Insulating for Cold Climates: Creating the Best Envelope for the Budget

It is amazing the array of insulation that is available today. If you wish, you can lose yourself in the debates about the pros and cons of spray foam (of which there are many types), batt insulation, blown cellulose, blown fiberglass, hay bales, etc. Because we decided to minimize the amount of wood framing in the house by using 2x4 studs, 24" on centers (except for the main floor, because the local code enforcer made us use 16" off center at the last moment), our strategy was to utilize the highest R-value material in the interior walls, while using exterior rigid foam sheathing to create a continuous insulated air and vapor barrier on the exterior. By using the rigid foam sheathing, we could also eliminate any plywood or OSB sheathing. A review of the house's envelope ground up:

Sub-Slab Insulation: If you've ever walked across your basement floor and noticed it's cool temperature, or even damp, this probably indicated that it is "slab on grade" meaning that there's nothing between the poured concrete and whatever substrate (hopefully a good amount of "fines" to allow for water to percolate down) your basement/first floor has. Based on the good work of the Building Science Institute, I learned that uniformly their research on various homes in cold climates has shown that sub slab insulation that provides at least an R-5 should be installed (in warmer climates, you might actually enjoy that cool air, and circulate it through your house). Rigid foam, spray foam, foil faced foam wraps and even above slab sub-floor products all exist that allow you to achieve different degrees of insulation and at different costs.

We specified 2" of sub-slab foam, and the builder selected Greenguard's (TM) Extruded Polystryrene Insulated Board. My builder would like me to point out that this was NOT easy to install over the fine gravel substrate, nor was it easy to cut to accommodate plumbing and radon piping (yes, we are in a high risk radon zone). Additionally, I was dismayed to visit the site when the guys came to pour the concrete (remember: early January, nice and cold!) and they were all standing on one piece of board to stay warm by the heater, without any idea that perhaps they should distribute their weight more evenly so as not to crack it. They had no idea what they were standing on, or its purpose. Good oversight for installing whatever product is being used is pretty critical, if you don't want it to be compromised. Also, the gap between the sub-slab foam and superior wall was an issue I discovered last minute. We should have cut the foam to fill this, albeit a very labor intensive process. However, the 2" of closed cell foam applied to the interior foundation wall above the slab that filled this gap should mitigate the potential vapor/air/water transfer. More flexible sub slab foil barriers or spray foam would have been MUCH easier to install in this foundation situation where the studs are built in. Anyone using Superior Walls needs to consider this!!

2" Greenguard EXP Board, taped at seams. Note: gap between Superior Wall cavity (gravel in recessed area). This area was filled in with poured concrete for slab, and then we foamed the cavity with 2" of R7 closed cell foam.

Superior Walls:  As discussed earlier, these have the rigid foam embedded in the concrete, creating an R-12.5 wall. We then applied an additional 2" of closed cell spray (Demilec's HeatLok Soy 200) foam to the cavity and at the joists between the foundation wall and first floor decking. By some accounts, a house can lose up to 30% of its heat through a poorly insulated foundation wall and joist. Our minimum foundation wall R-value is a 26.

Timber Framed Walls:  Remember, we used 2x4" construction, 24" off center for the first (walk out/basement--SOUTH SIDE, rest is Superior) and third floor, with 16" off center for the main floor as code dictated. I searched a number of sites to see what the R-value of the 2x4 board is, and found that the agreed upon number is pretty much an R4. So, wherever there are studs in the house, the maximum R value for these areas due to the 2" of exterior rigid foam we used (see below) is an R-14. This is why some truly advanced framing systems go well beyond standard practices (where code allows), because for every stud, you decrease your overall wall R.

We used a number of products to exceed our goal R-value (conservatively, we are at 34, but this doesn't include the windows or studs, so our total wall R is less), as well as to ensure an air tight barrier. It was really hard for me to decide on the interior insulation, as I was concerned about it's impact on indoor air quality (VOCs, Formaldehyde's), the ozone layer (many foams use harmful blowing agents that are employed to keep the foam liquid so it can be sprayed on), as well as they impact the ability of future inhabitants to modify the house (batt insulation or sprayed cellulose is easily removed/moved, if one wishes to modify the envelope). Changing our envelope is something that future occupants of our home will think twice about, as the entire wall may need to be cut away. Use of spray foams also impacts the future salvage potential of our building materials (unless new technologies emerge for recycling this stuff). Ultimately, the following materials were selected:

1. 1" Dow SIS Rigid Sheathing: R 5.5 per inch. This was placed on the framing before the walls were raised (see above), an easier process than our second inch, which was put up after the windows were installed.

2. 1" Foamular 150 XPS, an Owens Corning Product: Kudos to Paul for discovering this product in the intervening weeks between ordering the Dow foam, and the next "layer." It has an R of 5 per inch, but has the added benefit of containing 20% recycled content! I figure if our house is going to be made out of a certain amount of petro-chemicals, that we ought to try and have some percentage be reclaimed material. Hopefully, there will be higher percentages of recycled content in future products of foam, so that we are not relying on a diminishing petroleum supply. I'm sure the neighbors were amused by the colorful display the exterior foams provided throughout their application.

Exterior Rigid Foam. First layer, Dow SIS product. Second layer, Owen Corning Foamular. When installed in layers like this, all seams are offset and taped (taping on both layers) to ensure that there are no gaps. Windows are foamed around frame to ensure continuous barrier.

3. On the interior of the cavity, we ultimately selected Demitlec's HeatLok Soy 200, which is a closed cell (rigid), spray-applied polyurethane foam insulation. The material is Greenguard Certified, and contains approximately 16% recycled content (plastics), 6% soy, and uses a non-ozone depleting blowing agent, HFC 245fa (at least it passes for the moment, this standard is constantly getting more stringent). I've posted the spec sheet. The "cured" R value of the product is listed as R7 per inch, and it also serves as our moisture/vapor barrier as well as our air barrier, eliminating the need for house wraps. This product is sprayed on the interior of our "exterior" rigid foams. HeatLok Soy 200 Spec Sheet

Finally, the Attic. We used pre-fabricated trusses to create the roofline of the house (with near optimal solar angle on the south side). We went with a standard 30-year asphalt roof, over OSB, with 1" of the HeatLok Soy 200 applied over the ceiling gypsum (the floor of the attic if you will), and beyond the top plate, and then 16 plus inches of cellulose. The 1" of the closed cell creates the air/vapor/moisture seal at this level while giving us an R7, while the remaining cellulose gives us nearly an R60 value. Once they put up the ceiling board, they sprayed in the 1" and then added the cellulose through an access door in the ceiling. I didn't get photos--too messy and dark! Maybe when they put the solar evacuated tube plumbing in I'll sneak up to get a look. The photo below shows the HeatLok spray foam being carried up from the wall cavity (top floor) over the top plate and filling the elbow of the truss.

While I'm certain there are imperfections to be found within the envelope, we tried. Material application/installation needs to be done properly, and having experienced contractors who have handled all materials in various applications is pretty key. We had many materials on-site, particularly the rigid foams, that were new applications for the installers. This caused the process to be slower in the beginning, and in the case of the sub-slab foam, less than ideally installed. However, we did pass our blower test just recently (more about this in the future), which means that we achieved our goal of a tight envelope.