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Can I use rigid foam to insulate my house, or am I asking for trouble?

Spanky66 | Posted in Energy Efficiency and Durability on

I want to build an energy efficient and comfortable home. My thought is to frame 2×4 walls with 3.5″ of rigid foam between the studs and then using a modified “Mooney wall” technique run 2×4’s horizontally on the exterior of those walls with another 1.5″ of rigid foam to complete the envelope.

I was looking at using metal siding and roofing.

My question is: Am I going to be effective using this concept or am I cooking up a recipe for moisture issues, mold and rot (basically a structural disaster)?

Any and all suggestions and ideas are gratefully accepted! Thanks a bunch! (~8

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  1. iLikeDirt | | #1

    No need to reinvent the wheel. Put all that foam outside of the sheathing instead and fill the cavities with mineral wool or fiberglass batts, or dense-packed cellulose if you can find someone trustworthy to do it for not too much money and guarantee it won't settle. Way cheaper, easier, safer, and more effective. KISS.

  2. Spanky66 | | #2

    The reason for reinventing the wheel is economics... I've got access to a pile of rigid foam that basically will not cost me a dime... Using the modified "mooney wall" allows me to reduce thermal bridging while increasing R value but my concern is with moisture, mold and rot,,, especially with wanting to use metal siding and roofing... Is this a valid concern or am I worrying over nothing??

  3. Andrew_C | | #3

    Your main question is about insulation. Where you live will likely effect your decisions and the advice you receive. Where do you live? Use the map of climate zones at the start of the Q&A page.

    The cost of insulation is likely to be a small factor in the overall cost of your house. The rigid foam may not be the type of insulation that you need for under slab, on walls of basement/crawl (if applicable), or roof. And it may take a lot more money to stick those free square pegs into round holes. I'm not trying to discourage, but there's a lot to consider.
    If you want to build a comfortable and efficient home and are looking for good ideas, I always recommend using the Search function to look for Martin Holladay's (regularly updated) article HOW TO DO EVERYTHING. That should provide you plenty to think about for a few days.

    Back to basics: letting us know which climate zone would be helpful.

  4. Spanky66 | | #4

    My main question is will the foil faced rigid foam panels cause moisture issues, mold and rot??? I'm well enough read to understand that R value is increased and thermal bridging is decreased by going with the "mooney wall" concept. I wanted to "reverse" the usual method by placing my horizontals on the exterior and running 1.5" of foil faced rigid foam between them in addition to using 3.5" of foil faced rigid foam in between the 2x4 studs... This in theory would give me a fairly high R value wall with very low thermal bridging... However I also have read that rigid foam can cause some moisture, mold and rot issues... What isn't clear to me is with my panels (that are free of charge by the way) will the foil facing basically act as a vapor barrier and my design will work fine or will they act as a condensation point and cause moisture, mold and rot issues?? We are in zone 5 if that helps,,, I'm just not able to find the clarity on this issue so far... Thanks for any and all suggestions and information...

  5. user-4524083 | | #5

    Del - I'd put the 31/2" foam on the outside and mineral wool between the studs. The problem with foam between the studs is two-fold: 1. It's a lot of work to "cut and cobble" the foam to fit the stud spaces, and then spray foam the perimeter to get it tight. 2. If you're successful at this task, you now have wall sheathing with foil-faced, vapor impermeable insulation on both sides. If any water vapor gets onto the sheathing, it will have a very hard time drying. You need to have at least one side of the wall to be vapor permeable to allow moisture to escape and dry out the sheathing.I understand the insulation is free, but it will create trouble if placed on both sides of the sheathing. Good luck to you on your project! Oh - if you do as I and Nate are suggesting, probably better to cover the sheathing with the foam and strap it to create a rain screen and nailer for the siding, as opposed to putting it between horizontal 2X4's - the 2x4's water down the insulation and act as a "thermal break" since they don't insulate anywhere near as well as the foam.

  6. Chaubenee | | #6

    Del, I implore you to do the ultimate in reducing thermal bridging and employ the foam board on the exterior. This will keep the inside of the wall cavity warmer, thereby decreasing the chances of condensation that you might face inside the wall with the foam tucked in there as you plan to do. Also you effectively decrease the bridge on the headers, plates and over jacks and all framing lumber where yu could not gain benefit from the foam on the inside. Use inexpensive cellulose or even fiberglass inside and ditch the mooney wall concept. If you are hung up on the Mooney, do what I am doing and employ it with the cellulose dense pack on a north wall with no windows where yu needn't contend with the issue of extension jambs. I designed a second floor room with no Northern glass, 32x9 that has a long walk in closet on it and am strapping the Mooney on there to gain some added R-value. I have 2x6 walls 24oc and 2.5" eps foam outside. It is the wall where all the wind hits so I am doing some extra detailing to prevent wind driven rain damage and leaks too. I also went with two foot eaves and rakes.

  7. Spanky66 | | #7

    Ok,,, clearly the consensus is that rigid foam will cause moisture and rot issues... Thanks for the info folks!!! It's greatly appreciated!!! (-8

  8. Spanky66 | | #8

    Can you still use metal siding?? And if so how do you mount it over the rigid foam?? 5" screws?? Looks like I've got some more home work to do,,, LOL!!!

  9. GBA Editor
    Martin Holladay | | #9

    No one is saying that rigid foam will cause moisture issues. What everyone is saying is that you need to install the rigid foam in a continuous layer -- ideally, in two continuous layers, with staggered seams -- on the exterior side of your wall sheathing. That way works. If the foam layer is thick enough, you can even leave your stud bays empty of insulation (a method called PERSIST).

    Your suggestion is far inferior. Your method is called the "cut and cobble" approach. Here is an article on that approach, which is very time-consuming and which can easily lead to air leaks: Cut-and-Cobble Insulation.

    Here are three articles on the right way to proceed:

    How to Install Rigid Foam Sheathing

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    Getting Insulation Out of Your Walls and Ceilings

  10. GBA Editor
    Martin Holladay | | #10

    Q. "Can you still use metal siding?"

    A. Yes.

    Q. "If so, how do you mount it over the rigid foam?"

    A. In most cases, the siding is fastened to vertical furring strips. For more information on furring strips, see the following three articles:

    Fastening Furring Strips to a Foam-Sheathed Wall

    All About Rainscreens

    How to Install Rigid Foam Sheathing

  11. Spanky66 | | #11

    Thanks a bunch y'all
    !!! This is all greatly appreciated!!! (-8

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