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Community and Q&A

Wall Construction With Exterior Rigid Foam Questions

Jaccen | Posted in General Questions on

Long time reader, first time poster. I thank all those involved with GBA for the resources they provide.

-SW Ontario (Zone 5/6 border so we’ll go with 6)
-2 storey house (approx. 2500sqft total, even split per floor)
-standard 2″X6″, 16″ OC frame construction
-Zip sheathing planned

Construction Methodology:
-10″ thick basement walls
-2″X4″ sill plate
-4″ of surplus polyurethane rigid foam insulation against the Zip sheathing (I understand polyu to have a similar chemical formula & insulating properties to polyiso)
-2″ of surplus XPS rigid foam insulation on the exterior of the polyu
-1″X4″ vertical furring strips (12″ GRK screws)
-insulated vinyl siding
-perforated metal J-channel approx. 7″ deep to help keep out bugs, support foam, support furring strips, etc. at the bottom
-flashing between exterior of Zip sheathing and top of basement concrete wall
-Membrain or equivalent interior wrap( ie. not “regular” poly)

1 – Are there any foreseen issues with that construction methodology? I’m using surplus polyu simply because it’s quite cheap in my area ($0.33CDN sqft for a 4″ or 6″ thick piece 6′ to 8′ tall).

2 – Would the 2″ XPS be necessary? At 6″ thick, I calculate the absolute worst case scenario at an R-value of 12 (6″X2R per inch) based on the GBA article on sheathing, rigid exterior foam, and the dew point. Since 11.25 is required, I’d just squeak by (notwithstanding the most recent article on less being passable). I’m more concerned with the polyu becoming waterlogged/severely underperforming in the cold.

3 – Is the 2″ XPS recommended? I have a large amount of surplus XPS. Placing the 2″ outside the polyu is do-able from my perspective as I’m doing the installation myself. I prefer, unless told otherwise, having the XPS as it will help to serve as a WRB (*only to the polyu*) when tapped . Ie. the XPS will help keep the polyu from taking on water. Are there any technical reasons NOT to put up the XPS on top of the polyu?

4 – Would the J-channel need to be elevated slightly above the top of the foundation walls? Ie. 1/8″ or so? The window boxes will be flashed to the exterior XPS and the XPS itself will have its seams sealed with tape. I doubt there will be much water able to get into the polyu. I also feel that rainsoaked polyu would be able to drain even if the J-channel was resting on the concrete.

5 – Are longer GRK screws required or recommended? At a 10degree inclination, I’m calc’ing over 1.5″ of penetration into the 2″x6″ (plus, the 1/2″ of the Zip) and most of the documentation I’ve read recommends at least 1.5″ penetration for vinyl siding, 16″ OC. Any “real world” experience to back that up out there? Since I’m over 4″ of foam I’m guessing that I’ll need to consult an engineer.

6 – Any preference between GRK and Headlok screws using 1″x4″ furring strips?

7 – Any problems using polyu as the rim joist insulation? I’ll be using the “peanut brittle” or “tight fit & caulk” method and consulting my building official over what he wants in regards to fire code. I’ll likely have enough XPS scraps left over that I can use that if it’s a far superior choice.

Insulated vinyl siding was only chosen for aesthetics (my wife hates the way some vinyl goes “wavy” when installed; the budget cannot afford brick or stone veneer and I, surprisingly, have found quotes for Hardy board higher than expected in my area). I’m not counting it to actually insulate anything.

Thanks in advance for any discussion.


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  1. User avatar GBA Editor
    Martin Holladay | | #1

    Why install insulated vinyl siding over a vented rainscreen? The R-value of the siding is wasted.

    Perhaps you just like the stiffness of the product. That may be a reason to justify its use here.

    Note, however, that many manufacturers of vinyl siding don't permit their product to be installed on furring strips. For more information, see Can Vinyl Siding be Applied Over Furring Strips?

  2. Jaccen | | #2

    Agreed on the wasted R-value. My wife just hates "wavy" vinyl and that vinyl type provides the cheapest "stiff" vinyl. Randy Ross' replies and success with vinyl on furring strips in that article were what caused me to consider it.

