Back in the olden days, when energy was cheap, the Habs were winning Stanley Cups, and our knee joints were well-oiled and pain-free, batt insulation was all we needed to keep our homes relatively cozy inside.
Then came the term “thermal bridging”, which enlightened us to the fact that lumber isn’t all that good an insulator. And, considering wall designs featured a stud at every 16 inches on center, the overall R-value of the wall was reduced significantly. A typical wall assembly would have contained either an R-12 or R-20 batt of insulation, depending on whether the wall had been framed with 2×4 or 2×6 lumber. The weakness in this system, energy wise, is the solid lumber, which accounts for about 10 per cent of the wall’s mass overall. With a thermal resistance value of about R-1 per inch, between every 15-1/2 inch span of warm, protective insulation, you had 1-1/2 inches of solid wood that would effectively allow the cold to migrate through to the inside, hence the term “thermal bridge”.
So, how can we as homeowners, and home builders, reduce the cold thermal bridge inadvertently created by the wood studding? The answer is Johns Manville’s polyiso ridged insulation board. With a thermal value of R-6 per inch, polyiso ridged sheeting can be installed directly over the wall studs, or over the OSB (oriented strand board) or plywood sheeting. The JM polyiso board is not a structural sheeting, therefore, a stud wall would have to be re-enforced with steel bracing if it were to be nailed directly to the lumber.
How thick should a homeowner consider going with their JM polyiso? Minimally 1 inch, with a 1-1/2 inch sheeting being better, and a 2 inch ridged board better yet.
Essentially, with the cost of heating being what it is, there’s no such thing as over-insulating an exterior wall. Therefore, a standard wall assembly in 2017 would begin with drywall on the inside, then a 6 mil. vapour barrier, wood studding with batt insulation in between, then a plywood sheeting, followed by JM polyiso, then a house wrap, ending with the customer’s choice of siding overtop. The 6 mil polyethylene vapour barrier is always installed on the warm side of a wall assembly, and effectively stops heat and moisture from entering the wall system.
Many people question why a plastic vapour barrier couldn’t be installed on the outside of an exterior wall as well. Or, be installed on the outside only, since that’s the side that faces the elements, and is the most likely side to let water in. Two elements dictate why we install a plastic vapour barrier on the inside wall only — our colder weather conditions, and the resulting dew point. In a perfect world, with a dry and air tight exterior wall cavity, having a vapour barrier on both sides would be the perfect scenario. Unfortunately, we can’t build a home in the same way we build a fridge, or beer cooler, at least not yet anyway. So, moisture already in the wall assembly, or entering the wall cavity by some other means, has got to be able to escape somewhere. In our Canadian climate, that’s best accomplished by having moisture dissipate towards the exterior.
Without a vapour barrier on the inside wall, heat and moisture would get into the insulation, then hit the dew point (the line where warm meets cold) somewhere in between the studs. An ice mass would then develop in the insulation, killing it’s thermal value, while creating all kinds of mold issues once things thawed out in the spring.
Installing a JM polyiso creates a band of continual insulation, and moves the dew point to about half way into its foam core, well out of the wall cavity. As a result, the thicker the JM ridged foam board, the further cold is kept from the home, and that’s a good thing.