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Community and Q&A

Retrofitting new wall inside existing frame

Griffin728 | Posted in PassivHaus on

I have an interesting challenge in my 1941 1500SQFT energy retrofit in Minneapolis (Zone 6-7). I want to maximize realistic R-value while preserving the existing siding and sheathing, which we can assume has no vapor barrier (this will be verified when we open it up). While I would normally prefer to wrap the exterior, this is not possible at this time. There is no historic value to the plaster/sheetrock, and we’re demoing kitchen and bath anyway, so I intend to do all insulation from the inside. The home is all 2×4 16″OC construction with old yellow fiberglass batts. Adding wall thickness to the inside is acceptable, although 2″ more is preferable to 4″ more. I’m not looking to get certified, but want a very comfortable house with a chance of 15-20 year payback.

Here are some options I’ve considered:

1) Do nothing with the walls and focus on the roof and air sealing – this is the adivce that most local builders have given. Most have never heard of passive house and think US building codes are too strict. Some, however, have cited the valid reason of opening a can of worms with wiring, etc.

2) Remove existing sheetrock and fiberglass. Spray foam the 2×4 walls for R-21. This preserves interior dimensions and provides a real vapor barrier. Downfalls are it’s a lot of work just to get R21, and there is still a lot of thermal bridging going on.

3) Do everything in #2, but then add a inside layer of 2.0″ EPS foam to stop bridging and get us to R-33. I like this idea, but am thinking that it might make it hard to hang drywall and kitchen cabinets to the foam. Would this require framing work?

4) Don’t gut anything, but build a new stud wall inside the existing and spray foam that for R38ish Maybe this is crazy, but it might save some on the demo side, and would allow us to further insulate in the future from the outside (we will eventually need to touch the siding). It also limits the can of worms effect (good and bad). Is this even possible? It seems to me that it shouldn’t cause any moisture issues since there is a full interior barrier and no exterior barrier. There would also be no thermal bridging as the studs wouldn’t touch. Thoughts?

5) The ultimate solution is to gut everything, build the new double wall and spray foam the whole thing, or do a combination of foam and another material (cellose?). This could get us into the R-40 to R-50 range.

Please let me know what you think of these scenarios given the information provided, and the desire to keep this semi cost effective. Thank you very much for your help here!

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Replies

  1. charlie_sullivan | | #1

    Answer 1 is a legitimate answer....you can also consider things like adding PV and and air-source heat pump and seeing if you can have a better energy impact for the same cost that way.

    The other options mostly seem to include spray foam. Spray foam still uses high-global-warming impact blowing agents, so it's really hard to justify using that. You'd probably increase the global warming impact of the house with any of those designs.

    EPS does not have that problem. So what about doing something like #3 without #2: Add EPS inside with taking the drywall off or changing anything inside the wall? The wiring isn't a big deal--just add box extenders to bring the box through the EPS. Sure, those will be air leaks but not as bad as they are now if you do a little sealing while you are at it. You can attach drywall by gluing, add furring strips to attach it to, or use a product like R-etro that is EPS with built-in plastic support for drywall. Kitchen cabinets would require better support, so maybe that part has 2x4s turned sideways as the furring strips.

    With any interior insulation, you'll have thermal bridging where the interior walls go through but that's not a huge problem--you'll still be making it much better.

    You could also do a double wall, but I'd be inclined to leave the existing wall, sheetrock and all, in place, and just build the additional wall however far in you are willing to go, and fill with cellulose. The sheetrock perhaps adds some thickness without adding much R-value, but it's not hurting anything either.

  2. Griffin728 | | #2

    Thanks Charlie for the thoughtful response. I like the idea of using rigid foam, but have not seen it done on the inside like this.

    That's interesting about the spray foam impact. I always thought that the energy and GHG savings throughout its lifecycle would quickly offset the initial embodied energy/carbon. Are you aware of any studies that give a definitative answer to this?

  3. GBA Editor
    Martin Holladay | | #3

    Ryan,
    For more information on the global warming potential of the blowing agents used in some spray foam products and XPS, see:

    Avoiding the Global Warming Impact of Insulation

    Calculating the Global Warming Impact of Insulation

  4. Griffin728 | | #4

    Martin,

    Thanks for the links. I would also love to hear your opinion on using thick rigid foam on the interior side of the existing wall. I hear that Polyiso has the lowest impact, but then some have said that it will shrink, making it unsuitable for this application. EPS doesn't seem to have the R-value I'd like, but maybe it's a better choice? To be clear, I am considering adding 4.0" (two 2.0" sheets, taped) on the inside of a 3.5" wall with old fiberglass and no real vapor barrier (so it drys to the outside). Polyiso looks like it would give me an additional R23, whereas EPS an additional R15. I would still sprayfoam the rim joist, where the rafters meet the wall and any other crevices that I can access.

  5. charlie_sullivan | | #5

    For a more direct answer to Ryan's question about embodied energy: The issue with spray foam is not embodied energy. It is the gas the fills the bubbles in the foam. It is correct that the embodied energy of insulation is usually small compared to energy savings over the lifetime. However, the effect of the gas in the bubbles in the foam is >1000X that of CO2, so that can be significant, as discussed in detail in the links Martin provided.

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