Now that the drywall is complete (and looks great!) I thought it might be time to go back and take a look at the crowning achievement of this whole renovation – the roof assembly. It took a substantial amount of time and cost to complete and it was not something I had even considered at the outset of the project. However, I think the added loft space, the insulation values and the visual space that it creates in the bedrooms and hallway made it worth the effort.
In terms of R-value for the assembly it was clocking in at R7. That translates to heat losses of about 36 Watts per square metre during winter temps of -20°C (occurs annually about 0.4% of the time). The NS Building Code requires R40 in this type of roof configuration.
The new unvented roof configuration is more like a cathedral ceiling which requires a minimum insulation value of R31 in the NS Building Code. The original roof rafters are 4.5 inches deep, so even if we filled them up with spray foam at R6 per inch, we would only get a total of R27 for the total roof assembly. So we had to deepen the cavity by framing down a few more inches, which we did with 2x6s in the bedrooms and 2x4s in the loft in order to maximize head room. With that additional insulation cavity, the assemblies are R54 and R45 respectively. The attic heat loss during (abnormally low) -20°C winter temperatures is reduced to 5 Watts per square metre versus the 36 Watts in the original uninsulated roof. Keep in mind, that all this insulating and tightening up is only possible if you provide a mechanical source of ventilation into the house. We installed a new Heat Recovery Ventilator unit in the loft.
Initially I decided the newly framed down portion of the roof would be filled with R20 fibreglass batt. A layer of spray foam followed by a layer of batt insulation in deep cavities is a common application called “flash and batt”. The idea is that the closed cell spray foam is a high quality, high performance insulation/air barrier/vapour barrier, but it comes at a relatively higher cost. So if you use it to your advantage in the first few inches the rest of the cavity can be filled with a lower cost batt insulation to provide the remaining R-value since the air sealing and vapour diffusion have been already addressed in the first couple inches of foam. And that, folks, is what we call optimum value engineering.
One concern with this arrangement is the old one-third/two-thirds rule for placement of a vapour barrier in the insulation assembly. The old adage of vapour barrier goes on the warm side of the insulation still holds, but the 1/3 rule-of-thumb says the vapour barrier can be installed up to one-third of the way in the roof (or wall) assembly’s R-value. The purpose behind all these rules and axioms and adages is to prevent moist interior air from filtering too far through the assembly where it will eventually hit dew-point temperature. That can lead to water inside which can lead to big problems. The spray foam IS the vapour barrier and it’s buried underneath 5.5 inches of batt insulation. Is that one-third of the way through the assembly’s R-value? Well, I could do a quick check to see where that location was, but rather than rely on bumper sticker wisdom for the assembly I decided to calc it out. It’s a pretty straightforward exercise if you can find the thermal conductivity and the vapour permeance of the materials. I set it up in a spreadsheet and when I mess around with interior and exterior temperature and relative humidities I can see the condensation potential in the wall and roof assemblies. It’s a little more accurate than the rules-of-thumb and it makes me feel slightly more secure. The upshot is, there is potential for condensation in the assembly if I am running a hot tub in my bedroom during a hellishly long cold snap in January. I am ok with that risk.
My original calculations were for fibreglass batt in the roof cavity. Martin actually used Roxul (made in Milton, Ontario!) which is rock wool. That added a few extra Rs to my R-value. So the roof assembly R-value went from 50 to 54. That’s 74% more than code required minimum.
My Wall Wants to Party All the Time
The party wall on the main floor also received some insulation treatment. The concern here was to improve the sound barrier rather than reduce heat loss. Surprisingly a single brick wall is not quite enough to stop sound transfer. I did a little research and learned that you have to block air leakage, keep rigid material layers from touching when possible and fill all air spaces within the assembly. So we built a 1-5/8 inch steel stud wall just about a half-inch from the party wall with the intention of filling the cavity with batt insulation. Good old Roxul has a product for this called Safe and Sound that strangely enough doesn’t even list the R-value on the package. That means it probably isn’t as good compared to their thermal insulation products. A higher density batt would help with sound dampening but would have the disadvantage of increasing thermal conductivity.