Concept

A pretty smart fellow (Joseph Lstiburek – see references) says, when asked about how much insulation is good, "Double it and shut up." I guess he’s heard this question a few times, eh? Generally, his point is that more insulation is better.

However, there is a limit to the value of insulation but that limit is well above what code requires today. I have found a number of “extreme” examples that indicate the practical limits in a climate like ours are: Roof, R-60; Above Grade Walls, R-40; Below Grade Walls, R-20; and, below the slab, R-10. While I have come across some calling for even more, other areas of investment will probably yield better results. Consequently, I have adopted these numbers (R-60, R-40, R-20, R-10) as “world class” guidelines, for our climate; and these are the numbers I targeted in my design.

There is more to insulation than R-value, including: cost, thickness, greenness, and permeability. Also, the specific situation or application should be considered. For example, if you are planning radiant floor heating in a slab on grade, you want more than R-10; R-20 of fiberglass would not be a good idea. And finally, there are structural issues – how will you assemble it?

The bottom line is, "more is better, to a point, and you need to consider the overall situation."

Application

Here is what I used for insulation and why, starting from the bottom and working up.

Below slab: Below the Solar Battery, we installed three, staggered 2” inch layers of Owens Corning Blue Board. This gives an R-value of about 30. This is much higher than the 10 noted above because this foam insulates our Solar Battery from the ground. We expect temperatures in the lower part of the mass to exceed 90 degrees, so more insulation is a good idea. The situation is similar if you are planning to use radiant floor heating, except worse because you are probably heating the slab with some very expensive fossil fuels. The blue board will withstand up to 25 pounds per square inch (plenty for my application); it is vapor permeable (if it gets wet from the top or bottom it will dry out); but, it is not cheap - I don’t want to be second guessing this which would be difficult to replace, so I used what I considered the best!

Foundation walls: The Solar Battery and basement walls are constructed with ICFs (Insulated Concrete Forms) by Quad-Lock. They are 4 ¼” on each side and provide about R-40 insulation. Quad-Lock was the only manufacturer I could find with the thicker foam blocks – all others I found were an anemic 2 ¼” on each side. Once I found these blocks, I didn’t consider price, ease of use, or anything else different manufacturers were touting. As far as I was concerned, the R-value was essential and this trumped any other minor advantages another system might have. Furthermore, Quad-Lock has recently come up with block configurations that allow it to use considerably thicker foam insulation, though I don’t think greater than R-40 is necessary for our climate, they are going in the right direction. After getting the blocks, I found they were a real pleasure to work with – easy to install, easy to keep straight, very solid with the bottom metal track, and very little waste because they are assembled on site. This was the first experience I had with pouring cement in an insulated concrete form and Quad-Lock was rock solid – amazing!

Rim joist: This is the band where the floor joists rest on the foundation wall. It was initially insulated, before the first deck was installed, with spray-in-place, high-density, closed-cell foam. About two inches were sprayed on at that time. Later we will fill the cavity with low-density, open-cell foam. The reason for this timing is because it is impossible to spray this location from the basement later, especially on the ends where the blocks reduce the distance between the first floor joist and the foam block to almost nothing. Foam on the top of the wall, covering the plate is critical in reducing air infiltration.

Later, because the joists pass through the foam and connect directly with the rim joist, we will extend the exterior insulation up to the top of the 3 ½” slab to eliminate thermal bridging. This is a little excessive in the typical house, but in our case some of the dearest heat is in the first floor slab and we don’t want it leaking out unnecessarily. The rim joist will have an overall R-value of over 50.

Exterior walls: We built double 2x4 exterior walls, with a 5 inch gap between them. On the outside 3 inches there is spray-in-place open cell foam. The main reason for this is to take advantage of its air sealing capability. As a matter of fact, it is part of our building process to do blower door testing after the windows, doors and this first layer of foam have been installed. At this time it is possible to find leaks and seal them directly, rather than waiting until after the drywall has been installed, when it will be much more difficult to find and seal leaks.

The remaining 9 inches, including the 5 continuous inches between doors and windows that minimizes the thermal bridging, will be filled with cellulose. We are using cellulose because it is a bit greener than soy based foam in that it is recycled newspaper. Plus, it has the same R-value per inch and it costs less. Overall the walls will have an R-value of over 42.

With the combination of open-cell foam and cellulose there will be no vapor barrier, just drywall and vapor retarding latex paint. The reason for this is that this is sufficient with low infiltration rates to control moisture. If/when any water or vapor gets into the wall assembly, it will be able to dry out or in, depending on where it is located and the season.

Roof Assembly: You can choose to have a vented or an insulated roof assembly. The purpose of these designs is to be sure moisture does not condense on the inside roof deck (typically the plywood the shingles are nailed to). The vented assembly does this by facilitating mild air movement from the soffit to the peak that carries away any moisture in the air before it can contact the roof deck and condense. With more complex roof structures, using these subtle forces is increasingly problematic. The insulated assembly prevents the roof deck from getting as cold as the dew point. The latter is what I have chosen to construct.

Between the 10 inch rafters is 9 ¼” of spray-in-place foam. I have used ½” plywood on top of the rafters, and then two layers of 2 inch polyisocyanurate (closed-cell, foil-faced, sheet foam). Any ¼” or larger gaps were foamed and all joints were taped. On top of the foam is a 5/8” layer of Zip sheathing. I used the Zip System sheathing in both the exterior walls and roof to help minimize moisture entry and to help with air infiltration. A key characteristic of this fairly new building material is that it allows water vapor to exit but won’t admit water. In our particular installation, this material is perfect because it allows any moisture that may get between the 5/8” sheathing and the impermeable, foil-face insulation to dry to the outside. All joints are taped with special Zip tape. Overall the roof assembly provides an R-value of over R-62.

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