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I'm finishing a 14x14 shed that has 2x6 rafters in Austin TX, and I want a cathedral ceiling. The shed builder says they usually recommend filling the cavity with R-19 fiberglass insulation, leaving no room for airflow.

But I've also read many suggestions like this for vaulted ceilings:

Remember that a two-inch breathing space between the insulation and the roof sheathing must be included to allow for ventilation.

If I leave room, it's hard to fit more than R-13 without going to rigid foam board.

Do I really need the space? I don't have any venting. And I don't envision having much excess moisture in the shed, as it's going to be an office. Does that apply more to cold climates, where moisture becomes an issue as the hot inside air meets cold outer air?

Edit: The roof appears to be OSB.

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  • A bit of air would help in cooling down the roof deck. Austin TX, hot hot sun beating down, bet you could fry an egg on roof. Think what that would do to asphalt shingles. Commented Apr 17, 2021 at 22:50
  • Hmm it might help the roof longevity, but at the cost of heating up the interior, which is not what I want. Better to put elastomeric paint over the shingles if that's what I'm trying to solve. Commented Apr 17, 2021 at 23:05
  • If you use white paint, that will probably be more helpful than an extra R 7. Just need about an inch of air, so maybe just foam or a mixture. Commented Apr 17, 2021 at 23:36
  • The crux of my question is whether that inch of air is even necessary. Right now I'm thinking it's not. Commented Apr 18, 2021 at 0:56
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    @ThreePhaseEel Ah. Shingles (presumably asphalt). Commented Apr 18, 2021 at 18:10

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Yes, cold climates require the insulation in walls to dry towards the outside through breathing gaps in the sheathing, with a vapour barrier separating the insulation from the warm inside moisture.

Roofs are similar, where ventilation is provided through soffits and ridge or gable vents.

Also, the airflow from soffit to gable provides for cooling (!) of the shingles in the winter. Otherwise warmth from the house would melt the snow near the top where heat accumulates, and melt water would re-freeze towards the bottom of the roof, causing ice and ice-dams to form.

In a hot and dry climate I could imagine that the convection through the roof's air gap will more than offset the reduced insulation.

A compromise is to leave a gap and slightly compress the R19 insulation with strapping (length- not cross-strapping, applied in the direction of the slope).

The R value will drop a bit due to compression, but it will be better than R13. A rule of thumb is that for every 2 inches compressed you loose only 1 inch of equivalent R-value.

Additional insulation from heat can be obtained with reflective foil insulation (e.g. Reflectix) applied to the roof sheathing on the inside.

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  • Thanks! Unfortunately there is no venting. I can't find any 4.5" thick insulation, so I think my choices are (1) 5.5" with no air gap (R-21), (2) 5.5" but add an inch to the rafters for air gap, (3) add a few inches to the rafters and fill it with R-30, (4) add a few inches plus one for R-30 plus air gap. Right now I'm leaning toward (3) to balance insulation and ceiling height. What would you choose? Commented Apr 18, 2021 at 17:43
  • @A_P what do you mean with "add a few inches to the rafters", upgrade from 2x6 to 2x8? Commented Apr 18, 2021 at 17:47
  • The structure is already built (see first link), so I guess they'd have to add furring? Commented Apr 18, 2021 at 17:48
  • @A_P, ah, yes, ok. I am not familiar with building in your climate, but I still think the airflow will be better for summer cooling and for the life of your roofing. So (2). You can also ask a local roofing company for a 2nd opinion. Commented Apr 18, 2021 at 18:00

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