Deb Crossley is a Newton City Councilor and an architect. She comments on today’s post ‘A Newer Option for Attic Insulation and Ice Dam Prevention’ by Craig Forman. Craig’s post was previously published in the January/February 2017 edition of GN’s Green News.
I was pleased to read Craig Forman’s Green News article on his choice of foam insulation for his open attic. Installed properly, high expanding urethane based foam insulations is a good choice for many applications, and his is one. Craig is correct that evenly and continuously insulating the roof assembly can prevent the melting and refreezing that creates ice dams that can cause great damage.
However, as a decades long energy efficiency advocate, and one who regularly specifies such spray foam applications his article warrants clarification.
If your attic is open, accessible and dry like Craig’s, foaming the rafters and end walls is a great choice. By aligning the thermal envelope (insulated section) with the building envelope (the ‘shell’ – roof, walls, etc.) you can create a tight seal, and better performance. This means spraying the insulation onto the underside of the roof sheathing (which may not be the ceiling) and over the roof framing, over the rafters and down to the wall plates below. It must include gable end walls or dormers.
The installation must be continuous. Spaces that are inaccessible and therefore left uninsulated would require ventilation to remove moisture vapor that naturally migrates through porous insulation. The moisture may condense on the underside of a cold surface. Trapped moisture is very bad for buildings. Ventilating odd spaces is often not possible to do well.
If your attic is complicated, you have moisture intrusion issues, or your electrics need repair then you need to address these things first, and design a solution that works.
Prior to insulating any building element, attics included, first identify and remove sources of moisture intrusion. This includes making sure there’s no leaks through the roof, sidewalls, around chimneys or pipes, that your bath exhaust fans exhaust to the outside (not into the attic!) and that there’s no signs of mold anywhere. It is also important to check to be sure there is no exposed wiring. If you need to run new wiring, best to do it now.
Considering the cost, I prefer closed cell insulation at least for the first 3 inches or so because it is a moisture barrier. It is more expensive than open cell insulation but you need less of it. To achieve the same thermal protection (“R” value) most installers say the cost is the same. However, all foams are more expensive (though more effective) than blowing cellulose onto the floor.
Finally, many building products are toxic in their application. Even natural products can create dusts during cutting and fitting operations. Spray-on foams today cure quickly but best practices require aggressive exhaust ventilation during application and afterwards for a period of 24-48 hours. Residents should not be present during application.
By Deb Crossley, Newton City Councilor
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