Interior thermal bridging break at rafter location
dfvellone
| Posted in General Questions on
Location is zone 6. I’m wondering what the minimum required r value of rigid foam should be for a thermal break installed against the underside of the rafters on a cathedral ceiling.
Thanks, Daniel
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The minimum R values for rigid foam are for exterior rigid foam where it’s important to keep the indoor side above the dew point for moisture control. Assuming you have some other kind of insulation in the roof assembly already, any interior rigid foam doesn’t really have a minimum R value, but it’s probably going to be important to use something that is more vapor open to avoid creating a moisture trap. 1/2” EPS would be a pretty good option here, but we need to know more about your overall roof assembly to be able to offer much more info.
Bill
The rafters are full dimension 2x10 with 2" vent channel at underside of roof deck and cavity filled with ccspf.
There is no minimum-R for a thermal break on the underside of the rafters. It only depends on your performance goals.
>"The rafters are full dimension 2x10 with 2" vent channel at underside of roof deck and cavity filled with ccspf."
Has that ship already sailed? (Is the 8" of R-48 to R50 ccSPF already installed?)
If not, do the math:
https://www.finehomebuilding.com/membership/pdf/184243/021269086NRGnerd.pdf
R30 rock wool would leave ~3" vent gap and a continuous layer of 1.5" of foil faced polyiso on the underside would bring it up to code performance on a U-factor basis (assuming rafters no tighter than 16" o.c.), and would roughly double the R-value of the framing fraction to something greater than that of a fully-filled 2 x 12 cavity.
>"...it’s probably going to be important to use something that is more vapor open to avoid creating a moisture trap."
With a 2" vent space drying toward the interior becomes much less important, even if there is 8" of ccSPF in the cavity. A 2" or larger exposure on each side of the rafter to the vent channel is plenty of drying capacity for the 8" of foam-embedded rafter. Any additional drying capacity on the interior side rafter edge in contact with the rigid foam would be gravy, but not critical. If the thermal break is intended to double the R-value of the 8" of rafter (~R10) mostType-II EPS would be tight enough to qualify (or close to it) as a vapour barrier under Canada's NBC, which isn't much drying capacity to speak of.
Something like 2" of rigid insulation with R24 batts in 2x8 rafters (rest vent channel), is a decent colder climate roof (~R30 whole assembly). Depending on your building code, this might not be enough insulation though.
You can hang drywall directly over the rigid insulation with longer drywall screws. These are standard item at commercial drywall suppliers.
Make sure to tape the seams on the rigid insulation to get a good air barrier.
Daniel,
As bill said, there really isn't an easy way to quantify a minimum. One approach might be to just assess the whole-assembly U-factor and compare assemblies that way, instead of defining a minimum 'thermal break.'
Have you checked out this article: https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling
Also, have you considered the potentially negative environmental aspects, not to mention high monetary cost, of using ccspf to fill the cavities? There's a lot of information here regarding environmental concerns of ccspf; especially when its being used in a location where its not necessary from a moisture management standpoint (which, with the venting you describe, makes it unnecessary).
One issue is the global warming potential (i.e. blowing agents, polymer). It would also leave a gap at the edge of the framing since it can't be filled flush.
Is the structure already built / cast in stone design-wise?
Certainly in this case, the thicker the interior rigid foam, the better. (consider polyiso for highest R)
It is already built: the design is a cape with drop ceiling. Rafters are filled with 8" foam from the top plate to the intersection of collar-ties - 64" length. I'm trying to understand, given these factors, what the minimum r value might be for an effective interior thermal break.
There is no minimum R value. Whatever you add helps. Personally, I’d go with 1/2 or 3/4” EPS as a thermal break, and would put batts in to fill any remaining space between the spray foam and the EPS sheet.
Bill
Ok, so not sure how much space is left between the cured foam and the face of the rafter, but one consideration is that you can get added thermal bridging effects 'sideways' through the exposed rafter. see: https://www.greenbuildingadvisor.com/article/installing-closed-cell-spray-foam-between-studs-is-a-waste
(just for an explanation of the mechanism)
As Bill said, if it's possible to add some insulation to that space, then cover with foam (I dunno, 1/2 inch min? I'd do a bit more...) you're better off than just adding the interior foam alone.
Your current roof, assuming 8" of R7/inch foam, is around R36.
Using my design temp of 2F, 70F interior, a 1000sqft of R36 roof looses ~2kBTU, which is not that much.
1" of EPS bumps you up to R43, 1.6kBTU loss
2" of EPS bumps you up to R48, 1.4kBTU loss
Neither option is really worth the extra cost unless there is an energy target like passive house you are trying to meet.
I apologize in advance for my confusion...
Are you saying that furring out the exposed rather ends isn't worth the cost of materials and labor for the 1 or 2 inch eps, or are you referring to a continuous layer?
Also, I understood that the thermal bridging was a substantive factor and that simply furring the rafters would be beneficial.
The calcs above were assuming continuous layer of insulation. The furring strips do work almost as well (provided the SPF cover the edge of the rafter), but lot more labor.
If you are doing it yourself, on material cost alone there might be a reasonable ROI on that. If you are paying for the labour, ROI is going to be many many decades.
If you use furring strips made from rigid foam I’d use polyiso instead of EPS. This gets you more R per inch, and a slightly more rigid foam for less chance of drywall screw punchthrough.
I’d assumed you were planning on putting up a layer of continuous rigid foam over the entire inside of the ceiling.
Bill
A layer of foam is not going to be cost effective from a net-present-value of future energy cost savings to add much foam, but with an 8" x 2" wood thermal bridge it's going to yield a significant reduction of ice damming potential in snow country.
Simply furring the rafters with wood won't improve anything, but 4" wide strips of 1.5" thick (twice as wide as the rafters, centered on the rafter edges) would do almost as much as a continuous sheet of foam.
It's probably easier to just install a full continuous layer, seal it well, and on not worry too much if there's a 1/4" gap between the cavity foam & sheet foam.
Daniel,
Thermal breaks aren't really thermal breaks. They just add to the R value of whatever they are covering. So continuous interior insulation adds to the existing r value of the cavities, and that of the wood framing. The rafters have an R-value of about 13, an inch of foam board is somewhere around R-4. So you can see why it's hard to say there is a minimum amount required, or what that would be.
Something didn't feel right with this setup, so I ran it through Therm. Not my day job, so I wouldn't necessarily trust my results.
As is, with just drywall over the ceiling, I got an R34 ceiling assembly (less but close to my spreadsheet calc).
The surprising thing was adding in the foam block over the rafters. 2" EPS foam strips only bumped up the R value to R37. Filling the cavity fiberglass bumped it up to R44.
No easy answer to thermal bridging. Best to design it out than try to deal with it later.
Got it, and thanks very much for the replies and great info. Martin's article was also very informative.