Fasteners and thermal bridging with exterior insulation
DWest
| Posted in General Questions on
I’am planning on residing my house and have decided to add some exterior insulation while I’am at it. I live in climate zone 6 in Maine and the house has 2×4 walls with kraft batts. I ‘am planning on using cedar shingles for siding and will use zip R 9 or similar for my nail base with a rainscreen matrix. My question is will sending hundreds of siding nails through the exterior insulation when I apply the shingles negate some of the insulation value of the Zip R sheathing? I’ve noticed a possible corollary with thermal images showing the interior side of walls with cold spots where the drywall screws are located. Thanks!!
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Replies
DWest,
From what I can see from the discussion here on GBA, at the worst the thermal bridging could reduce R-value of the foam by 10%, but probably far less. Here is what John Straub said: "The effect [that fasteners have on the thermal performance of rigid foam sheathing] is on the order of 1 to 2 percent. Variations of this have been modeled repeatedly. 45 per cent [reduction in thermal effectiveness is] technically preposterous."
https://www.greenbuildingadvisor.com/question/do-screws-through-exterior-insulation-reduce-the-wall-r-value
Malcolm,
Thanks for the prompt and great reply! Some math is in order but I was particularly focused about the application of cedar shingles and the sheer number of nails I'll be sending through the foam. Granted if i used 1 1/4 or 1 1/2 nails they wouldn't penetrate more than a 1/2 into the foam.
I estimate with a typical 5 inch reveal one square foot of shingle would have maybe 6-10 nails depending on shingle width. Cedar shingles is a classic New England detail but its application with exterior foam is a bit more nuanced than I would like. Thanks for link and I'll keep digging for more info.
The fact that the nails don't make it through the foam makes a difference as well as the rest of the foam mitigates the thermal bridge. It also means if you ever go to reside you won't a thousands of tiny holes through your air barrier.
DWest,
Is there any other realistic alternative for fastening either sheathing or cladding that doesn't include some thermal bridging?