Insulating above a garage roof
Location: Western NC, zone 4, mixed humid
Elevation 2000 ft
Roof: slope 2:12, standing seam.
Attic: formed with trusses and unvented
House roof insulation: Polyiso nailbase R15 min above the deck, netted BIBs below.
We are building a house with a garage attached for a short distance at the adjoining door. The roof is also continuous from house to garage. The architect shows the nailbase insulation continuing from house over the garage to maintain the plane of the roof. The garage will be unconditioned and there will be no BIBs below the deck here. The ceiling will be dry-walled and penetrations sealed.
Am I more likely to have problems with this approach than simply framing the roof higher to match the house nail base? Part of the reason for insulating above the roof and below the slab was to to turn it into a workshop one day by adding insulation to the walls.
Thank you.
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Replies
If the plan going forward is to condition the space, you definitely want to run the nailbase polyiso over the garage as well, since you wouldn't be able to easily install under the roof deck ventilation on the garage roof. With R15 nailbase over it you won't need to.
Even before insulating the walls or heating the garage the average temp at the roof deck will run warmer in winter (== lower moisture content) if it has R15 above it.
In zone 4A with R15 above the roof deck the safe max R-value of the BIBs layer is R35-ish (~9" if cellulose, ~8.5" if 1.8lb fiberglass). If it's substantially thicker that it will need a interior side vapor retarder much tighter than standard latex paint on gypsum.
Thanks Dana. You have also answered the first part of my next question about the need for a vapor retarder.
The second part is that the polyiso manufacturer suggests a vapor retarder if only because of the additional water vapor caused by fresh concrete. By the time we make the house tight the concrete foundation walls and slab will be a year old. Do we need to be concerned about this? Should I monitor the indoor humidity for a time and keep it below a certain level?
Don't sweat the vapor retarder at all if the slab is a year old (or even younger, if the garage is well ventilated.) A half-inch CDX or OSB roof deck is jtself a Class-II vapor retarder when dry.
Seriously, this is the first time I've heard of recommending vapor retarders under nailbase panels other than roofs over special cases like steam rooms, hot tubs, and heated swimming pools. Concrete takes up water as it cures, and needs an excess for the first hours & days to ensure maximum strength, but after you've stopped wetting the amount of stored residual moisture isn't huge, and the rate at which it would release it would be slow unless heat is applied. It's nowhere near as high as over a swimming pool
If you're worried, leave the garage door open for a month after you've installed the roof, (eliminating ongoing rain wetting of the concrete), but I seriously doubt that's necessary.
Whose nailbase panel are you using?
We are using Hunter H Shield NB. Their spec sheet only mentions excessive vapor drive during the construction phase, I extrapolated that to mean concrete. Due to the storms that are sweeping in we are now thinking of putting roofing felt down temporarily to protect the framing and sub floor before the roof proper goes on. Would there be any harm from leaving it on when we come to put down the nail base?
The short-sheet brochure for H-Shield shows a vapor barrier between the structural roof deck and the panel. In a zone 4 climate I wouldn't sweat that too much. #30 roofing felt is also a class-II vapor retarder when dry, so go ahead with it.
During the construction phase there is a lot of stored water in the framing lumber, and water is used in all sorts of finishes, drywall mud, etc. so the RH is elevated during construction compared to 2 years after occupancy. Keeping the wintertime RH under 40% by adjusting the ventilation rate is usually pretty easy and a good idea in a brand new house. Keeping the house under 60% during the summer with air conditioning/dehumidifiers the first year after construction would also be a good idea to keep the vapor drive direction pulling residual moisture out of the building materials.
Most of the time during summertime construction crews keep the windows open (or even run fans) to stay comfortable, and the interior humidity will pretty much track the outdoors, which is fine. Unless there are security issues, leaving some windows open at night or actively ventilating while the crews aren't there is a good idea, independent of the nailbase panels.
Dana, thanks again for clearing up my questions and doubts.
Not sure if I should start a new post but will try this first
This is a follow up question relating to the ratio of above deck to below deck insulation mentioned in reply #1 where Dana wrote:
" In zone 4A with R15 above the roof deck the safe max R-value of the BIBs layer is R35-ish (~9" if cellulose, ~8.5" if 1.8lb fiberglass). If it's substantially thicker that it will need a interior side vapor retarder much tighter than standard latex paint on gypsum".
An energy rater who performed a Manual J recently recommends we do not go above R23 for the BIBS unless we increase the above deck insulation proportionately to reduce the risk of condensation.
I want to increase the BIBS to R30 and keep the nailbase polyiso at R15 as it is already installed. Is there a relatively simple way to reassure myself that R30 is safe such as a dew point calculation?
Dana's number's are based on code 2012 minimums, which is R-49 (see https://www.greenbuildingadvisor.com/articles/dept/musings/combining-exterior-rigid-foam-fluffy-insulation). You need at least 31% rigid foam above the sheathing and that equates to R-15 of rigid foam. R-30 would be decreasing your insulation. Did you mean to type R-40?
Yes: Install an air tight interior side "smart" vapor retarder, such as 2-mil nylon (Certainteed MemBrain), Intello Plus, or a class-II vapor retarder such as vapor-barrier latex primer on air-tight gypsum board. Any of the above would limit the wintertime moisture uptake to tolerable levels with R15 on the exterior, even if there's R40 in the cavities. A smart membrane type vapor retarder is preferred, since it also allows for rapid drying if it ever needs to, whereas vapor retardent paints do not. VB paint is cheap, but 2-mil nyon won't break the bank even at box-store pricing. Intello is ~3x the cost of 2-mil nylon, but (unlike 2-mil nylon) is rugged enough to be used as the blowing mesh, and might be a cheaper solution than mesh + separate vapor retarder. See:
https://www.youtube.com/watch?v=hOhE-UP72xg
(The vidi demonstration is a wall, but it works the same in dense packed roof decks.)
In your climate zone you should able to run up to R35 on the interior with only a class-III vapor retarder on the interior side (standard latex paint on air tight gypsum.) The moisture buffering capacity of cellulose in BIBS offers considerable forgiveness on dew point issues (but not outright absolution, as some vendors/installers have claimed in the past.)
Steve:
Our county still works to IRC 2009 so R15 + R 23 = R 38 meets code. I wanted to exceed the code and go to R15 + R30 = R45. As I mentioned our energy rater feels this is risky and we should increase the rigid foam to R20.
But if I understand the 2012 code correctly, clarification was added to say what % of the roof's total R value needs to come from the rigid foam layer which for our zone it is 31%. If so, we could increase the sub deck layer to R 30 safely with a max of R34. Is my understanding correct?
Jeremy,
Here's a link to an article that discusses this issue: Combining Exterior Rigid Foam With Fluffy Insulation.
In Zone 4A, this type of roof assembly will work (from a building science perspective) if the rigid foam layer provides at least 31% of the R-value of the entire assembly. When it comes to your local code, however, you'll have to ask your local building department for an interpretation.