Dense packing cavities within cathedral vault
uberaustin
| Posted in Green Building Techniques on
I’m preparing to have my attic spray foamed and decided to dense pack the cathedral vault cavities with cellulose. The existing cavities contain batts. Two questions:
1. Is it ok to blow in cellulose on top of the batts or should I fish them out?
2. Should I seal off all the soffit vents (with spray foam) under the eves of the cathedral vault since I’m dense packing the cavities to further prevent any airflow into the cathedral cavities?
3. Should I also spray foam all the soffit vents around the entire house to further promote sealing off any airflow from entering?
Thanks,
Lee
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Replies
Lee,
First of all, it's always a good idea to tell us your geographical location or climate zone so that we can provide the best answer to your questions.
Second, it's a violation of building codes (and, according to building scientists, a bad idea) to install cellulose insulation in rafter bays unless you provide a ventilation channel between the top of the insulation and the underside of the roof sheathing. To create this ventilation channel, you need to install rigid baffles in each bay.
(There is an exception to this rule: If your roof assembly includes an adequately thick layer of rigid foam on the exterior side of the roof sheathing, you can install cellulose insulation in the rafter bays without any ventilation channels.)
For more information on this topic, see How to Build an Insulated Cathedral Ceiling.
To address your specific questions:
Q. "Is it OK to blow in cellulose on top of the batts or should I fish them out?"
A. If you are certain that there are rigid ventilation baffles above the existing batts, you might try to install cellulose between the top of the batts and the ventilation baffle. But this is usually a bad idea, because in most cases, either (a) there is no ventilation baffle, or (b) the ventilation baffle is a flimsy product made of polystyrene (a product that can't withstand the pressure of the dense-packed cellulose).
Q. "Should I seal off all the soffit vents (with spray foam) under the eves of the cathedral vault since I'm dense packing the cavities to further prevent any airflow into the cathedral cavities?"
A. No. Flowing ventilation air between the top of the cellulose and the underside of the roof sheathing is desirable -- not just desirable, but code-mandated.
Q. "Should I also spray foam all the soffit vents around the entire house to further promote sealing off any airflow from entering?"
A. No.
-- Martin Holladay
In US climate zones 1 & 2 dense packing cellulose into unvented cathedral ceilings is pretty safe, even in zone 3 it can be, depending on the roofing type, color, and shading factors. Further north than that it gets a bit dicier, though it's still done fairly often, it's courting some moisture risk. See the second column in Table 3 of this document:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf
Dense packing over batts is fine if there's enough space to snake the dense packing hose in there. With low-density batts that are falling apart & bunched up, or less than a full code-minimum 1" of air gap between the batt & roof deck it can be pretty difficult to get there with the batts still in in place, especially if there are 1001 shingle nail points interfering.