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Musings of an Energy Nerd

Using Rigid Foam As a Water-Resistive Barrier

Some builders of foam-sheathed homes have decided to omit the housewrap

How do I flash the windows? There is no way to tuck the top flange of a flanged window under the WRB when you’re using foam sheathing as a WRB. This illustration from the EEBA Water Management Guide shows one solution: extending Z-flashing from a horizontal sheathing seam above the window down over the window flange.
Image Credit: Building Science Corporation

Do foam-sheathed walls also need housewrap? There’s no simple answer to the question.

It is possible to use foam sheathing as a water-resistive barrier (WRB). However, those who choose this route should know:

  • Some brands of foam have been approved for use as a WRB, while others have not.
  • Even if you choose a code-approved foam, you can run afoul of your local building inspector if you don’t follow strict fastening and seam-sealing details.
  • According to some building experts, even excellent installations (using an approved foam and approved seam-sealing details) may not be durable.

The code calls for asphalt felt

By now, most builders know that wall sheathing needs to be protected with a WRB — for example, asphalt felt or housewrap. (For basic information on WRBs, see All About Water-Resistive Barriers.)

According to the International Residential Code, builders must install a layer of number 15 asphalt felt or paperbacked stucco lath over the wall sheathing or studs of every new home. The code requirement (section R703.2) includes a qualification: if you don’t want to use number 15 asphalt felt, you can use some “other approved water-resistive barrier” (WRB).

The code requirement calling for walls to be covered with number 15 asphalt felt is very odd, because every manufacturer of asphalt felt declares unequivocally that the product is intended for roofs, not walls. In spite of this curious anomaly, asphalt felt performs well when used over wall sheathing as a WRB.

Other approved water-resistive barriers

So, what does the code mean by “other approved water-resistive barriers”? Almost anything, as it turns out — as long as the product (or system) has been accepted by the International Code Council Evaluation Service (ICC-ES). If a manufacturer can present adequate evidence to the ICC-ES that a material or system meets “acceptance criteria” established by…

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  1. John Linck | | #1

    foil faced vs non foil face and cell phones
    I recently built a garage using foil faced foam. My cell phone completely dies inside. If I follow Building Science's recommendation to use double layers of foil faced foam outside sheeting will my phones and over the air TV reception die? I suspect foil faced foam holds tape much better than non-faced foam, but since a reflective surface must face an air space to add R value is the much more expensive foil faced foam worth it? Should I tape non-faced foam carefully and just use a plastic housewrap when I build homes.

    thanks, john the toymaker

  2. GBA Editor
    Martin Holladay | | #2

    Response to John Linck
    John Linck,
    I'm interested to hear whether any other GBA readers have experienced cell-phone reception problems in buildings sheathed with foil-faced polyiso.

    Polyisocyanurate insulation is available with a wide variety of facings. Commercial roofers use polyiso all the time, and they don't usually require foil facing. If you like polyiso, you don't have to buy it with foil facing.

    Concerning your last question -- "Should I tape non-faced foam carefully and just use a plastic housewrap when I build homes?" -- that is the topic of my blog. I hope I provided you enough information to make a decision.

    I believe that taping foam is worthwhile; it should help reduce air leakage. Of course, the tape may fail if the foam shrinks.

    I believe that housewrap is a more dependable WRB -- and is easier to integrate with window flashings -- than rigid foam.

  3. Bob Ellenberg | | #3

    Owens Corning and other requirements

    A reading of your article would leave any of us saying, how do we know what to do?!

    I recently had looked at using Owens Corning products and came to the conclusion that you did--not approved. I wrote them and received a reply saying that is was approved with several documents attached. I would like your take on this in light or your opinion that they have not been approved,

    Thanks for your on going information--Bob Ellenberg

    The email cover letter:
    Attached are several documents to back up the weather-resistive barrier claim. In Legacy Report 9727A there is note 4.3.1 that a water resistant barrier is not required if fomular is installed. Included is ICC-ES AC-71 which details the criteria to be a weather resistant barrrier.

