How to properly insulate an unvented cathedral ceiling
In your recent “Musings of…” you refer to an unvented roof assembly as follows:
“The 2009 IRC (Section R806.4) allows unvented roof assemblies insulated with a combination of rigid foam insulation above the roof sheathing and air-permeable insulation in the rafter bays. (The 2009 IRC defines air-impermeable insulation as “an insulation having an air permeance equal to or less than 0.02 L/s-m² at 75 Pa pressure differential tested according to ASTM E 2178 or E 283.” Although spray foam insulation and rigid foam insulation can meet this standard, dense-packed cellulose cannot.)”
I am designing an addition which I was hoping to use 4″ of rigid over the sheathing and dense pack in the bays. What would you recommend instead of dense pack? (I hate using fiberglass and spray foam is expensive)
Thanks, Ralph Hill
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"The 2009 IRC (Section R806.4) allows unvented roof assemblies insulated with a combination of rigid foam insulation above the roof sheathing and air-permeable insulation in the rafter bays."
This describes your intended assembly. When you have a sufficient thickness of rigid foam above the sheathing, the code allows the use of air permeable insulation (e.g. cellulose) in the rafter bays. If you didn't have the exterior foam, you would have to use an air impermeable insulation such as spray foam in the bays.
Torsten is right -- it's perfectly OK to use cellulose insulation in the type of roof assembly you are describing. Just be sure that the rigid foam meets the minimum R-value requirements in the code. Four inches of rigid foam has an R-value ranging from about R-16 (for EPS) to about R-25 (for polyiso).
If you are in climate zone 4 or warmer, R-15 is enough.
If you're in climate zone 5, you need at least R-20, and if you are in climate zone 6, you need R-25.
If you're in climate zone 7 or 8, your plan won't work, because you need a minimum of R-30 or R-35 (respectively) of rigid foam.
There are many healthy options to use in your cathedral bays: sheepswool, cotton insulation (bonded logic), cellulose batts (http://cmsgreen.com/), mineral wool (roxul), etc. As was noted, this insulation is not airtight and therefore prone to convective looping. In such cases, to optimize the insulating value, the insulation needs to be protected by a windtight/airtight layer outboard and airtight layer inboard.
In addition to airtightness, moisture management is an issue, so we have started to import Intello - an intelligent vapor retarder - that is completely airtight when properly tape - preventing bulk vapor from entering your insulation - while being vapor closed in winter (below 0.2 perms) and vapor open in summer (allowing any humidity that had gotten in anyway, to dry out the interior, providing maximum drying potential.... this is important as your foam will be vapor closed and therefore the cavity will never be able to dry outwards.
Also, because of the Intello's vapor management characteristics, if you are able to achieve the required insulation value with batts or dense-pack alone, then the foam can be responsibly eliminated altogether - providing what could be an even more robust assembly. (because it can then potentially dry to the outside in addition to the inside.)
For more info on this product, please visit our website http://www.foursevenfive.com where Pro Clima's Intello and airtightness tapes are available.
Don't attempt to follow Ken's advice -- "if you are able to achieve the required insulation value with batts or dense-pack alone, then the foam can be responsibly eliminated altogether" -- unless you provide a ventilation channel above the insulation layer. Otherwise your roof assembly will violate code requirements.
The variable-permeance membrane that Ken is importing is one type of "smart" retarder, but there are several others, including MemBrain.
Martin.... good point.
here is the comment I was working on ...before I saw your comment
I agree with your not-so-foamy,vapor open, Euro-inspired suggestions.
You did not mention "over-venting".
Almost every European compact roof assembly that I have viewed (online) also includes some type of battens or venting space below the roof cladding.
Here is a Thorsten Chlupp quote:
"...and I for sure would not put an unvented hot roof on a building.
Ralph, Martin and John,
Yes, I should have been explicit. If you can, vent your roof and get rid of the foam. It is better for the assembly and the planet.