Cottage Q&A
QUESTIONWhat is the best way to replace the subfloor insulation in a cottage elevated on posts? The floor joists are 2" x 6" and it is currently insulated with fibreglass that has deteriorated and become a home for mice. Are fibreglass batts the best material to use? How can we keep the mice out? How should we install the vapour barrier from underneath the cottage?
ANSWER
Blown-in polyurethane foam insulation was the first choice, hands down, of the cottage-country contractors we consulted about your crawl space. "It has all the advantages for a situation like this," says Rod Thurston of Rodco Enterprises in Haliburton, Ont. "The foam adheres to the joists and subfloor, moisture doesn't damage it, animals don't get into it, and you get a high R-value without taking up much room." Polyurethane foam also acts as its own vapour barrier. Each inch of thickness gives an R-value of 6, so three inches of foam would insulate the crawl space to R-18. (A 6" fibreglass batt provides R-20 of insulation.)
Blowing in foam insulation is a job for a licensed contractor. The price of a particular job depends on several factors, including the accessibility of the crawl space, suggests Steve Clarke of Quinte Urethane Insulation in Madoc, Ont. But as a general rule, insulating an area 10' x 20' (200 sq. ft.) with three inches of foam would cost about $600. Fibreglass batts may be the pros' second choice, but they are relatively simple for cottagers to install themselves. The vapour barrier presents an interesting problem, though. There's no point in using one at all if it's going to be broken at every floor joist; Thurston says the floor will develop cold spots, and moisture will migrate to every place the barrier is discontinuous. He suggests that it's better to have no vapour barrier than one that is poorly installed and will ultimately expose your joists to excess moisture and rot. To keep those mice from moving back in, Thurston suggests you staple a barrier of 1?4" hardware cloth, available in rolls at building supply stores, below the batts.
* Published in the March 2002 issue of Cottage Life