    I also included a letter written for another customer by Herb Slone, the commercial Technical Manager for Foamular. If you need additional information, please contact me at 419-248-7894.

    Thank you,

    Marc Keenan
    Product Specialist
    (419) 248-7894

    What they attached was a copy of the AC71 requirements and 2 ASTM reports prepared by Architectural Testing of York, PA. One report is :
    ASTM E 331-00, Standard Test Method for Water Penetration of Exterior Windows, Curtain Walls, and Doors by Uniform Static Air Pressure Differences

    The other was:

    The letter he referenced was:
    Dear Mr. Customer:
    In follow-up to our discussion this morning, this letter is to confirm that Foamular® Extruded Polystyrene Insulation (XPS), with joints properly sealed, will serve as an air and water resistive layer in a steel stud wall assembly. Foamular XPS with joints and transitions sealed, not only serves as a continuous insulation layer, but, can also take the place of an extra layer of building wrap. Owens Corning produces Foamular XPS and can provide data verifying performance claims stated below in this regard. Owens Corning does not produce joint sealing materials. Data regarding joint seal material must be obtained from the manufacturer.
    For resistance to water penetration, Foamular 150 has been tested as required by the International Code Council (ICC) Evaluation Service, Acceptance Criteria 71 (AC71), entitled, “Acceptance Criteria for Foam Plastic Sheathing Panels Used as Weather-Resistive Barriers. The acceptance criteria defines performance for foam plastic panels, when installed on an exterior wall with joint sealing methods or treatments, that are intended to prevent water intrusion into the wall cavity. For conditions of acceptance, AC 71 states, after weathering, specimens shall not exhibit water leakage on the underside. The following test reports specified in AC71 are on file. Copies can be provided if necessary.
    • ASTM E 331-00, Standard Test Method for Water Penetration of External Walls, Doors by Uniform Static Air Pressure Difference, ASTM International. FOAMULAR RESULT: No leakage. This test method seals the test specimen into one face of a test chamber, supplying air to or exhausting air from the chamber at the rate required to maintain the test pressure difference across the specimen, while spraying water onto the outdoor face of the specimen at the required rate and observing any water penetration. (Report for Owens Corning by Architectural Testing; Report No. 01-46081.04)
    • AATCC, Test Method 127-1998, Water Resistance: Hydro Static Pressure Test, American Association of Textile Chemists and Colorists. FOAMULAR RESULT: After UV exposure and accelerated aging, all samples held the required hydrostatic head for the required 5 hours with no leakage. This test method exposes samples to ultraviolet light for specified cycles, and accelerates aging via 25 alternating cycles of oven drying and water soaking. Once cycles are completed, in accordance with Test Method 127, the specimens are held at a hydrostatic head
    of 21.6 inches (55 cm) for a period of 5 hours. (Report for Owens Corning by Architectural Testing; Report No. 01-46080.06)
    In addition to the described testing for water resistance, Owens Corning has also completed testing to determine the air permeance of Foamular XPS at various pressure differentials.
    • ASTM D 2178, Standard Test Method for Air Permeance of Building Materials. FOAMULAR RESULT: The average result for Foamular specimens tested is equal to 0.0001 L/s/m2 at 75 Pa. Samples meet the performance requirement of the Canadian National Building Code and the Massachusetts Energy Code with an air permeance lower than 0.02 L/s/m2 at 75 Pa. The purpose of this test is to determine an air permeance rate of the material at the reference pressure difference of 75 Pa. The results of this test are useful in determining suitability of Foamular as a component of an air retarder system. Please note, this data does not address installed, in system, air leakage performance of Foamular. (Report for Owens Corning by AIR-INS inc.; Report No. AI-02433-A)
    Foamular Extruded Polystyrene Insulation is manufactured to comply with ASTM C 578, Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation. Foamular 150, 250, 400, 600 and 1000, ASTM C 578 Types, X, IV, VI, VII and V respectively, differ primarily in density and compressive strength. The lowest density/compressive strength is Foamular 150. Other products are increasingly stronger and more resistant to water absorption.
    Also included with this letter is a summary of readily available tape and sealant products recommended by their manufacturer for use in sealing joints against air and moisture intrusion. Data sheets for all of the referenced products are readily available via an internet search of the product and manufacturer name.
    I hope this information is useful and informative. Thank you for specifying Foamular XPS and please contact me if you have additional questions.
    Herbert Slone
    Registered Architect
    Technical Manager
    Commercial Insulation