    "we build mostly ICF homes 30 - 40% have viynl siding
    vert straping 16" oc 12 years + and have seen no problems so far
    personaly i wouldnt use viynl on a dog house but customors choice"

    "never had a problem with wavy siding as long as the plane of the straping is true
    the siding should remain nice and strait the 1/2 " bend on the butt should hold its shape
    as for straping i rip 5/8 or 3/4 whatevers on sale presure treated plywood into 3 inch strips
    i have found plain staping cups and warps and throws out the wall
    (word of caution dont use iron or galvinized screews with pt lumber)
    stainless steal is the best fastener or ceramic coated"

    As the manufacturers will never seem to give a straight answer, I've decided that we'll just install it and take our chances. Most warranties are only for the product themselves, not the installation anyways.

    Thanks for the prompt reply.

    Anyone out there built a house with cheap, surplus polyu rigid insulation with a story to tell? :)

  3. Brendan Albano | | #3

    "2 - Would the 2" XPS be necessary? At 6" thick, I calculate the absolute worst case scenario at an R-value of 12 (6"X2R per inch) based on the GBA article on sheathing, rigid exterior foam, and the dew point. Since 11.25 is required, I'd just squeak by (notwithstanding the most recent article on less being passable). I'm more concerned with the polyu becoming waterlogged/severely underperforming in the cold."

    Why are you calculating your insulation at R-2/inch? If the surplus polyurethane boards have similar performance to polyiso, your 4" of polyurethane ought to be more than adequate to keep your sheathing warm and free of condensation issues. You might prefer to have 6" of foam for even more R-value, but 4" seems like it should be plenty from a condensation management perspective.

    On a related note, with that much exterior foam, you probably don't need the membrain vapor retarder on the inside (unless its a code requirement).

  4. Walter Ahlgrim | | #4

    You may want to put the foam on in 2 layers with staggered seems so you have some insulation everywhere in one layer you have no insulation over the joints.


  5. Jaccen | | #5

    Hi Brandon,

    I'm basing that R2 rating on Dana's comments here:

    It's a worst case scenario for the Rvalue based upon expected winter conditions here in SW Ontario. "Plan for the worst and be happy when things are better" type mindset. My hope is that the 2" of XPS insulate the polyu enough that its Rvalue is higher. I would go with all surplus XPS, but it costs significantly more in my area. I'm fairly set on using XPS under slab and on the basement walls as I don't like the idea of polyu subslab or, really, subgrade. I'll spend the extra for XPS there, but what with the real estate explosion up here forcing contractor rates up, I'm trying to save a few bucks where I can.
    The Membrain is in anticipation of what the building inspector will want. Poly is fairly common on new builds in my experience and many people seem to expect it. I'd rather dig in my heels on some other battlefront.

    Hi Walt,

    Excellent suggestion. Thanks for the link. I plan to ship lap the polyu edges with a router setup, but layering should also help.

  6. User avatar GBA Editor
    Martin Holladay | | #6

    In spite of Dana's comment, I stand by the recommendation in one of my articles (Cold-Weather Performance of Polyisocyanurate). In that article, I quoted John Straube, who said, "Stick with polyiso and just make it thicker. If we do that, let’s call polyiso R-5 per inch."

    Dow, the manufacturer of Thermax, has released a document that points out that the problem with the cold-weather performance of polyiso affects mostly roofing polyiso rather than wall polyiso, since these products are manufactured differently. We don't yet know whether this is true, or whether the statement can be generalized to other manufacturers of wall polyiso. Unfortunately, polyiso manufacturers are keeping their cards close to their chest.

  7. User avatar
    Dana Dorsett | | #7

    Though the polymers are chemically similar, rigid polyurethane does NOT have the temperature derating issues found with polyisocyanurate (which has been a selling point for that product in some markets.)

    In SW Ontario the winter temperature averages are nowhere near cold enough for a -15C mean temp through the foam layer where performance of 2lb roofing iso might drop to R2/inch. It might not even get there even the your historical all-time record low temperature in your location.

    Outdoor temps hit -15C quite often, but that's the cold side temperature of the foam, not the mean temp through the foam. If the warm side if +5C (warm enough be at or above typical indoor dew point), the cold side temp would have to be -35C for the mean temperature through the foam layer to hit -15C. Even if the warm side of the foam hit 0C it would require the cold side to be -30C for the mean temp through the foam to hit -15Cm which may or may not happen in your area more than once in 25 years.

