Mineral Wool in Ceiling Cavity With Rigid Foam to Interior
AlexTinyBuilds
| Posted in General Questions on
Hi,
Is it okay to use mineral wool insulation in ceiling cavity between framing with a few inches of rigid foam bellow on the interior side of ceiling framing? The house will also have an intello vapor retarder. Just trying to get a higher R value in the ceiling. Is this an ok ceiling design?
Zone 5. With a small attic space. Vented at the ridge and will try to put in venting at eaves as well.
Anything wrong with this plan?
Thank you,
Alex
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Replies
If you'll have a vent channel of at least 1" (which is code minimum, 1.5" is better) between the top of the insulation and the bottom of the sheathing, then your plan should be fine. Remember that you need soffit vents at the opposite end of those vent channels from the ridge vent.
If you did NOT have a vent space, then your proposed assembly could be a problem.
Bill
Ok thanks , and what it we do a flat ceiling with trusses and gable roof. You just insulate the flat ceiling and don't insulate the roof sheathing in the attic space correct ?
Thanks !
Alex
It's fine to add a layer of foam on the interior side of the flat ceiling under a vented attic, but it's usually much cheaper & easier to hit the same performance point with deeper fluff in the attic, skipping the foam. In new construction specifying an "energy heel" truss with sufficient depth for the desired performance level is usually pretty cheap upgrade compared to a few inches of foam.
https://www.greenbuildingadvisor.com/article/raised-heel-trusses-make-better-enclosures
If the plan is/was to put ducts & air handlers in the attic, a "plenum truss" to acccomodate the mechanical systems fully inside the pressure & thermal boundary of the house is also cheaper than insulating at the roof deck:
https://www.greenbuildingadvisor.com/article/use-plenum-trusses-to-keep-ducts-out-of-your-attic
The concepts are not mutually exclusive- most plenum trusses would also be energy heel trusses. Just be sure to specify the target insulation depth & density to the truss designer.
Awesome,
Raised heel trusses ! Great info as a potential second option.
Thank you,
Alex
If you'll have attic space above (I was thinking you'd have a cathedral ceiling arrangement), I would use trusses like Dana suggested, then blown cellulose over the attic floor. This is usually the cheapest type of insulation you can use in this application, and it's also one of the best performing.
Bill