  4. GBA Editor
    Martin Holladay | | #4

    Response to Bob Ellenberg
    Bob Ellenberg,
    Good questions. Since it's Labor Day weekend, we'll all have to wait until Tuesday to get a response from Owens Corning.

    You're right that at one point, some Owens Corning rigid foam products were approved for use as a WRB. However, after these products were recently reformulated, the foam products were unable to pass the tests necessary under the AC71 criteria. They are now re-testing.

    When I was researching this article, I spoke with Chris Hines, the technical manager for Owens Corning Foamular. Hines told me, "Concerning AC71, we just had a new ICC-ESR for Foamular issued back in May. It did not recognize us as a WRB, while under the previous ICC-ESR report, we did get approval. It was a routine update -- a conversion of the legacy reports. Our expectation at that time was that it would carry over. The new testing for ICC was necessary because of a blowing agent conversion. So now we're undergoing new testing to have that addressed. The results were disappointing, but we made a strategic decision not to delay the release of the report."

    I will contact Owens Corning on Tuesday and ask them to respond to your questions.

  5. mapnerd | | #5

    Housewrap isn't that expensive
    I'm not sure I see any advantage to "taking the risk" of using polyiso foam as a WRB. The seams and the tape concern me. "...must be installed with details identical to those used by the lab that performed the AC71 tests complicated" - sounds complicated. I plan to stick with housewrap for now. It's not very expensive, relative to the overall wall costs.

  6. Rock | | #6

    foam sheathing
    This blog is both fantastic and frustrating, perhaps because I'm a late comer. RE:foam sheathing - I'm at a decision point, about to start new roof/roofing, and some wall rebuilds. Information I've gathered from several sources implies (regarding the roof) that what I need to do for my area (I'm an hour south of Buffalo NY in ski country, about 1800-2200 ft above sea level, on the top of a hill) is put a 1 inch layer of foam directly on the rafters to break the thermal bridge, add 5/8 osb sheathing on top of that, install 3/4 inch strapping crosswise to the rafters on that, and install metal roofing on top of that. Many of the local crews are Amish, and they want to add a layer of double bubble on top of the strapping below the metal. I'm wondering if that would prevent air drying of any moisture that gets under the metal and caught on the strapping when trying to run down. I didn't mention taping the foam seams, but would somehow address that. Underneath all of this I plan to install 4 inches of solid foam in the rafter cavities, foamed or caulked on the edges for sealing, and then nailers up on the sides of the rafters to attach plasterboard strips. The rafters will be 3X10s on 24 in centers, with 4-5 inches exposed below the plasterboard strips - the ceiling is 'cathedral'. Is this a workable plan for this region?

    RE: walls - wow, this changes monthly (at my level of background). It looks like I need 2X6s on 24 centers, 2-3 inches solid foam as sheathing, taping, special window jam boxes, vertical strapping, insect shield at the bottom, some type of strapping/webbing at the corners for shear management, a whole lot of extra care sealing windows and doors, and then I can install the clapboard siding. That takes me from the outside edge of the studs out to the great outdoors. Inside I still need to address more insulation in the cavity and make sure I don't use any vapor barrier on the inside. I will probably look at more solid foam to get the better R-value. You may have covered these questions/discussions in previous blogs, and a simple citation would direct me where I need to look.
    Again, this blog is very helpful. Rock

