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I'm turning my 3rd floor attic into a usable space. It's a hip roof, 25' x 23', with one dormer. We just had new asphalt singles put on it and it was about 12 foot of ridge vent on the dormer / peak. The rafters are 6 inch actual.

Should I build a hot or cold roof?

If I go cold, I would have to install soffit vents (circular ones), baffles, and drill through a lot of rafters to make air ways to the ridge vent. (I didn't know about hip roof ridge vents before we had the roof re-shingled). Then I would only have room for 4 or 5" insulation.

If I go hot, I could get 2 layers of 3" 4x8 rigid foam board cut to size and seal the gaps with great stuff. This seems much easier than the spray foam kits and easier than the cold roof option.

This project is all DIY, and I don't want to add depth to the rafters and lose attic space. I've heard a cold roof can add lifetime to your shingles, but I've also heard that it doesn't matter. For energy savings I've heard that sealing and more R value is the way to go.

  • Typically a cold roof is to mitigate dealings with snow such as ice dams, slide offs, etc. in places that get large amounts of snow and ice. (usually in the measure of feet, not inches) With no mention of such things my question would be why go through the trouble of a cold roof otherwise? BTW life time of shingles is probably linked reducing freeze thaw cycles. – Damon Jul 14 '15 at 19:38
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Oh what a shame you already did the roof. That would have been a perfect opportunity to add rigid foam insulation boards above the roof sheathing.

A vented roof is definitely better from the perspective of shingle life and moisture resistance, but with such shallow rafters and an unwillingness to lose any ceiling height, that's not an option. If you go with a vented roof, you will need to use 3-4" or so of rafter depth for the ventilation channels (depending on roof pitch; lower pitch requires deeper channels), giving you not much room to work with for insulation between the rafters. If you are unwilling to reduce the ceiling height by installing foam or mineral wool boards across them, then the room will be very uncomfortable as a result.

Your best bet at this point is to get someone to apply a full 6 inches of closed-cell spray foam to the underside of the roof deck between the rafters. This stuff is R-7, so you'd wind up with nominal R-42 between the rafters, which isn't bad. Taking into account the thermal bridging of the rafters, the actual performance would be more like R-37--not terrible, but not great. If you go the spray foam route, hire it out. The DIY kits wind up being about as expensive as hiring out the job in most locales. If you thought the upcharge to add foam to the roofing job was high, wait till you see what the spray costs. Expect $9-14 per square foot of attic space.

You can try doing this yourself with cut-to-size foam and Great Stuff filling the entire faster bays; this is referred to as "cut-and-cobble." You will definitely save money, but it is a messy, time-consuming, un-fun, thankless job, and it is more difficult to get a perfect air seal (which is absolutely required) with this approach than with spray foam. Furthermore, the R-value you'll achieve will be lower because there is no R-7 rigid foam commonly available. XPS is R-5 (yielding a whole-roof R-value of R-26) and polyiso is R-6.5 when it's hot but R-4.5 when it's cold (yielding a whole-roof R-value of between R24 and R-34 depending on the temperature).

I strongly recommend accepting a 3" loss in ceiling height and putting polyiso under the rafters. If you can live with that, you can detail the polyiso as your air barrier by taping all the seams with housewrap tape. Then then fill the whole rafter bays with cheap cellulose, which is hydroscopic so it will help wick water away from the roof deck if it does get wet. This should yield you a whole-roof R-value of R-39. Then cover that in drywall. You can do it all yourself and it won't break the bank. To improve the R-value, just add more inches of polyiso under the rafters; each extra inch will add another R-6.5. The cost difference between this and 6" of spray foam should be dramatic--maybe 5x cheaper or more.

  • Thanks for the quick answer! Would I really need 4" of vent space? Baffles are only 1 or 2" deep... Yeah adding foam above the roof was too expensive on an already expensive job. – Jess Jul 11 '15 at 15:12
  • Can you say why a perfect air seal is important? – Jess Jul 11 '15 at 15:15
  • Depends on the pitch. 6:12 would need 4" to effectively move enough air. 1" won't do much. Regardless, even if you could only get away with 2" channels, that only leaves you with 4" for insulation. Even closed-cell spray foam wouldn't be good enough with that little space to work with. You'd be looking at R-24 whole-roof, which is pretty bad. – iLikeDirt Jul 11 '15 at 15:15
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    A perfect air seal is important because it prevents humid air inside the house from touching the roof decking, which will get cold enough in the winter to make water vapor condense into liquid water. This liquid water will rot the entire assembly with no air movement to evaporate it. That's why a vented roof is more robust against this risk. But without ventilation to carry away such moisture, you need to make absolutely sure that those components stay dry in the first place. – iLikeDirt Jul 11 '15 at 15:17
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    Got the project done and feel really good about it. Spray foam FTW. – Jess Dec 22 '15 at 3:25

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