Attic insulation
We have just built an addition to our house. It is two floors and a basement. The 1st floor is a kitchen and is open to the existing house. The second floor is a master bedroom and a bath. The only connection with the existing house is a door. There is an attic space above the bedroom/bath. It is an engineered truss roof so the space is not usable. We have sprayed the underside and ends with closed cell foam. to R38 or better. The only penetrations into the roof cavity are 1) the bathroom fan,which vents out of the gable end and 2) a required panel in the closet for access to the roof cavity. There is no insulation in the ceiling. In fact, as I write this, there is no ceiling. It will be installed very shortly (5/8″ drywall).
Questions:
1) Do I need to worry about condensation in the roof cavity?
2) Should I use a vapor barrier in the ceiling (poly sheet)?
3) Should I insulate the ceiling? If so, how: foam or blown in material?
My gut feeling is that I should insulate the ceiling with more closed cell foam. Logistically this is a nightmare as it will be very difficult to access. At least with blown in I can get up and shoot it to areas I really can’t reach. However, if there is a condensation issue it will collect in the blow in material and sit on my drywall ceiling.
Thank you for your help.
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Replies
Joe,
It's always a good idea to mention your climate zone.
If I understand you correctly, you have created an unvented conditioned attic by installing R-38 of spray foam on the underside of the roof sheathing (and perhaps against the gable wall sheathing). Is this correct?
If so, is it open-cell or closed-cell spray foam?
Of course, you should verify that this R-38 insulation meets minimum code requirements. It is probably sufficient in Climate Zones 1, 2, and 3, but it may be insufficient in Climate Zones 4, 5, 6, 7, and 8.
In most cases, there is no need to install insulation on the attic floor if you have already installed insulation along the roof slope.
Q. "Do I need to worry about condensation in the roof cavity?"
A. If you used open-cell spray foam -- maybe. If you used closed-cell spray foam -- no. For more information on this issue, see High Humidity in Unvented Conditioned Attics.
Q. "Should I use a vapor barrier in the ceiling (poly sheet)?"
A. No.
Q. "Should I insulate the ceiling?"
A. Probably not.
Martin,
Thank you for your fast reply. I am in Upstate NY, between climate zones 5 and 6. The foam is closed cell. The underside of the roof (2x4 construction) and the gable ends are coated to the full thickness (and, in places, more) of the 2 x 4's. R38 is local code. Personally, I'd go much higher but after talking with numerous foam contractor this is all they would put in. Please don't get me started on the general lack of understanding of those putting in foam.
I would have preferred to have the ceiling insulated. Regardless of the insulation, the roof cavity will get cold and the bedroom ceiling will "see" this and lose heat accordingly. I'd like to blow in chopped glass on top of the ceiling to about a R50 but I'm worried about entrapping whatever condensation makes it into the roof cavity. I still have time to put in a vapor barrier. Please advise.
Thank you, Joe
If there is no way to properly vent that space to the outdoors, in zone 6 you'll have to put R25 of closed cell foam on the underside, or R25 of rigid foam on the top side of the roof deck, and another R25 of fluff below that. You can't just put a vapor retarder on ceiling for an unvented attic space.
Dana,
I don't understand your answer. Joe has installed 3.5 inches of closed-cell spray foam on the underside of his roof sheathing.
Joe,
If you have installed 3.5 inches of closed-cell spray foam, that's about R-22.7, maximum, not R-38.
You don't need a vapor retarder at your ceiling. If you want to increase the R-value of your insulation, it would be best to install the additional insulation directly under (and in contact with) the cured spray foam.
I guess my confusion is about the condensation. Inevitably some moisture will make its way into the roof cavity. My guess is that it will come from the bedroom/bathroom side, which is why I'd opt for a poly sheet under the drywall ceiling. This will slow moisture migration but not eliminate it. I think moisture from the roof side should be comparably negligible. If I put the polysheet vapor barrier in then I have also reduced any escape paths for the moisture. My fear is that if I blow in insulation it will eventually become moisture laden and cause issues.
Joe,
In winter, indoor air is always warm and humid. That's normal.
In winter, outdoor air is always cold and dry -- also normal.
So what? Why are you worried about your roof assembly?
If you have installed 3.5 inches of closed-cell spray foam on the interior side of your roof sheathing, then there is no easy way for your warm, humid indoor air to contact any cold surfaces. Closed-cell spray foam is both an air barrier and a vapor barrier.
Joe. You have a ventilation fan in your new bath and an exhaust fan for your new kitchen, correct? Be sure to run them whenever any occupant is showering/bathing and/or cooking.
You can increase the r-value of your roof assembly to r-49, which is the minimum you want in your climate zone by adding rigid foam on the exterior or air permeable insulation on the interior (in contact with the closed cell foam.
I found the original explanation of the space in question a bit confusing. If there is R38 closed cell foam at the roof deck, it doesn't matter if you have a ceiling enclosing a space below that roof as long as it's vapor permeable (say, standard latex ceiling paint.) The temperature & humidity in that enclosed space will track that of the room below, and if it's humid enough to condense in the enclosed mini-attic you would also see condensation in the room.
DO NOT NOT INSTALL POLYETHYLENE SHEETING in the ceiling, since that would prevent the enclosed space from drying, creating a moisture trap between the very low permeance R38 foam & roofing layup (definitely a Class-I vapor retarder) and the poly sheeting (also a Class-I vapor retarder.