  7. GBA Editor
    Martin Holladay | | #7

    Response to Rock
    1. It's a good idea to provide a thermal break for your rafters, but not the way you're planning. Either (a) install continuous rigid foam on top of the roof sheathing, followed by one or two layers of stapping and metal roofing, or (b) install continuous rigid foam under your rafters. If you want to see your roof framing from inside your house, as it appears you do, it makes sense to choose option (a). It would be best to install all of your foam on top of your roof sheathing -- that way you won't have to cut any foam into thin strips and try to insert the foam between the rafters. In your climate, R-60 is a good idea.

    2. Bubble wrap is basically worthless. Anywhere you are tempted to install bubble wrap, rigid foam would be a better choice.

  8. HandMadeByRock | | #8

    Martin, Thanks for getting back so quick. OK, so I put all the foam on top. There's a practical limit thickness wise. I was content with 5 inches of exposed rafter - should I put anything above the plasterboard in that space?

  9. HandMadeByRock | | #9

    Ongoing point
    Martin, Currently, the roof has 4X4 rafters on 24 - 30 inch centers (some with rotted tails, most probably cracked based on the sagging and the drop in the ridge line) which a previous owner 'super' insulated with 2 layers of R-11 foil faced fiberglass (both vapor barriers intact). We have had tremendous ice dams with the heat loss. Someone also installed 3 dormers with incorrect flashing, so the cause of leaks is a combination. Energy wise, we heat significantly with wood (oil bill on hydronic radiator system - some baseboards remaining, but I've been replacing all with new wall hung units - runs about $100 a month) and have been able to handle the cost easily. We also keep the thermostat low and wear wool a lot. Where I'm going here is R-60 is like 10 inches or more of solid foam - how do I get close with a practical limit of 4-5 inches (plus strapping) to screw through? (and screw straight?) To put into perspective, this is a 100-200 year old Post and Beam - much of the structural wood is hand hewned - and so there are some hurdles that are insurmountable. And there are other factors, e.g., I wanted 3X12 rafters, but we couldn't find enough large diameter Scotch pine to cut and mill for 3X12s. Rock

  10. Randell | | #10

    What can I do reasonably when re-roofing a Homosote roof?
    When I next need to re-roof, what should I do to minimize heat loss and expense, with a reasonable payback (the big catch with energy remodels)? Similar question for the walls, which are mostly standard 2x6 or 2x4 (ugh) I think with foil or paper-faced fiberglass, exterior sheathing with T-111.

    I have a roof which (in two large sections of the house) consists of 4x12 rafters, 48" on center (yes, really to both), with 3.5-4" of Homosote on top, then sheathing (I think) and 1" of some sort of insulation board (built in '73 if that helps; I haven't seen the insulating board myself). Obviously, the insulation level is meager - I'd guess R-12 to maybe R18, depending on the "insulating board". The good thing is no thermal bridging, but that's about the end of the good portion. When I next need to re-roof, what should I do? If for some reason I had to tear off the Homosote, I imagine that replacing it with a SIP roof might make the most sense given the existing structure (it would drop right on I'd guess). I live in the Philadelphia area, so mostly heating (5000 degree days I think, lows in single digits), with significant cooling (lots of 90+ days and high humidity - luckily I'm on the top edge of a 330' wooded steep ridge, so we get nice breezes and it's 2-6 degrees cooler than the valleys below much of the time). Lots and lots and lots of glass and walls, so those are actually the worst of it from a heat loss perspective.

    In one section (master), it's classic rafters, with a 4x12 main beam, and two 2x10 cross-ties at each rafter in free space (cathedral ceiling). Horizontal span is ~22', so those are probably 24' 2x10's, with the rafters probably 14', and the main beam a 30' 4x12. (All rafters and the main beam continue out through the walls and support the overhangs).

