Concerns about double vapor barrier in a manufactured home
Hi! I am thinking of purchasing a manufactured home and renovating it to a higher level of energy efficiency. The home was built relatively recently (2010), and the builder has a good reputation in the area for solid, well constructed homes. In particular, they emphasize the quality of their air sealing. The house already has a HRV. I am hoping to add sufficient insulation to take it to passive house standards (or thereabouts – I am not worried about certification).
I have attached a cross-section of the existing construction as described on their website. Based on this, I have two questions:
1. The construction diagram shows (from inside out) a polyethylene vapor barrier, fiberglass insulation, and exterior foam above grade, and a vapor barrier, fiberglass insulation in studs, and a layer of foam on the basement wall. Based on what I have read on this website, having a vapor barrier on the inside and well-sealed foam insulation on the outside creates a double vapor barrier situation, which is not ideal. Is this something I need to be concerned about, and if so, what should I do about it?
2. Our plan at the moment would be to approach adding additional insulation from the exterior above grade, and from the interior below grade. The interior of the house is in good shape; the vinyl siding needs replacing; the basement walls are not finished with drywall but ARE insulated with foam and fiberglass and have vapor barrier installed (and from what I can see, it is indeed a very thorough job, including at the rim joists). Given the concern about the double vapor barrier above, what kind of insulation should I be adding to get close to the passive house standard, and where?
We are at the border of Zones 4 and 5. Thanks very much for your help!
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Replies
Hi Sarah,
You are right that it is not a good idea to have an interior vapor barrier on foam sheathed walls. It's also not a good idea to have too little R-value on the exterior, which can cause condensation issues. These articles should answer your questions about the double vapor barrier issue and the Passive House insulation levels.
https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
https://www.greenbuildingadvisor.com/article/passivhaus-for-beginners
The foam board on the exterior is not a true vapor barrier unless it has a foil or plastic facer. Most foam board labeled R7.5 is 1.5" XPS, which would come in between 0.5- 0.8 perms. While that would past muster under Canada's National Building Code as a "vapour barrier", it's still an order of magnitude more vapor-open than the 6 mil polyethylene on the interior side of the stackup, and the assembly would still be able dry toward the exterior.
With R22 cavity fill and R7.5 on the exterior the assembly would still be moisture safe even without the 6 mil polyethylene in climate zone 4 and all but the coldest edge of zone 5. But with the 6 mil poly on the interior there is effectively no interior moisture drive reaching the susceptible sheathing layers.
The IRC prescriptive in climate zone 5 for 2x6 framing ith only a class III vapor retarder on the interior (such as standard interior latex paint on wallboard) is R7.5. While that presumes R20 in the cavities (not R22), and a true R7.5 (XPS is only warranteed to R6.8, and will eventually drop to R6.3 at full depletion of it's blowing agents), there are tens of thousands of houses in Canada with 6 mil poly on the interior and only 1" (labeled R5) XPS on the exterior that do just fine, existence proofs that the stackup presented is not a particularly risky approach, even if it's arguably not as resilient as some other idealized stackup.
> a vapor barrier on the inside and well-sealed foam insulation on the outside creates a double vapor barrier situation
Nonsense. It may or may not and even when it does, it can be better or worse than some non-double vapor barrier assemblies.
Follow the recommendations here. And verify air sealing with testing.
Also review the economics of adding more insulation. At some point, there are other options that can be more cost effective and better for the environment.