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Community and Q&A

Reducing thermal bridging without creating moisture issues

sickofscreenname | Posted in Green Building Techniques on

I am currently working on an estimate for an exterior remodel of a high end home in Northwestern NJ (E5 zone…I believe?) and need some advice. I have been a remodeler for 30+ years, and have tried to keep up with current technologies, but as you know, insulation techniques are ever evolving and difficult to get a straight answer on. The homeowner is somewhat knowledgeable in regards to building practices and wants to try to “tighten-up” his home. The project involves replacing all windows and doors with upgraded units, strip the exterior of the walls to the studs (currently the house only has 3/4″ blue foam as sheathing, except for the corners, which is 3/4″ plyscore. In addition, the foam is riddled with holes from mice…), install new foam to prevent thermal bridging, new sheathing, then Hardee plank siding (or similar). The home was built in the early 1980’s and has 2×4 wall construction with, I assume, R-11 or R-13 fiberglass insulation (paper faced).
After spending hours searching the web for some information, I have become thoroughly confused as to what to advise the homeowner to do. Most techniques discussed are for new structures or complete gut remodels and don’t take into account pre-existing conditions that we are forced to deal with, while trying to upgrade without creating new problems and dealing with budgetary constraints.
If I am understanding things correctly, I could create more moisture issues within the wall cavity by not having enough R value in the foam. As the existing F/G insulation is staying and there is no poly vapor barrier on the interior side of wall, what thickness foam should be used and should I install plywood sheathing over studs first, then the foam? Or foam then sheathing? My other concern is the weight of the cementious siding. If it is only attached through foam to sheathing with ringshank nails, the weight can loosen or bend them over time and create a mess of a callback. I really don’t want to add more layers of strapping due to making the wall thickness so deep that window and door attachment becomes problematic. I have read many answers in your forum, with many idea’s that seem to contradict each other. Also, let’s try using some common terminology for those of us that are not engineers and are trying to educate ourselves and our clients.
Thanks for the help!

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  1. iLikeDirt | | #1

    If mice ate through the foam, you might want to replace it with a type of exterior insulation that can't suffer the same fate, such as rigid mineral wool (e.g. Roxul ComfortBoard). As for the thickness, this article will tell you:

    In Zone 5, you need at least R-5 for 2x4 walls and R-7.5 for 2x6 walls. That works out to 1.5-2" of mineral wool. The preferred siding attachment method particularly over mineral wool is to fasten vertical furring strips over the exterior insulation, into the studs. The strips don't even have to be very thick; 3/8" or 1/2" is plenty (particularly over mineral wool which is water-repellant and drainable), so you can use strips of plywood instead of 2x lumber for the strips. Fasten them with screws. Then you nail the siding into the furring strips.

    Regardless of how thick you extend the wall, you shouldn't run into window attachment problems; just put your new WRB over the sheathing and fasten the new windows to the same plane, on the level of the sheathing, under the mineral wool, rather than having them flush with the wall surface. Then just trim out the windows on the exterior.

  2. Expert Member
    Dana Dorsett | | #2

    There's no such thing as "preventing" thermal bridging unless all of the insulation is outside of the framing layer. What the blue foam is doing is providing a thermal break, mitigating, not preventing that thermal bridge.

    A 2x4 wall in US climate zone 5 needs a minimum of R5 on the exterior of the sheathing for dew point control, keeping the average temp at the sheathing above the anticipated dew point of interior air. At 3/4" the blue foam is only R3.75. This is per the IRC chapter 7:

    That would also meet IRC 2012 & 2015 code min for wall-R, per chapter 11:

    There's good, better, best ways of hitting that mark, and ways that are risky. The short list:

    An inch of foil-faced polyiso would be labeled R6 or R6.5, but would underperform it's labeled R during the winter months that count, and the foil facers would block drying toward the exterior. If going that route, a minimum of 1.5" should be used to have any margin.

    An inch of XPS (blue, pink, green) would be labeled R5, but would slowly lose performance, and in 50 years would be doing no better than R4.5, and possibly as low as R4.2. XPS is also manufactured with a climate damaging blowing agent, HFC134a (automotive AC refrigerant, about 1400x CO2 on a 100 year global warming potential.)