    The other section (great room) it's a long, tall room with rafters installed cross-wise every 4', with 4x8's (4x10?) supporting each end, and smaller supports triangulating the rafter->column connections, of varying angles (depending on height). Starts at 8', rises to about 16', then jumps up to ~22-24' and ramps down to around 8'. A picture is worth 1000 words:

    Heating is three 2-stage modern heat pumps, one for each section (bedroom wing (2-story, 2.5 tons(?)), center (5 tons), great room (4 tons)), and in the center section (not described, very different) I run a small woodstove 12-18 hours a day in the main part of winter. Windows are very-good-for-1973 Anderson double-pane or Pella double-pane for casements, and double-pane for the floor-to-ceiling sections of the sunroom and greatroom.

    More photos:


  11. GBA Editor
    Martin Holladay | | #11

    Another response to Rock
    You are correct that there is a practical limit to foam thickness. You might want to look into nailbase or SIPs -- either product includes a layer of roof sheathing bonded to the rigid foam.

    If nailbase or SIPs don't provide all the insulation you need, go ahead with your original plan and add a little more insulation between your rafters from the inside.

  12. GBA Editor
    Martin Holladay | | #12

    Response to Randell
    Only you can decide how much insulation to add to your roof. Your decision will be based on your budget and your assumptions about future energy prices.

    If you are planning a re-roofing job, you have a once-every-30-year opportunity to get the details right. If I were you, I'd do my best to install insulation complying with minimum code requirements (R-38 in your climate zone).

    Installing rigid foam on top of your existing roof sheathing is probably the easiest way to proceed.

  13. HandMadeByRock | | #13

    Metal roof in cold climate with wood siding
    Martin, my original idea was a spin off from the Strategies, etc log number 4-01103. Am I misinterpreting that drwg - it seems to have solid foam on the rafters with sheathing on top. So I should go ahead with the original plan but increase the insulation between the rafters. I can reduce the amount of exposed rafter, or eliminate it entirely and have 10 inches to play with. I've worked with SIPs . This is two houses joined somewhere close to the hip. The 'back' house has no basement and originally was in the dirt (we're on a hill). I pulled up the flooring, pulled out the log joists on 3 ft centers, hand dug a crawl space, and hung SIPs from the foundation beams for an R-24 sub floor. Thanks for the ongoing help. Rock

  14. GBA Editor
    Martin Holladay | | #14

    Another response to Rock
    Drawing 4-01103 does not show foam above the rafters with sheathing on top.

    The drawing shows rigid foam under the rafters (between the rafters and the drywall ceiling).

  15. Joe W | | #15

    nothing's simple
    Quote Martin: “For anyone worried about foam shrinkage or the longevity of tape adhesive, there’s a simple solution: just use housewrap as your WRB.”

    Well, isn’t that just a fine howdee-do. Here I was thinking the wall I planned for a new house here in Georgia (Zone 3: Mixed Humid) wasn’t only energy-conserving but simple and cost-effective. My definition of “cost effective” includes simple to build (i.e., fewer labor steps = less room for error) and sustainability.

    I was envisioning two inches of tape-sealed shiplap foam over studs. Inside the rigid foam: Simpson shear bracing, studs with cellulose, ADA gypsum. Outside the rigid foam: Tyvek or asphalt paper, 1x3 rainscreen, fiber cement planks.

    I understand the tyvek/asphalt is still the WRB, but I like redundancy and I wanted the tape-sealed foam to “back it up” – literally, and just in case – and I wanted the tape-sealed foam to also back up the ADA drywall with an exterior air barrier.

    Even so I think I see how to supply the redundant WRB/air barrier. I could apply two sheets of foam, overlap them and tape seal both. That way shrinkage would expose only another layer of foam. Right?

    But there goes “simple” down the tubes. The extra layer doubles the time for the foam sheathing install, and doubles the number of places to make installation mistakes.

    Martin, am I right on all of this? And if I am, is there an installation technique or a product I could use instead that would restore the simplicity of my vision of a single sheathing layer?

