Instead of a plastic film for a vapour barrier between the Roxul insulation and the drywall, John is installing 2 foot by 8 foot sheets of 1.4 inch thick Styrofoam. The Styrofoam adds an additional R value of 7 to the wall, bringing the total insulation in the exterior walls to R21. And, by laying the sheets of Styrofoam horizontally, the areas where the wood studs in the wall would normally create a break in insulation are now also covered and insulated. (Thanks to my brother Michael for that idea.)
These rigid sheets of foam insulation boast a lap joint edge, making a better seal than would be the case with straight edged pieces simply butted together. After the sheets of Styrofoam have been fastened to the wall, Tuck Tape is used to cover all the joints to complete the vapour barrier.
Like the Roxul, the Sytrofoam sheets are mildew and mould proof. Similarly, no insects or animals will be interested in eating it or using it for nesting materials. Sunlight can deteriorate Sytrofoam but the insulating sheets shut into the walls won’t be getting any of that!
Because Styrofoam is flammable, the building code requires a firewall between it and the
living spaces of the home. Happily, five-eighths inch drywall qualifies as a firewall. (Who knew? Not me, but I am learning many things I didn’t know before.) Half-inch drywall wouldn’t be enough but, fortunately, by watching for sales and specials, the cost differential between the two thicknesses can be quite minimal.
(And as a sidenote, one of the things I learned in writing this post is that Styrofoam is actually a brand name and therefore always spelled with a capital S.)
Google “most important place to insulate” and the whole first page of results from a variety
of sources and websites make it clear: the attic is the place. In the ceilings of the upstairs bedrooms, John is laying down two layers of six inch Roxul, perpendicular to each other. The six inch Roxul has an R value of 22, compared with the R value of 14 for the four inch batts used in the walls. A total of 12 inches of Roxul, combined with the Styrofoam
added across the bottom of the ceiling joists will bring us to a total insulation factor of R51 in the ceilings/attic.
Our old home at Prairie Woods will be a nice cozy one indeed. I’m almost eager for winter just to try it out — almost, but not quite.