Less is More
Simple building methods that shrink energy consumption
Article and photos by Steve Maxwell
When it comes to choosing an insulation strategy for your new or renovated home or cottage, a bit of little-known information and the right kind of action can save you a lot of time, money and energy, and deliver these benefits for as long as your building stands. The key is to understand the truth about energy performance in the real world, and more specifically something strange that observant builders see all the time.
The fact is, not all insulation products perform equally well, despite similar, or even identical, R-values. The reason has to do with the three different ways energy transfer happens. One way is conduction, the movement of heat between objects or materials in contact with each other. Convection is another. This happens when differences in air density cause warmer and cooler currents to circulate, moving energy as they do. The third form of energy transfer is radiation. It happens without air movement or physical contact. The sun’s heat, for example, warms the entire earth by radiation only.
Why is it that a building’s R-value doesn’t always correspond with real world energy performance? In some cases, homes with the same R-value in walls and attics show huge differences in energy consumption, and a striking 2001 Canadian study by Dr. Tony Shaw of Brock University, Ontario first highlighted this fact. He compared two homes built in the same place by the same crew, using identical floor plans. One had R-20 wall insulation of one type, and the other R-17 of a different wall insulation. Yet somehow the R-20 home used much more energy during the depths of winter. Why the disconnect?
R-value insulation ratings only account for a product’s resistance to conduction, and that’s just one way heat leaves a building. R-values are determined in a lab where all outside air movement is eliminated. In the real world, however, it’s a much different matter. There’s always at least a little air movement within hollow house frames and attics – sometimes a lot – and this is a big part of the reason for Dr. Shaw’s discoveries. R-values are particularly deceptive when it comes to open cell insulations, such as fibre-based batts. Air movement through these is unavoidable, and when it happens it causes convective energy transfer, something not accounted for in R-value ratings. That’s why closed cell insulations that don’t allow air to pass through them are so much more effective, even when they carry the same R-values. Two great options for optimal energy performance are structural insulated panels (SIPs), and spray-foam stud frames in renovation projects.
If you’re interested in a building that goes up fast, strong, and is energy efficient,
SIPs are worth thinking about. Several studies have found SIP construction to be roughly twice as energy efficient as equivalent wood frame structures carrying the same R-values, and three times as strong. These factory-bonded sandwiches of dense, rigid foam are sheathed with sheets of OSB or plywood, usually 7/16-inch-thick. Panels are typically four feet in width, and can be as much as twenty-four feet long. They’re also strong enough to be horizontally self-supporting, and can even be used to form entire roof or floor structures, without the need for rafters, trusses or joists.
Send your plans to the nearest SIP manufacturer, and they’ll produce drawings that show exactly where each panel in your shipment goes. Once your SIPs arrive, cut holes for windows and doors, then tip panels upwards to form walls, plumbing each panel with a level before nailing it to the plate bolted to your foundation. Factory-applied recessed edges allow you to install special connectors between neighbouring panels for extra support. Make sure to lay a medium sized bead of low-expansion polyurethane foam along the bottom plate before panels go up. This extra seal should also be applied to every joint in a SIP structure, as well. It’s one reason this construction method prevents air infiltration so effectively. While truss roofs typically need to be well ventilated to avoid condensation and mold growth, for example, SIP roof structures don’t need ventilation, and are completely impervious to condensation buildup.
SIPs prevent all internal frost, and there’s no chance the insulation will settle over time. Building this way typically adds between one to four per cent to total building costs compared with other methods that deliver the same R-values, but a few years of energy savings more than pay for the difference.
Another closed cell insulation option is polyurethane spray-foam. It costs more than batts, but the money you’ll save on your winter heating bill quickly overcomes the difference in price. Applied properly, spray foam is the best option for boosting the performance of existing stud frame structures so they approach the performance of SIPs.
There are two possible options for spray-foam sealants. You can hire a professional foaming service to come out with a truck, or order the foam in portable canisters and do the work yourself. A Canadian company called Tigerfoam makes the best DIY foaming kits I’ve used so far. They come with two disposable tanks, hoses, a spray gun, replaceable plastic tips, and a protective suit. The foam needs to be applied when it’s warm (82ºF or 28ºC), and typically hardens less than a minute after leaving the tanks. Being portable, these kits are ideal for using in cottage country. Next-day courier deliver is possible to most Canadian building sites.
Tidy application of spray-foam in open wall cavities takes a bit of practice. The key is to keep the tip of the gun about a foot from the surface you’re foaming, and make sure the foam is distributed evenly. It’s also important to fill each cavity only an inch or two at a time before giving the foam a chance to harden. If you try to fill a cavity all at once, the foam starts clumping together and drips from the wall.
Keep in mind that spray polyurethane expands about 10 per cent or so after application. To minimize trimming back excess hardened foam with a handsaw, try to fill each cavity just enough to allow for expansion without ballooning out past your studs, or leaving empty spaces in the wall. If you do need to add more foam later, it’s easily done with a gentle squeeze on the spray gun’s variable-flow trigger.
Slow rise spray foam kits are the best-kept secret in the renovation world. They expand and harden much more slowly than regular spray foam, and they’re made especially for insulating hollow stud wall cavities without removing interior or exterior siding.
Small changes in building strategies can yield big differences in energy performance. And while professional builders don’t always understand how to achieve optimal insulation results in the real world, you’re now one step closer to making sure this happens on your own projects.