    Thanks in advance, Joe W

  16. GBA Editor
    Martin Holladay | | #16

    Response to Joe W
    Joe W.,
    Only you can make this decision. You need to install whatever details make you sleep well at night.

    According to Dow Building Products, their rigid foams can be used as a WRB. They tell you how to do it, and it's been code-approved. If you're comfortable with these assurances from Dow, you are all set.

    If the photos of Timothy Lenahan's house make you nervous, then it's probably worth installing housewrap over the foam. I think that housewrap over foam is a belt-and-suspenders system.

    I don't think you need housewrap over taped foam over taped foam (in other words, a belt and suspenders and suspenders system). But it's your house -- so build it the way you want to.

  17. Joe W | | #17

    Thanks to Martin
    And I'll let you know if my pants fall down.

  18. Bob Ellenberg | | #18

    2" of foam on the exterior
    Joe W.

    Putting up the 1st layer of foam will not be too difficult with 1.5" staples. But the 2nd layer, the rain screen and finally the sidin could be tough. 1" of foam over studs requires 3" siding nails for code required pentration--can you find 4" siding nails? I don't think so but if you have thought through how to do this I'd be interested to learn.

  19. Rob Susz | | #19

    To Rock - Please go with more rigid on the outside
    Hi Rock,

    I know exactly where you are. I did lots of consulting in your area and worked with one contractor down there quite a bit. You are in a microclimate that is literally punishing relative to anyone just one hour away in any direction. It is your extreme snow accumulations that are causing the ski condos to have liquid water pooled against their siding all winter long. Literally 2 deacdes of "traditional methods" has caused millions of dollars of damage. All of it lurking just below the surface of those quaint, cute ski condos.

    Anyways, I would suggest moving all your foamboard to the outside. I have been doing this since 1997. On my own house and on customers homes. You asked what the practical limit was for accuracy while screwing through foam. I recently ran several hundred 9 1/2" screws through 7 1/2" of foam board and a 3/4" furring strip into conventional roof trusses. Roughly 1.2 square of roof.

    I had to hit a 1.5" wide target. Maybe 12 screws missed their mark. I was much more consistent than the framers guys, but once you get setup and snap a line, it's very easy. Even screws that miss the truss or rafter will be VERY difficult to pull out. They may "strip" in the OSB, but they still have lots of pullout strength.

    These types of roofs have been built in your area and are performing very well.

    The roof you originally proposed to do was built in a project I was involved in at Hollimont a few years ago. All work was done as a retrofit from the outside though. Lots of work, lots of gun foam, and still won't work as well as throwing down full 4x8 sheets as fast as your helpers can throw them up to you.

    I haven't registered here yet, so I don't think you can send me an e-mail, but if you ever want to contact, I'd love a reason to head down there again.


  20. GBA Editor
    Martin Holladay | | #20

    More information for Bob Ellenberg
    Bob Ellenberg,
    I just got an e-mail from Chris Hines, a technical manager at Owens Corning, responding to your questions. This is what Hines wrote:

    "Your article and the conversation with Bob Ellenberg, caught us in an unusual period between: legacy report, first ICC ESR, and revised ICC ESR. I fully expect the more up-to-date ICC ESR 1061, section 4.3, will clear up the questions regarding the OC approval as a WRB and conditions of approval. While it is acceptable to provide proof of compliance in the form of test reports directly to a building official, it is certainly easier with an ESR. The revised ESR will be published in approximately 30 days and the referenced section 4.3 contains specific language to address: board fastening, seam sealing with 3” wide construction tape, window installation, sealing of penetrations with expanding spray foam or sealant. Thank you for the opportunity to comment and respond to the question.

  21. Joe W | | #21

    Reply to Bob Ellenburg
    I can't pretend to be expert, but there seem to be several screw products available 4 1/2 inches and longer. (Rob Susz's post, immediately following your question to me, is a typical reference to products available and used for SIP installations.) In addition, Joe Lstiurbek (in an article I can't put my hands on at this second) recommends attaching fiber cement through furring strips when using foam > 1" thick.