If it's only 3.5-4" thick you're really looking at only R25 (at ~4"), best case, and it may be only R20, definitely not R38. But in your climate zone you could safely install R13-R20 fiber insulation directly against the foam to bring performance up to something akin to code. If you blow loose insulation into the ceiling, with an air space between the fiber and foam it wouldn't meet code, and would likely underperform from a total R point of view, but it would still be pretty safe up to about R20 from a moisture handling point of view.
Another approach would be to add 5-6" of half pound density open cell foam directly onto the 3.5-4" of closed cell foam, which would bring it up to roughly code level performance, and would be moisture safe without interior side vapor retarders.
Correction: The foam thickness in the roof is 5.5" according to the contractor. That puts us at about R38. None of the contractors up here who did closed cell would put in more even if I offered to pay. I was told that the added insulation would not be measurable. I think my thermo classes tell me otherwise.
My concern about the moisture in the roof cavity is from the interior of the house, not from the outside. Also, in the summer here we have at least 10 or so days of very high humidity. So moisture is inevitable. Venting the roof cavity would be defeating the purpose of the insulation. In any event, once the moisture is up there it will eventually condense on a cool surface, which will be the underside of the roof. If it drips onto my ceiling I have a problem. If I blow in insulation it will delay the problem and make it bigger as it's going to act like a big sponge. Am I paranoid?
I feel that insulating the ceiling with foam is probably the best option. (Theoretically, filling the entire cavity with foam would be the solution, but I don't think I have the budget for that.)
The roof is on so I can't add foam to the outside.
The ceiling drywall has not been installed. I am going to try to talk the drywaller into letting me foam as he puts up the sheets. He was not at all open to that in prior discussions.
!!
Joe,
If you have R-38 insulation along your roof plane, then the interior surface of your cured spray foam will be at room temperature -- just like your drywall downstairs, or your kitchen countertop, or the dishes in your kitchen cabinets. There won't be any cold surfaces. Why do you worry about condensation?
If you want to beef up your R-value, the best place to put additional insulation is along your roof plane, on the interior side of your cured spray foam -- not on the attic floor.
Thank you all for the good information and advice. My current plan is to get some spray foam kits and bolster up the underside of the roof and gable end (the contractor did an absolutely minimal job and he missed a lot of the gable end). Code here is R38 for the roof. I'm pretty sure that when I'm done it will be at least that all over. Then we'll put up the ceiling, no vapor barrier.
If you have 5.5" of closed cell foam you can safely add up to R20-R25 of fiber insulation direct contact with the closed cell foam before it would need even latex paint as a vapor retarder. Cap-nailing unfaced R15 or R23 rock wool batts onto the closed cell foam layer and it a day, would be a lot easier, greener, and cheaper (under half the cost) than adding 2-4" of closed cell foam with a DIY kit.
Assuming 5.5" of closed cell foam you're already at ~R30-33, and R15 rock wool would get you close to the IRC's R59 prescriptive. If that R15 covers the rafter edges as well, it beats code performance on a U-factor basis.
Part of my challenge is logistics: As we stand there is no ceiling. Once the ceiling is in place it will be all but impossible to move around in the crawl space above the bedroom-tough enough as a small person-tougher with a protection suit and spray equipment (whether spraying foam or blowing in insulation) . Whatever is being done has to be done at this point. I think we're all in agreement that the maximum amount of foam should be applied to the roof cavity (roof and two ends). There is no cap or eave vent. I have about 7" of foam in most places in the roof cavity. I can't add bat or anything to the foamed layer as the structure underneath it is not accessible (it's covered in foam). There are 3 companies in my area that spray foam. One never returned my call, the second refused to do closed cell foam sighting many disadvantages of it, and the third got the job. Sadly, he refused to put as much foam as I would have liked. I added 4 200 kits from Lowes this past weekend. It's as good as it's going to get. With any luck the dry wall crew will have installed the ceiling today. They did not use a vapor barrier. In any event, at this time I feel much more confident of the structure after conferring with this group. Many, many thanks.
Joe
It's fine to have them spray 4-5" of OPEN cell foam (R15-R19) onto the closed cell, if they would actually come back.
Cap screws long enough to penetrate the closed cell foam 2-3" would likely be more than sufficient for hanging 3.5" rock wool.
I don't think "...we're all in agreement that the maximum amount of foam should be applied to the roof cavity...", quite the contrary. I don't agree with that proposition (at all!)
There is nothing magical about spray foam on performance, and it's not really very green, especially closed cell foam. Closed cell foam uses twice the amount of polymer per R as open cell, and uses (with rare exceptions) HFC245fa as the blowing agent which has a hefty environmental footprint, including (but not limited to) a ~1000x CO2 global warming footprint.
Open cell foam uses water as the blowing agent, and roughly half the polymer at any R-value. Once you have installed sufficient closed cell foam for dew point control (which you have), if it has to be foam, open cell foam has by far the lowest environmental hit. But any type of fiber insulation is even lower impact.
Six hundred board-foot kits for closed cell foam can be shipped for about $700, which is quite a bit cheaper than three 200 board foot kits, but if you needed it in a hurry...