    The larger the R-fraction the exterior foam is, the higher the mean temp through the foam. So if the ONLY insulation is the foam, with a 20C interior it would take an outdoor temperature of -50C (-90F) for the mean temp through the foam to be -15C. That probably hasn't occurred in your location since the last ice age.

    With used XPS, derate it to the performance of EPS, which would be about R4.5/inch when the mean temp through the foam is +5C, about R4.7/inch when the mean temp through the foam is -5C.

    In most locations on the zone 5/6 border the mean OUTOOR temperature for the coldest 5 weeks of winter is only -5C to -7C, not colder, and with a 20C interior the mean temp through the foam would average more than 10C, and would be near the PEAK performance on the derating curves, at about R6.5/inch even for 2lb roofing polyiso.

    If you're adding 6" of exterior foam, there's no point to using ZIP, or ZIP-R( which is more expensive per R than rigid foam ,new or reclaimed), since you would almost certainly be installing the windows "outie", with the WRB on the exterior side of the foam.

  8. Dave B | | #8

    I'm doing a similar build this spring in Ontario East of Toronto. I'm using 2 layers of used 3" Polyiso, no zip, regular plywood. because I plan Outie windows, I also dropped the stud size to 2 x 4,as 2x6 seemed overkill in R-value with the 6" exterior.. these 2 things should save some good amount of $$$
    I bought GRX screws for a garage I'm building (3" exterior polyiso) they worked out the cheapest, although if you watch Amazon sometimes you can get good deals on others.
    Also I wouldn't bother with the insulated siding, it's expensive and limits your choices. (father in-law is in the business and at his cost it wasn't worth it)

  9. Jaccen | | #9

    Hi Martin,

    Reading your and John's reply of keeping construction simple and just making the foam thicker seems to make sense from a constructability and economic sense. 1 type of foam, one construction method. 2 staggered layers of 3" polyu seems to be the "sweet spot" in my situation.

    Hi Dana,

    Thanks for the calcs. You are correct in that we will have outie windows. I believe it's enough fun getting them lined up in a 2D plane, let alone making it a "3D" job. We plan on using plywood boxes and flashing them to the exterior foam.

    I'm curious as to your statement "there's no point in using ZIP." I had planned on using it as it seemed a relatively easy way to create the WRB and air barrier underneath the foam (ie. similar to PERSIST or REMOTE, but I would have a blown cellulose attic with the air barrier transitioning to the interior drywall). The window plywood boxes would have been flashed to the rigid foam, but it would have been taped (thus, acting as a "secondary" WRB). In this situation, what would the advised alternative be? I can see the following options:

    1. Taped/caulked OSB (air barrier) with taped/caulked polyu (WRB)
    2. Taped/caulked OSB (air barrier), Tyvek on OSB (WRB), and taped/caulked polyu (secondary WRB) --> would "soggy diaper" be a problem with polyu here?
    3. Taped Tyvek on the exterior of the polyu (WRB) and drywall done as the air barrier
    4. Taped/caulked polyu (WRB) and drywall done as the air barrier
    5. Taped Tyvek on the exterior of the polyu (WRB) and taped OSB done as the air barrier

    I'm slightly leery of using the polyu as a WRB or air barrier. I have little faith in the tape/caulk holding out for a long time. I've also read the article where OSB isn't necessarily a great air barrier when taped so I'm sceptical of it being the sole air barrier. I'm always interested in first-hand experience, though.

    Thanks again for all the response.

  10. Jaccen | | #10

    Hi Dave B,

    Thanks very much for the experience. I agree that the 2"x6" cost is unnecessary; however, I feel that it will pass the sniff test of the building officials easier if included. While an additional cost, I take solace in the fact that the house will be structurally more sound.

    I agree completely on the vinyl insulation. Now if I can just convince my better half of that.........

    I'd be interested to "compare notes" with you as we're both in the same relative area. My email is my user name here at gmail. Please feel free to drop me an email if you like.

    Thanks again for all the responses.

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