CLT walls, insulation, and dew points
Hello,
I am looking for clarification/help/advice about where a dew point can be within a CLT wall assembly. I am designing for a cabin in climate zone 6. IBC has required insulation R value of 15/20 of continuous insulation (ci) for Mass walls (EC 402.1.3). However, if you only put R-15 of insulation outside a 3-ply CLT (4-1/8″), that places the dew point threshold of 55deg F (based on 70deg interior and -9deg winter dry bulb) inside the CLT, running risk of mold and delamination. But if I try to keep the dew point in the exterior ci, it then requires over 6″ of the exterior ci (Rockwool 80 at 4.2/in. is the best that I have found that is vapor permeable) because of the inherent R-value of the wood (4.125″ x R1.25/in.=5.16 for the 3ply CLT), bringing the overall wall assembly R-value to over 34. Is that typical for CLT wall assemblies in colder climate zones?
Oddly, if I increase to a 5ply CLT (6-7/8″), that requires even more insulation because the additional thickness of the CLT pushes its exterior surface further out past the dew point, requiring more exterior ci to pull the dew point out further within the overall assembly. Is this counter-intuitive result correct?
The attachment is screenshots of my calculations for both the 3 and 5ply. Thanks in advance for any feedback provided.
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Replies
There is a lot of midrise CLT around me and it is all insulated with exterior rigid mineral wool. This allows the assembly to dry to the exterior, so a bit of condensation near the insulation/rigid interface is not an issue. Most also use a fully peel and stick over the CLT, so there is little chance of interior air leaks making it there so condensation won't happen. You will get some diffusion through the wood, but that much wood is pretty close to a class I vapor retarder, so not much will make it through.
I also think you are using a bit too cold of a dewpoint for your calculation. It should be the average temperature of the coldest 3 months. Of course some of the time, it will spend bellow this temperature, but the moisture buildup is small enough that it quickly dries up once the weather warms. I would re-run your numbers with a higher dewpoint and see how much rigid you need.
Thanks for making me review how to read the psychometric chart. I was actually at too HIGH of a dew point. Going up from 70deg interior dry bulb to 35% relative humidity actually drops me at 40deg on the saturation edge (dew point), which is much more believable and manageable.
So I am curious as to why you are saying I am at too COLD of a dew point. A higher dew point is making me need MORE insulation. Are you assuming the moisture is moving out-to-in (which would use a higher relative humidity curve, but would not be in climate zone 6) or in-to-out (which is what I am doing, so choosing the 35% for interior humidity control)? Just curious.
Thanks again.