    If I'm wrong, I hope someone will clarify for us both!

  22. GBA Editor
    Martin Holladay | | #22

    Response to Joe W
    Joe W,
    There is a certain amount of scrambling going on right now to determine fastening schedules for vertical strapping installed on the exterior side of thick wall foam. How many screws? What type of screws? Can we make the engineers happy? Industry experts are conferring as we speak.

    Real world tests (as well as engineering calculations) confirm that ordinary common-sense methods are working fine, but we may end up soon with some specific fastening schedules that we'll all have to follow.

    In the meantime, buy some long screws and fasten your strapping securely through the foam to the underlying studs. Then attach your siding to the strapping. You should be fine.

  23. Bob Ellenberg | | #23

    Siding Fasteners, Foam Shrinkage, Tapes and Window sealing
    These are both comments and questions and I'll put them in the above order.

    I focus on Hardi Smooth 4x8 panels in 5/16" thickness because that is what I am using but I think most of their specifications are similar. First or all they have lots of documents and you have to read many of them to find all of the install specifications, but in general the following apply: you can't install it over foam thicker than 1" and the penetration of the nails into the wood is about 7/8". They only list screws being approved for installation to metal studs or directly onto OSB--everything else is by nails. Nails must be 3/8" from the edge and 6" oc around the perimeter of the large sheets (different for lap siding.)

    Martin--has anyone come up with any verifiable explanations for the foam shrinkage? I saw that this house was in Ohio but was it uniform shrinkage or more on the sides of the house that received more intense sun?
    Even if we assume the new foam formulations won't shrink, is there anything to make us think the tape will last? If you read the Dow Weathermate specifications, they say it only has a shelf life of 12 months. What happens after 12 months on a shelf in a conditioned lumber supply house out of the sun? Or does it need to be applied during the first 12 months after being manufacturered to bond properly to the Dow foam? And you can't use Tyvek tape on any foam as they disclaim it's use on foam anyway. Lastly on foam, since the code requires taping of seams, why aren't taped seams part of the AC71 tests?

    In all of the discussions about window sealing with tape and how to do the WRB, I see no mention of the fact that the fins should be sealed in a bed of high quality sealant. If you aren't using cheap caulk, the tape over the fins should only be an insurance policy against water getting behind the window.

    Lastly, if you are allowing a small space for a rain screed, even if your foam shrinks and your WRB taped seams turn loose, the water is likely to dribble out the bottom. I believe that space behind the siding is your first line of defense in keeping water out of your structure.

  24. GBA Editor
    Martin Holladay | | #24

    Response to Bob Ellenberg
    Bob Ellenberg,
    1. As far as I know, the foam shrinkage on Timothy Lenahan's house occurred on all orientations. While I'm sure that it is possible for a chemist to come up with an explanation for the Styrofoam shrinkage, I'm not sure the explanation matters much. Clearly, the Stryofoam sold to the builder of Lenahan's house was not stable -- just as the polyiso sold to many roofers over the last few years was not stable. As the foam aged, it shrank.

    These anecdotes help builders make decisions about what products to buy. Regardless of the chemical reasons for the shrinkage, we either have to trust that today's manufacturers have solved the problem, or we have to assume that foam panels can shrink.

    2. Many builders share your questions about tape adhesive longevity.

    3. You asked, "Since the code requires taping of seams, why aren't taped seams part of the AC71 tests?" The answer is, they are. Every type of foam that has passed the AC71 tests has used taped seams, and that's why manufacturers require builders to tape seams the same way if they are using foam panels as a WRB.

    4. I disagree with your statement, "If you aren't using cheap caulk, the tape over the fins should only be an insurance policy against water getting behind the window." I think that vinyl windows move a lot due to thermal expansion and contraction, so I would be very reluctant to assume that caulk will stay waterproof for the life of the building.

