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Rick
Rick , General Contractor
Category: Home Improvement
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Experience:  Licensed construction supervisor with 35+ yrs. experience.
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How do we insulate our cottage roof from the outside What

Customer Question

How do we insulate our cottage roof from the outside? What kind of insulation? In what order? Insulation, strapping, plywood. membrane, ridge vent and vents on botton, aluminum metal panels?
The roof was done 9 month ago by Classic Products, out of Mississauga. We had selected their Aluminum Click Lock system. The installers put on 2x4 strapping HORIZONTAL, one inch white styrofoam, next horizontal strapping etc. On top 4x8 half inch spruce plywood, a membrane and ride vent, the aluminum panels.
This past winter we had a 6 to 8 feet ice built-up. No air circulation due to the closed horizontal strapping. Now the roofing company agreed to take the whole thing off and start all over. While at it we want to change the insulation. We don't think one inch white styrofoam is enough.
Anneliese Jamnik
Submitted: 7 years ago.
Category: Home Improvement
Expert:  Rick replied 7 years ago.
Hi Anneliese
To answer your last question first; 1" foam is better than nothing but that's about it. Typical recommendation for roof insulation in southern Canada is 6" of foam but that is not practical in this application. Insulating a sloped roof in the manner you describe is not standard practice. Ventilating the space beneath the roofing would require that the strapping be installed vertically.
The objective in preventing ice dams on a roof is to isolate the warm air in the home from the roofing surface keeping the roofing at the same temp as the outside air. To accomplish this a layer of insulation is used to isolate the warm house air from the roofing and a ventilated spaced is maintained above the insulation with outside air circulated through the space by installing a ridge vent to exhaust air and a vent at the bottom of the roof to draw in cold air.
At a minimum your should have 2" of rigid foam insulation in your application not the cheap white styrofoam insulation. The foam and the sleepers (2" thick "strapping") would be laid horizontally then vertical strapping on top to create the air space then plywood. It is important that the foam layer is tight so as to prevent gaps where warm air could penetrate this layer.
This insulation method helps alleviate ice dams but won't do much for lowering your heating bills.
Rick and 2 other Home Improvement Specialists are ready to help you
Expert:  Rick replied 7 years ago.
My recommendations may not be applicable to all metal roofing systems. Your roofing company should follow their manufacturer's instructions for installation.

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