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WRB and Rigid Foam

Hi Martin/GBA Community,
I'm thinking about installing rigid foam on the exterior with "innie" windows. Would like some thoughts on this set-up, from interior to exterior

Drywall -> 2x4 wall with R-15 Roxul Comfortbatt ->3/4 exterior sheathing -> Tyvek HouseWrap -> 1" R-5 rigid insulation -> Obdyke HydroGap -> Siding

I'm not sure if a WRB behind the foam and in front of the foam is a good idea. WRB behind the foam because that's the drainage plane. WRB in front of the foam is meant to provide a small air space behind the siding.

I can install 1x2 batts in front of the foam if needed, but hoping to avoid the extra depth on the exterior.

Climate Zone Marine 4.


Asked by Mdubya
Posted Nov 9, 2017 5:56 PM ET


9 Answers

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I have a similar stackup on my home. The Roxul will allow drying to the inside. Also consider taping the rigid foam and the sheathing if that's not part of your plan. It's a little overkill but provide a bit of extra protection.

Answered by Steve Knapp CZ 3A Georgia
Posted Nov 9, 2017 10:11 PM ET


I'm here to learn as well. Why did you decide to use HydroGap rather than a rainscreen? Is due to the siding material you're choosing? Ease of install?

Answered by Brad S
Posted Nov 9, 2017 10:28 PM ET


Which type of siding are you using? They may have rainscreen air gap requirements.

Answered by Tyler LeClear Vachta
Posted Nov 10, 2017 10:36 AM ET


Brad, I purchased the HydroGap early on, and wasn't intending to use foam. Would like to use HomeSlicker plus Typar but trying to balance cost and ideal assembly. HydroGap seemed like a reasonable balance between the two.

Answered by Mdubya
Posted Nov 10, 2017 3:22 PM ET


Tyler, I'm toggling between reusing the 1x10 cedar beveled siding (I'd need to purchase some new as some old boards have cracked) or using a Hardie lap siding (or similar). Looks like Hardie says their siding can be installed directly against foam, but I'm thinking it would be better to have some type of drainage plain.

Answered by Mdubya
Posted Nov 10, 2017 3:24 PM ET


I'm wondering if there is any drawback to using a WRB behind the foam AND in front of the foam. So the foam would be sandiwched between Tyvek and HydroGap.

Answered by Mdubya
Posted Nov 10, 2017 3:25 PM ET


Other than the additional expense, there isn't any drawback.

BTW: Which R5 foam?

XPS is usually labeled R5 @ 1", but some of that performance is due to the (high global warming potential) HFC blowing agents. Over a few decades performance drops to EPS of similar density (R4.2) as the HFCs leak out.

There are some graphite-loaded EPS products out there that are R5 @ 1", as well as several polyisocynurate products rated 6 or more @ 1". While some of those polyiso will underperform their labeled-R in your stackup a Marine zone 4C climate, most will beat R5, and Dow Thermax will perform at over it's labeled R6.5.

Answered by Dana Dorsett
Posted Nov 10, 2017 6:59 PM ET


Thanks Dana, I'll take a look at alternate foams. I was thinking XPS but was unaware of the HFC agents. I'll find something more eco-friendly.

Answered by Mdubya
Posted Nov 13, 2017 12:58 PM ET


Using current new box store pricing near me, 2" EPS is far lower cost per R than any other rigid foam. Even more so once you properly de-rate polyiso. Reclaimed prices are higher than the box stores!

Answered by Jon R
Posted Nov 13, 2017 2:01 PM ET
Edited Nov 13, 2017 2:13 PM ET.

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