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How can I make my exterior walls more energy efficient?

tonysmithe | Posted in Energy Efficiency and Durability on

I would like to make the exterior walls of my home more energy efficient. My home is, as shown on your climate zone map, in zone 4a. It is about 1 hour south of St. Louis on the Illinois side. I currently have a 100 + year old home with no insulation in the 2×4 stud bay. The home currently has the original shingle siding, then a layer of 1/2 inch xps, and finally vinyl siding. Removing the siding is not an option in my foreseen budgeting plans so I would like to add to the interior of the home. I am planning to remodel most of the interior anyways so removing the sheet rock/ left over plaster to access the stud bay would be ideal. I have read a great deal, on your website forum, about the option of first insulating the stud bays with a bat style insulation and then adding rigid foam boards to the interior of the stud bay. I do like the idea of this but i’m concerned of moisture build up in the wall bay. I have been unable to find my specific wall situation, meaning the shingles, xps, and vinyl siding, on your forum. Could you please give me your professional recommendations for increasing the efficiency of my home walls? Thank you very much in advance for your time.

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  1. GBA Editor
    Martin Holladay | | #1

    The biggest unknown in the wall assembly you describe is the vapor-permeance of the 1/2-inch rigid foam. If it really is XPS (rather than fan-fold insulation with a vinyl facer), then you are in luck. The vapor permeance of this layer is probably around 2 perms, which means that some outward drying can occur. That's good.

    Considering the fact that you have a layer of exterior rigid foam, it's best if you don't install another layer of rigid foam on the interior side of your wall assembly. I would advise you to fill your stud bays with dense-packed cellulose, blown-in fiberglass, mineral wool batts, or meticulously installed fiberglass batts.

    If you want to increase the R-value of your wall beyond what is permitted by the depth of the existing studs, the best approach would be the so-called Mooney Wall approach. (For a Mooney Wall, you install horizontal 2x4s on the interior side of the wall. The 2x4s are installed 16 inches on center, flat ways, so that the 2x4s increase the depth of your stud cavity by 1.5 inch. This cuts down on thermal bridging through the studs.)

  2. tonysmithe | | #2

    I was mistaken. I popped a few pieces of siding off today to double check. The foam board under my siding is actually a 1/2 inch thick layer of polyiso board. Please let me know how this changes my situation. If it is not possible for me to install a layer of foam board insulation to the interior of my walls, to increase their R value, then I am interested in the "Mooney Wall" you mentioned. I understand how installing 2x4's flat, horizontally, and on 16 inch centers increases the wall cavity by 1.5 inches and helps to prevent thermal bridging. Would it be more beneficial to just frame an interior 2x4 stud wall, on the inside of the existing wall, and stagger the studs on a 16 inch layout. My theory is that this would use a similar amount of lumber, increase the wall cavity by 3.5 inches, and prevent thermal bridging. If i'm thinking correctly, the staggered stud layout would cause the insulation to cover both the interior surface of the existing studs and the exterior surface of the new studs. Then my week point would be my top and bottom plates and the window framing. If this is correct. would you recommend doing what I have stated and also, how could I reduce thermal bridging at the wall plates and window framing. Thank you so much for your quick reply. I appreciate your advice greatly.

  3. GBA Editor
    Martin Holladay | | #3

    The vapor-permeance of the polyiso depends on the facing. If the polyiso is foil-faced, its permeance is essentially zero. That's not ideal, but your wall will still probably be OK if you pay close attention to airtightness as you proceed with your work. You can gain an added layer of safety by installing a so-called "smart" vapor retarder like MemBrain on the interior side of your wall assembly.

    Your proposal to build a double-stud wall will work fine. The only downside is that you lose interior space. If you go ahead with your plan, don't worry about the inevitable thermal bridging at the plates. The heat flow in these areas won't be enough for you to lose sleep over.

  4. Expert Member
    Dana Dorsett | | #4

    Half inch polyiso is ~R3, which makes it on a dew point control basis for 2x4 framing in zone 4, but not for deeper fluff. The IRC prescriptive minimum is R2.5, so there is not a lot of margin there. If you bumped up the cavity depth to 5.5" the IRC's prescriptive minimum is R3.75, which is negative margin- you would need an interior side "smart" vapor retarder to bring the wintertime vapor retardency to under 1 perm while retaining a reasonable drying rate during the spring shoulder season.

    Better than a Mooney wall would be 2" polyiso nailbase panels cut into 1.5" wide strips, applied to the edges of the studs with a bead foam board adhesive and 3.5" bugle-heads every 16". The wallboard can then be hung on the nailbase in the usual fashion, no need to through-screw it to the studs.

    That would give you a standard 5.5" cavity depth, but unlike a 2x6 wall you would ~R8-9 of additional thermal break over the studs, and would allow you to put R23 rock wool, R21 fiberglass, or R20 cellulose or open cell foam in the wall cavities. Caulking the studs to he exterior sheathing inside every stud bay would be critical for performance for any of the fiber solutions, but would not be necessary with open cell foam. With any of the above a bead of polyurethane caulk between the doubled-up top plates, and where the bottom plate meets the subfloor is called for. With the open cell foam solution you can probably get away with half-perm paint instead of MemBrain as the interior side vapor retarder, since there would be zero air-transported moisture, but at ~12-15 cents per square foot MemBrain isn't usually a budget buster.

    IRC code-min for zone 4 is 2x6 / R20, and with the thermal break of the nailbase AND your continuous exteior R3 your thermal performance would beat the R20 + R5c.i. performance called out by the IRC for zones 6-8

    It's a bit tedious, but instead of 2" nailbase you could rip 1.5" polyiso into 1.5" strips (use a 4" wallboard taping knife with the edge sharpened), and rip some half-inch CDX for the edge strips. That's cheaper than 2" nailbase panel, and easier to find.

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