Worried about dew point
Salibonz
| Posted in General Questions on
I’m in climate zone 6a. I plan on building a new home and using zip sheathing as a wrb and halo exterra 2 inch insulation, which is an r10 eps. This is also vapor permeable.
My question: is it better to build 2×6 or 2×4. I am worried about the dew point where the exterior needs to be 35% of interior. I prefer to do 2×6. Can I just use interior vapor barrier like siga majrex with a service cavity to not worry about the dew point.
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Replies
Salibonz,
If you want a risk-free wall assembly stick to the advice and ratios in this blog:
https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
If you have some risk tolerance follow the advice in this one:
https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness
I have read the article, they are claiming that if you use an exterior insulation that is not vapor permeable. Halo exterra is, and if I use furring strips to create a channel for air to pass wouldn't that be effective? If condensation did happen wouldn't the osb have the ability to dry. Also I get a 2x4 wall with r 15 interior and r10 exterior is safer- would this just be the best option. Would this create a well built house
Salibonz,
Much of the evidence for using less than the usual ratio of foam to permeable insulation came from Eastern Canada where houses are regularly built with 2"x6" walls and 1" of EPS. With a good warm side air and vapour barrier they seem to perform fine.
The question is what happens when you increase the amount of foam to a point where it becomes much less permeable, but at the same time isn't quite sufficient to prevent the sheathing accumulating moisture? I suspect the risk is quite low, but it isn't zero like using enough foam.
This long discussion on this Q&A calls into question the true perm rating of a product that relies on perforations like Exterra:
https://www.greenbuildingadvisor.com/question/vapor-permeability-of-polyiso
Point is: the Exterra may not be as vapor permeable as the spec sheet says. You might want to go up to a polyiso type of insulation to increase the R-value per inch and get closer to the safe ratios of exterior to interior insulation. Your exterior insulation does not have to be vapor permeable, although arguably it is a safer assembly when it is. It will be able to dry to the inside as long as you don't install an interior vapor barrier like poly and there is some drying that can happen through the Zip and the micro-gap between it and the foam. Risinger put zero-perm polyiso against Zip directly at his personal home build, for example.
The thing with polyiso is it has shrinkage in cold climates so I would have to do 2 layers. Trying to avoid the extra labor and do 2 " exterra, one and done.
What do you mean by "shrinkage"? Atlas's website shows less than 2% linear expansion/shrinkage according to ASTM D2126 which ranges from -73 to 150 °C and relative humidity from 50% to 97%.
If you are referring instead to the reduction in R-value in cold temperatures, keep in mind that even at the coldest temperature you would ever run into, polyiso will always have a higher R-value than XPS and EPS.
https://wall.atlasrwi.com/products/residentiallight-commercial/energyshield-cgf/#tap-properties
https://wall.atlasrwi.com/products/residentiallight-commercial/energyshield/#tap-properties9a65-42b4
I've heard that polyiso contracts in cold climates leaving gaps at joints. So it is best to install 1st layer vertical and 2nd horizontal to deal with that issue.
Matthew,
Drying to the inside only works as a strategy in heat dominated climates if the exterior foam is thick enough to protect the sheathing against accumulating moisture. If you have less than the required R-value, then the whole thing flips and an interior vapour-barrier is necessary to limit the amount of moisture that get to the sheathing.
I gotcha, so if I use siga majrex and a service cavity this would greatly reduce any issues?
Salibonz,
Yes I think so - and you aren't very far of the required minimum foam for your climate (R- 11.25) even with a 2"x6" wall, so the times your wall may accumulate a bit of moisture during the year won't be very frequent.
Thank you Malcolm I appreciate your help
Salibonz,
Good luck with your build!
This house I am planning is a spec home so 2×4 framing , r 15 interior, r 10 exterior would be cheaper. But I don't want the fact that it's a 2×4 house to deter buyers. Am I taking a risk building this way even though it would still be a better build than a standard 2×6 with no exterior insulation