Air Barriers

One of the benefits of blogging is to organize thoughts about next steps. When Sarah visited, she advised me to finish the external barrier first and that was a helpful comment in a sometimes overwhelming project. I am taking apart the insulation layer in the main portion of the house. That means tearing out the drywall, and the fiberglass insulation.

Insulation Piles

Insulation Piles

I’ve been packing the used insulation up into construction trash bags. Another messy and itchy job. I have quite the getup for this task, long sleeves, slick nylon pants, neck kerchief, hat and a good quality breathing mask. I figure we will reuse the insulation that is not damaged by mice. The house was empty for quite some time and although there was not much evidence of animal intrusion, part of the ceiling was definitely inhabited by a large colony of little pooping critters. I’m glad to get all of that out of the house. But most of the insulation is in good shape and seems that it could be reused, if not in the house, then in an eventual outbuilding.

Considering the infiltration of animals, however, the house barriers definitely need to be beefed up. I’m still researching the final design of the exterior walls. An in-depth resource for that project has been the Building Science information website. I have downloaded and read not only their informative construction principles, but also their presentations and seminars over the years. These give a better explanation of their design principles as they have evolved over the years. The two people whose names are seen again and again are Joseph (Joe) Lstiburek and John Straube. These men have worked on building envelope design for decades and have changed thinking regarding air  infiltration and moisture management in walls ceilings and floors. In one marathon session, I downloaded 57 presentations and papers on thermal envelope design from this site. Since there is no audio, it is necessary to read several presentations since the same cases are cited again and again and the prinicples are more thoroughly explained over time.

Basically there is an ideal wall construction and that construction can be translated just as well to roof and floor. There must be a waterproof layer and a vapor barrier, an air proof layer and lots of insulation as a thermal barrier in between. The idea is that the thermal envelope should be a contiguous and continuous barrier between the inside and the outside of the house. This ideal can be somewhat difficult to implement in a home built out of dissimilar materials like ours.

deal same construction wall, slab and roof

Ideal same construction wall, slab and roof

Ideal wall, roof and floor

Ideal wall, roof and floor

We have a passive trombe wall, a mostly cathedral roof, an earth bermed north wall, window walls facing south and small portions of the earth bermed walls that are framed for windows. How does a continuous air barrier extend across all these surfaces?

For now I am concentrating on the wood framed areas, walls and roof. The original construction consisted of from the outside in, either cedar siding or stucco (water barrier), 1″ of foil faced polyiso with the foil facing inwards, (this is an air barrier if properly taped) then there is a window wall facing south that consists of 2 x 4’s surrounded by small areas of fiberglass batt, or 2 x 6 walls with fiberglass batt, these are covered inside by 4 mil poly (that would also be an air and vapor barrier if properly sealed). Finally there is 1/2″ drywall on the interior surface. So the existing walls are pretty well constructed to meet Energy Star guidelines, although the window openings were not taped and it appears there was not any building wrap used on the outside and the poly barrier is broken and not taped around windows and electrical outlets.

I am most interested in eliminating the intrusion of critters and birds (as well as moisture, heat and cold) by creating  new wall and roof barriers. For this purpose, it seems as if sprayed foam would be a good solution. Spray fills in any voids and is pretty impermeable to critters. I would get a couple of inches sprayed throughout the ceiling and frame walls and seal that well where these walls connect to the masonry walls. Then the remaining void could be filled by rock wool insulation which is less comfortable for critters to nest in. You would think fiberglass would be uncomfortable too but that did not seem to be the case. Finally, there needs to be a chase for electrical cable and the gas pipe and maybe even some plumbing vent pipe that goes through the roof. As another option I was thinking about installing a membrane air barrier under the insulation and taping that, then putting up a grid of 2 x 2’s on the ceiling and walls to hold the electrical outlets and wiring before having the drywall installed. That would protect the membrane from nail holes and other issues as the construction ages.

I am a bit confused about whether the foam on one side and the air barrier on the other would cause a wall and roof that could not dry out if it become damp for some reason. UPDATE: Now I know that the foam forms both an air barrier and insulation layer.

Foam under the roof is highly recommended by the building science folks, and if the air barrier is vapor permeable the wall should be able to dry out from the inside. The one problem with foam in the walls and roof are that it is capable of outgassing over time and I’m not sure if the air barrier will contain that as well as the concrete in the floor is expected to contain any outgassing from the foam under it.

Here is a wall construction diagram that I found at the Gregory La Valderra Architect website.

USA Wall System

USA Wall System


There are a few products that could be used for the interior air barrier. The Swiss Siga air sealing products are popular with the Passive House builders in Colorado. These can be ordered from a few small suppliers in the US online or imported by builders in your area. Similar products are made in the USA, I’m also looking at the Membrain sheeting from Certainteed that claims to allow vapor permeability and air sealing. This can be purchased from Menards and shipped to the house and may be available on order from Home Depot or Lowes. Menards also carries Thermofiber rock wool. But the local Lowes carries Roxul, a Canadian brand.

I finally decided that foam insulation was too expensive and ordered the interior SIGA air barrier materials and Roxul from Lowes. The local company A&E Building Systems is a dealer for both the SIGA products and high energy efficiency windows from Alpen, a company in Longmont, Co. I could find nothing but praise for the SIGA products for ensuring a vapor permeable (dry) air barrier, so I ordered the barrier, application tapes and caulk from these Passive House experts.



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