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Community and Q&A

Modified Mooney Wall (SPF)?

kjhkjh | Posted in Green Building Techniques on

Is there a way of achieving exterior foam board performance without the faming hassle by using a modified mooney wall?

The challenge of closed cell foam sprayed into interior cavities is that thermal bridging from the studs robs some of the benefit.
Our current project is an elaborately detailed house (lots of dormers and bumpouts) and although we’d intended to go for exterior rigid foam, we’re still open to ideas.

Would a modified mooney wall give us the benefits without all the hassle associated with getting the exterior detail ‘right’?

We would
– put 2×3 strapping horizontally across interior studs
– Apply closed cell spray foam 2-3″ thick into cavities and over the studs (including the space between the horizontal strapping)
– Rewire and then dense pack the bays and remaining space with cellulose.

It seems a lot simpler than trying to convince a local contractor into applyling the exterior foam!
(the house will need to be reshingled so that component is a wash)

I’ve been learning about Mooney’s but not found info on this configuration.

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  1. Expert Member

    Perhaps coincidentally Mooney walls have just come up in another thread here:

    If you are going with a Mooney wall, is there any great benefit to incorporating the spray foam, over just dense-packing the whole cavity?

  2. GBA Editor
    Martin Holladay | | #2

    Your suggested method has at least two disadvantages:
    1. With your method, the exterior sheathing stays cold and damp during the winter. With exterior rigid foam, the sheathing stays warm and dry.

    2. Closed-cell spray foam is made with a blowing agent that has a high global warming potential, so it is much less environmentally friendly than EPS or polyiso.

  3. Dana1 | | #3

    "The challenge of closed cell foam sprayed into interior cavities is that thermal bridging from the studs robs some of the benefit."

    Not "...some of the benefit..." but rather "...a LOT of the benefit..." due to the fact that the wood is about 6 times as thermally conductive as the foam. You'd be a lot better off with about the same amount of foam spraying it as a continuous layer on the exterior where its not being robbed of performance, and put the cheap stuff in the framing cavities.

  4. Chaubenee | | #4

    Edward, I think you are using a scheme (interior CCF) to flash and batt your walls, that is now in need of mitigation. Put the foam on the outside and then you won't need an expensive mitigation strategy.

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