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How to vent and insulate a hipped-roof loft living space?

Can anyone refer me to a "primer" for how to best vent and insulate the "vaulted" ceiling of a hipped-roof loft room? I have three major questions: venting a hipped roof, insulation type, and vapor barrier.
I'm turning an unconditioned attic into livable space, as a part of a complete cottage remodel (strip interior to studs and start over). The roof I'm working with is long-hipped (longer than wide), with a ridge along the center. There will be kneewalls and built-in closets/drawers.
The retrofit of this small cottage (22’x33’) has included installing a “doghouse” dormer on one narrow end, and a shed dormer extending from the full length of the ridge on one side. The roof has been re-shingled (dark asphalt shingles) with ridge vents installed along the doghouse dormer and the 11’ long central ridge.
The outer envelope is fully accessible (exposed studs) from the inside but not the outside. At the outside envelope in the loft, there’s about a foot of vertical wall before the roof slant begins. The highest peak is at just over 7’. Alas, presently the roof rafters and ridge rafter are only 6” deep (the exterior wall studs are only 4” deep!). This house is located in Midcoast Maine: long cold winters, plenty of snow, often followed immediately by saturating rain.
First, how do I best vent the ridges of the hip? I’m guessing one should install attic rafter vent spacers along each inter-rafter space, from the vented soffit up to the hip ridge, then drill vent holes at the tops of those rafters (if so, how big?), at the top end near where they meet the hip ridge, to vent up the angled ridge to the center ridge and its vent space. Very scary idea, though: wouldn’t those vent holes compromise the structural integrity of the roof?
Next, re. insulation choice. Alas I am reticent to use spray foam because a) roof shingle/sheathing failures escape notice and can permit extensive rot, and b) I’m passionate about recycling and hate the idea that a foam-sprayed roof, upon its ultimate future demolition, must become landfill fodder rather than wood (chippable, burnable BTU’s) plus insulation. So, what is the best insulation choice? Cellulose blown between the rafters?
Thanks in advance for your advice or referral.

Asked by Anita Brosius-Scott
Posted Nov 11, 2011 2:23 PM ET


3 Answers

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1. Consult an engineer to be sure that your rafters aren't undersized.

2. It's a little late to decide how to vent and insulate your roof. Ideally, that decision would have been made before your shingles were installed, since the best solution in your case would be exterior rigid foam on top of the existing roof sheathing.

3. Your rafters aren't deep enough for ventilation channels plus insulation, and your ceiling height is so low that you can't build down.

4. Hipped roofs are hard to vent, so an unvented roof would be best in your case.

5. The best solution in your case would be closed-cell spray polyurethane foam, for two reasons: (a) it doesn't require venting, and (b) it has a high R-value per inch.

Answered by Martin Holladay
Posted Nov 11, 2011 3:05 PM ET
Edited Nov 11, 2011 3:06 PM ET.


Thank you, Martin. My constraints are problematical!
Does a roof that is spray-foamed inside against the sheathing have a shorter life span due to summer heat load?

Answered by Anita Brosius-Scott
Posted Nov 11, 2011 5:20 PM ET


Hot shingles probably won't last as long as cooler shingles. If you want cool shingles, choose white shingles -- they will stay much cooler than dark colored shingles. Shingle color is far more important than ventilation in determining summer shingle temperature.

That said, there are many factors that affect shingle life, and shingle temperature is only one of them. The difference in longevity is probably not huge.

Answered by Martin Holladay
Posted Nov 11, 2011 5:26 PM ET
Edited Nov 11, 2011 5:28 PM ET.

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