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We are wanting to convert our attic space into living space. The challenge is that the rafters are 2x8 and the boards are on two-foot centers.

The last time we had the roof replaced, they added a ridge vent to improve air flow and we have soffit vents. We can't buy insulation to fit the space and if we do, it blocks any air flow after we install sheet rock to the ceiling. We are not sure what to do.

We want maximum insulation but adequate air flow.

I can add pics later if it lets me.

Thoughts?

  • Why can't you buy insulation to fit the space? – SteveSh May 3 at 14:30
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Assuming modern 2x8s:

Purchase sheet foam insulation (you want "best" so polyiso gives the most R per inch as far as I know at present)

That's commonly available in 2 or 4 foot wide sheets 8 feet long, perhaps different or perhaps not so different in metric areas.

Trim to (measure to be sure, your roof may be less precise than would be convenient) 22-1/2" to account for "modern" 1-1/2" wide by 7-3/8" deep "2X8" on 24 inch centers. Either buy 6 inch thick sheets, or buy enough sheets to add up to 5 or 6 inches thick for this part, leaving a 1-3/8" to 2-3/8" gap for ventilation between the insulation sheet and the roof sheathing.

For best results, use another 1/2" to 3/4" full sheets of Polyiso over the face of the rafters before sheetrock to provide a thermal break.

This will cost considerably more than cellulose or fiberglass, but it has better insulation values and provides ventilation.

There are other (less expensive to install, less insulation value so more cost to own long-term) approaches, such as using ventilation baffles stapled to the roof sheathing (underside) in each bay for the ventilation, and then using "6-inch" rockwool or fiberglass - cellulose is also an option, but trickier to install - I would still suggest spending for an overall layer of foam covering the face of the rafters as suggested above. If your local retailer does not stock 24" wide batt insulation, discover the joys of online ordering, or consult a real building supplier rather than certain blue and orange retail box suppliers.

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