5ACH50!  This is the lowest blower door test I have seen to date.  I will be discussing the AeroBarrier process in depth in a future blog.

Can these measures make a difference?  Yep!  Comfort, indoor air quality, durability, and energy costs can all be improved.  Some of these techniques will add significant costs to a home, some, not so much.  In the end, what is important is that the houses we build today perform better and last longer than the home we built just a few years ago.  We are moving in the right direction.  Want to read more?  A great article by Jake Bruton and Steve Baczek at the Journal of Light Construction’s web site can be found here.

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