Retrofitting cottage insulation
How to choose the right insulation system to take the chill off your old cottage
Julia Stratta and Jeff Armstrong bought a cabin on Big Mink Lake, north of Bancroft, about two years ago and immediately started ripping out the insulation. That may seem an odd thing to do when so many cottagers are converting their summer haunts for year-round use. But this particular cabin was so infested by mice that the walls had to be opened up and the old fibreglass insulation removed.
“We didn’t find any live mice,” Armstrong says. But they found lots of “evidence,” and the acrid stench of mouse urine was almost unbearable. They washed the foot-wide plank walls with bleach and water to remove the odour. It was “a nasty, smelly job,” but at the end of the day, the couple had a cabin to call their own. They’d also taken a shine to the rustic look of the exposed planks.
Armstrong and Stratta were left with a dilemma, though: How should they re-insulate their cottage?
Insulating an old cottage so that it will be a cozy retreat, even in the depths of a Canadian winter, is no easy task. A wide variety of materials exists, each with its own drawbacks and advantages, and not all can be installed by a do-it-yourselfer.
Because the couple’s mouse problem forced them to rip open their walls, they have plenty of insulation options. Insulating can be done without ripping the cottage apart, but it’s more difficult, with fewer choices of materials.
Anywhere exposed framing can be found, batts are the easiest, cheapest, and most DIY-friendly way to insulate a cottage. Fibreglass is the most common batt material, although there are alternatives such as mineral wool that perform comparably.
Installed properly in a 2 x 6 stud wall, R20 fibreglass batts cost about 55 cents per sq. ft. (R value is a measure of resistance to heat transfer; the higher the number, the better.) More common in older cottages are 2 x 4 stud walls, which take an R12 batt at about 32 cents per sq. ft. The Ontario Building Code has no insulation requirements for cottages, which are defined as seasonal-use buildings. (For houses, it depends on where they are and how they’re heated.) In most situations, cottages in southern Ontario (south of North Bay) will be okay with R20 insulation in walls, and R32 in attics, but Ernie VanKoughnett, deputy chief building official for Seguin Township, suggests beefing up the attic to R40. “The more you put in, the more money you will save in heating,” he says.
For islanders, batt insulation is a good choice, as the bags fit easily in a boat. When buying batts, try to get them in bundles, which are more compressed than individual bags (but at about 40 kg, a six-bundle multipack of R20 is also fairly heavy, so get help loading and unloading the boat).
While cost and ease of installation are two big benefits of batts, there are a few disadvantages, not least the unpleasantness of wriggling around crawlspaces or attics, clutching itchy batts to your chest. Also, as Armstrong and Stratta discovered, mice live quite happily in fibreglass batts. Wind and moisture also penetrate batts easily, so a vapour barrier is essential, ideally combined with an air barrier on the outside.
As warm, moist air meets a cool surface, the water vapour condenses. When this happens inside a wall, it can lead to rotting timbers and mould. To avoid this, a vapour barrier must be installed on the warm side of the insulation. Ideally, it should be hung inside while the interior walls are off. If you’ve removed the exterior siding and decided to insulate at the same time, put the vapour barrier in first from that side, tucking it around the studs.
Another option is to put a vapour barrier over the existing walls inside the cottage. Then just put a new layer of drywall or wood panelling over top of it.
In a pinch, a third method is to seal any cracks and use a vapour-retardant paint on the interior walls.
Rigid foam insulation
For cottagers like Stratta and Armstrong, who like the look of exposed plank walls inside their cottage, the insulation has to go on the exterior of the cottage, before adding a new layer of exterior cladding. For this situation, rigid foam is the best bet. It’s easily laid over existing studs or planks, then covered by siding. And if sealed properly, it creates an effective air barrier. “If the air can’t blow in from the outside, then you’ve eliminated most of the problem,” says VanKoughnett. “Rigid foam just doesn’t have the problems that batts do.” Moisture can’t easily penetrate boards made of polystyrene, polyurethane, or polyisocyanurate, three common materials, and mould won’t grow inside them. Because it blocks moisture as well as airflow, a thick layer of rigid foam is a particularly good choice for insulating basements.
Sheathing the outside of a cottage with 1 1⁄2″ of rigid foam will provide roughly R7.5 insulation and make it snug for three seasons. More foam will increase the R value, but those additional inches may mean that exterior flashings need to be extended before re-siding the cottage.
All foam insulation must be covered, preferably by a fireproofing material such as drywall on the inside, because when foam burns, it emits a very toxic smoke. Keep in mind, too, that your materials costs for rigid foam will be at least three times as much as for fibreglass batts.
This article was originally published on January 1, 2007
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