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hardyh 01-18-2010 12:53 AM

Exterior XPS insulation
I am in the process of figuring out the insulation options for a new construction in a mixed climate (southern BC, snow but not too cold in winter, hot in the summer, raining the rest of the time). I had originally intended on putting 1 inch XPS on the outside of the 2x6 wall (7/16 osb, tar paper, cedar siding...) with vapour retarder paint rather than poly on the inside. I looked up the mixed climate info on building science, and the rest of the interwebs, and also did some math. I figured out that with 40% RH the backside of the XPS would be below the required dew point for the three coolest months of the year. Besides adding more styro, are there any other suggestions? What about a 'rainscreen' type setup between the styro & the wall, so that any possible condensation uses the drainage plane provided by the tar paper? What about not sealing the XPS so that it becomes more permeable, or even adding spaces between the sheets? Will it help that the majority of the winter heating will come from a wood burning fireplace, would that reduce RH to any degree? Any thought would help, thanks in advance!

Gary in WA 01-18-2010 08:25 PM

You know the foam board is sealed to stop air/moisture from going through the joints, perimeter, right? And the foam is on the building side of the rain screen: Be safe, Gary

hardyh 01-18-2010 10:29 PM

I checked the links, thanks. I didn't think that the foam went underneath the tar paper, I've only seen it applied in residing applications where the stucco was just left on with the XPS overtop. I guess my confusion is just with the climate circumstances that I'm dealing with, it gets a bit chilly in the winter months but the rest of the year it's warm if not hot. What are some suggestions for this mixed climate situation? Most of the info I found shows design principles for one climate or another, but not both...

Gary in WA 01-18-2010 11:25 PM

Read this on MIXED CLIMATES, right after “cold climates”, then read the rest:

Be safe, Gary

hardyh 01-20-2010 09:36 AM

That was a good link on mixed climates, it didn't seem to come up on my google searches earlier. I've decided to go with the 1 inch styrofoam on the inside instead, and use the rainscreen on the outside as well. I wanted to reduce the thermal bridging, but it seems like my climate over all will require more drying to the outside. I'll also make sure that the mechanical ventilation keeps the RH steady through the winter. Thanks for the help, any other thoughts?

Gary in WA 01-20-2010 04:08 PM

Here’s more reading to quench your thirst, an assortment.

Compare your heating days to these sites…..

Be safe, Gary

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