Interior ripped 1/2 inch rigid insulation on studs
Build new custom on the water in ct and plans are done now. They call out for Osb sheathing, spray foam walls. We will also be using a Siga wrb and then vent grid or roll on rain screen. Recently I have become very interested in the reduction of thermal bridging and ways to reduce it on this house.
the engineer is pushing back from zip r because of shear and wind Load because the house is on the water in Connecticut and complete then a complete re-engineer. So thinking about exterior insulation and cellulose in the wall cavity. But the compression of rain screeen into the 1/2 foam concerns me with test I have done. So want a rigid osb or plywood to nail our rain screen to for our cedar shakes. Is 1/2 going to do anything for thermal bridging?
so now thinking of just ripping 1/2’ rigid foam and attaching it to studs with pl adhesive( not full sheets) so not to trap moisture, even thou cellulose would dry in or out or closed cell flash and bat would dry in. My rain screen would then be Directly on the sheathing and wrb. I like this hard smooth straight surface siding. will 1/2 rigid foam on stud face work in reducing the thermal bridging? Would not place it on ceilings only walls and roof rafters to de-couple sheath rock from transmitting studs… any help here and direction welcome!
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Replies
Kai-Uwe,
Thermal Breaks only function in the way we imagine them in very rare cases. Something like say, the break between very conductive metals introduced into a window frame, where the Break may represent almost all the R-value in the assembly.
Really they don't really "break" anything, they simply adding insulation to poorly insulated parts of an assembly - which is the case in what you are proposing. The studs in your wall will be about R-7.5, and adding the 1/2" foam will give them another R-1.6 for a total of R-9.1. Given that the surrounding cavities are about R-20, the 1/2" of foam over the framing hasn't got you much for all the effort. If you want to increase the insulation levels at the framing it's probably worth using something a bit thicker.
+1 on what Malcolm said. Don't focus on thermal bridging - do whatever results in the best $/ton of carbon reduction. If this makes your head spin, then focus on whole wall R value and cost.
From a practical point of view I would be worried.
1 The foam may shrink over time and the screws would pop out of the drywall.
2 Driving the drywall screws to be flush without breaking thru the paper is difficult enough without spongy foam behind the drywall.
3 Whenever you ask the subcontractors to do something abnormal they demand more money.
Be sure you understand heat flow always take the path of least resistance and moves from hot to cold. Heat will flow around your foam in the sides of the studs and out to the cold.
Walta