Adding foam board to ceiling rafters?
user-5520998
| Posted in Green Building Techniques on
Hello,
The rafters in my ceiling are only 2 x 6. Is it acceptable to lay polyiso sheets across the top of the rafters, or should I be concerned about the foam acting as a second vapor barrier? The fiberglass batts have a kraft liner facing the sheet rock.
If the answer to the above is yes, can I place the foam in between the rafters against the sheet rock under the fiberglass batts?
Thanks!
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Replies
Questions that might help you get a better answer:
1) What climate zone are you in?
2) Is the assembly you have now vented at all? e.g., a vent space between the top of the fiberglass and the roof deck.
Donald, from the context I'm guessing you've used the word "rafter" where you meant "joist". You asked if it's OK to lay polyiso "above" the existing insulation, so it sounds like you're proposing adding insulation to the floor of a conventional attic. If this is the case, you should probably consider blowing cellulose on top of the fiberglass for a fraction of the cost. Be sure to airseal all cracks in the ceiling first, as best as you can. If you're actually referring to the rafters of a cathedral ceiling, that's a whole nother thing. Lots of articles in the archives...
Donald,
You suggest installing "polyiso sheets across the top of the rafters." Do you really mean "across the top"? It sounds like a new construction job, where you want to install rigid foam above the rafters, followed by plywood or OSB above the rigid foam. Is that your plan? (If so, the answer is no -- you can't do that. The plywood or OSB roof sheathing needs to be fastened directly to the rafters for structural reasons.)
Perhaps you meant to write something else. Perhaps you are thinking of installing polyiso under the rafters, not across the top of the rafters. Please clarify.
While awaiting clarification on the actual construction details...
A kraft facer is NOT a vapor barrier, but rather a "smart" vapor retarder. As moisture builds up behind a kraft facer it becomes more vapor open- it doesn't/can't form a moisture trap.
But a foil facer on either foam or batts is very much indeed a true vapor barrier.
This is a 1-1/2 story house in climate zone 3. As a result, I have two attics, which mainly exist for the HVAC equipment. Because a large portion of the attic space is decked, I'm looking to maximize the R-value given the limited joist (thanks for the clarification) depth. On places where it is not decked I plan on cheaper methods of adding insulation (blowing cellulose or rolling out additional batts)
Good to know on the Kraft paper not being a vapor barrier, thanks! On one of my knee walls, there is plastic sheeting under the batt against the sheet rock. Do I need to remove this sheeting before placing foil faced polyiso on the attic side of the knee wall? I don't want to end up with a vapor barrier sandwich.
Can the foil faced polyiso also be used as an insulator/radiant barrier on the underside of my roof deck?
Thanks everyone for the help. This site is a treasure trove of information!
Donald,
Q. "On one of my kneewalls, there is plastic sheeting under the batt against the sheet rock. Do I need to remove this sheeting before placing foil faced polyiso on the attic side of the kneewall? I don't want to end up with a vapor barrier sandwich."
A. Is this a wall? (A kneewall is a type of wall.) If it is a wall, I'm confused to by the word "under." I assume that this is a wall, and the plastic sheeting is on the interior side of the batt. If you have access to the polyethylene, it would be best to remove the polyethylene before you install polyiso on the attic side of the wall.
Q. "Can the foil-faced polyiso also be used as an insulator/radiant barrier on the underside of my roof deck?"
A. I suppose it could, depending on your goals and details. Ordinarily, this wouldn't be done unless you wanted to transform your vented unconditioned attic into an unvented conditioned attic. What R-value are you anticipating putting in this location?
Thank you for the reply.
Yes, I am referring to a knee wall, so I probably should have said behind vs under.
I'd probably use R6 1" thick polyiso on the roof deck unless it is a bad idea?
Thanks!
Donald,
It usually doesn't make any sense to put some of your insulation along the roof line (creating an insulated sloped roof assembly) unless you are creating an unvented conditioned attic, and aiming to install something close to the code requirements for R-value. So that would mean that you need to install R-38, not R-6. For more information, see Creating a Conditioned Attic.
If you aren't aiming to create an unvented conditioned attic -- and if your attic remains vented -- then installing R-6 insulation under your rafters is a waste of R-value.