    1.5" of EPS would be labeled R5.8 to R6.3, and would have stable performance over time. The R5.8 @ 1.5" stuff is ~1lb per cubic foot density fairly vapor permeable but sometimes has a high shrinkage rate over time. It's also soft, and is often sold with facers that end up lowering it's vapor permeance. The better stuff is 1.5lbs or higher nominal density (aka "Type-II") has lower shrinkage problems, and is usually sold without facers.

    1.5" thick rigid rock wool labeled R6, is highly vapor permeable, completely fireproof, and has no shrinkage issues, but is usually quite a bit more expensive than 1.5" EPS or polyiso. It's also much more compressable which can be an issue for making the siding look flat.

    To take advantage of the drying rates of rock wool EPS, or 1" XPS requires back-ventilated siding, not fiber cement smack up against the insulation or housewrap. That ventilation gap doesn't need to be as thick as 1x furring- 1/4" will do, but it needs to be there.

    At 1.5" thickness you're not going to be long-nailing fiber cement siding to the sheathing, so you have to really figure out whether it's in the budget going to bring it up to code or not. If you're going to cheat code on it, a layer of 3/8" PERFORATED fan-fold XPS siding underlayment and some 1/4" thick OSB ripped into strips for maintaining the vent gap would work from a moisture point of view, and add about R1.5-R2 to the whole-wall performance (instead of the code prescribed R13 + R5 2x4 wall). eg :

    While the sheathing is exposed is a good time to air-seal the sheathing layer, and fix any defects in the cavity insulation by blowing insulation from the exterior. It may take a blower door and IR camera to find all of the defects and leaks, but this is an opportunity moment that is not to be passed up. Thin spots air leaks and insulation gaps were rife in 1980s construction (and sadly, not rare even today). Fixing those defects is more important to comfort and performance (both moisture & thermal) than another R3 of exterior foam to bring it fully up to code.

  3. GBA Editor
    Martin Holladay | | #3

    Nate and Dana have given you good advice. There are lots of article on the GBA site that will answer your questions.

    An article on determining exterior foam thickness: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    An article to give you guidance on installing furring strips that are strong enough to support fiber-cement siding: Fastening Furring Strips to a Foam-Sheathed Wall.

    An article that explains how to install rigid foam on the exterior side of walls: How to Install Rigid Foam Sheathing.

    How to handle windows: Installing Windows In a Foam-Sheathed Wall.

  4. sickofscreenname | | #4

    Thank you all for your input. Yes... The previous contractor did not " cap off" the blue board and the mice absolutely love tunneling through it. The homeowner has had it with this problem. I'm much more clear on this now thanks to your answers. I just read the article in Fine Homebuilding about the Roxul board but was hesitant on choosing that with the compression issues and order of application not being clear in my mind at the time. Unfortunately, I have learned over the years that in the remodeling industry, you can try to bring things to code(most of the time) but there are those few times that you just try to make it better than it was. With evolving technology today, that thought process can sometimes come back to bite you in the tush. One solved problem can lead to creating another. I realize that I have quite a bit to catch up on so I will do my "due diligence" and look at all of the info you have provided links to. Sometimes finding the right search words is more of a problem than the building issue! Ha-ha! The other issue that nags at me is the availability of products or services that are discussed in the articles. There are some suppliers that will flat-out refuse to order certain products because " no one uses that stuff" or " we have to buy whole units in order to get that"... which is a common problem in this area. As far as the blower door and IR camera... Good luck with finding that around here! Great idea but very hard to follow though with. Rest assured that I will do all that I can to give this client the right job. After all, that's my job!

  5. Expert Member
    Dana Dorsett | | #5

    You can do a lot with a $50 pistol-grip infra-red thermometer and a large reversible window fan. If the mice were boring through the sheathing it's highly likely that there will be missing cavity insulation, not just the 1.5" diameter air leak that the mouse made.

    If the sheathing is super-clean you can air seal the seams with the appropriate tapes. If it's dirty, cleaning it up a bit and using housewrap tape overlaid with 1/8" of duct mastic applied with a trowel or putty knife for redundancy will probably go the distance, especially if overlaid with fan-fold XPS or full-depth foam.

    It occurs to me that 2-3 wide strips of 3/8" fan fold at the studs could provide the necessary vent space for the siding, and you'd have an R3-R4 thermal break (at least at the studs, if not the headers and studwall plates) instead of R1.5-R2.

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