    5. I certainly agree with your statement, "if you are allowing a small space for a rain screen, even if your foam shrinks and your WRB taped seams turn loose, the water is likely to dribble out the bottom." That's the key to keeping walls dry: a ventilated, draining rainscreen gap.

  25. Sean McClintock | | #25

    Conflicting Advice from

    Rigid foam can be the insulation, air barrier, and drainage plane all rolled into one.

    We are in the middle of construction and the design includes using taped and sealed polyiso as the WRB and the air barrier. Now you're saying it shrinks and shouldn't be used as WRB, let alone an air barrier.

    So now we need to add the expense of caulking/sealing the sheathing as the air barrier and adding wrap as the WRB?


  26. GBA Editor
    Martin Holladay | | #26

    Response to Sean McClintock
    Sean McClintock,
    1. Yes, you are right -- you will occasionally find conflicting advice on the GBA Web site. We have many regular bloggers, and occasional guest authors, and we don't always agree. However, hopefully every author marshals enough evidence, and cites enough sources, to provide you the information you need to make your own informed decision about the best way to proceed for your building project.

    2. The link you provide is to a GBA article by Gary Bergeron, who advocates using rigid foam as the insulation, air barrier, and drainage plane all rolled into one. It can be done.

    3. If my read the article on this page -- "Using Rigid Foam As a Water-Resistive Barrier" -- then you will discover that I explain exactly what you need to do if you want to use rigid foam as a WRB. It is certainly possible. The manufacturers of the foam say you can do it; they provide information on how to do it; and it is code-approved. However, as with any building technique, the decision about whether you want to do it that way rests with you.

    4. I didn't say "it shrinks." I reported on one case, that of Timothy Lenahan, who has firm evidence that the Dow Styrofoam installed on his walls in 1977 has shrunk. I also reminded readers that roofers have experienced many problems with shrinking polyiso panels. Now that I have presented this evidence, it's up to you to decide whether to trust the manufacturers, and hope they have licked the shrinking problem, or whether it's safer to install housewrap over the rigid foam.

    5. You lament the added expense of "caulking/sealing the sheathing as the air barrier." But such caulking or sealing would always have been a good idea, even if you weren't worried about foam shrinkage. You'll get a tighter house! That's good.

  27. HandMadeByRock | | #27

    Rob Susz, I would like to continue our discussion. Don't know how. To Martin and Russ, been busy and haven't checked in in a while. Working on the front house where no new deck/rafters are called for so am installing closed cell foam on top of the deck. Let me backtrack. Not removing the deck there, but adding .5 inch plywood to it to 'level' things out and installing solid foam on top. I have solved the potential shrinkage problem - I'm re-using 2'X4"X3" panels already used somewhere else and I assume shrunk the limits they will shrink. (can't beat the cost either, though will use more sealant.)
    What are the options on sealing panels together? Everyone talks about tape (but I see in reviewing the blog that tyvek tape is not 'good' for solid foam), but what about spray foam as a 'glue', latex/silicone caulk as a 'glue', etc. Chemically, you want to make sure anything in a can will not 'melt' the foam, but if 'like' substances come together, the resulting union can be beneficial. (I'll overlook Martin's comment about us chemists) Regarding screws, have in my possession now 4", 5", 6", 8", and understand I can go higher. There is a cost factor here, but what's the cost of a mistake?

  28. GBA Editor
    Martin Holladay | | #28

    Response to Rock
    1. You can use a variety of tapes to seal the seams of rigid foam. Here is my blog on tapes:
    Air-Sealing Tapes and Gaskets

    2. You can also use canned spray foam (or even caulk, as long as it is foam-compatible caulk) between the joints if you want.

    3. If you have access to a variety of screw lengths, you should choose a screw that is long enough to penetrate at least 1.5 inch into the sheathing and rafters.

    4. I have nothing against chemists.

  29. pfDY4Dnqoa | | #29

    Re: Siding Fasteners, etc by Bob Ellenberg
    Martin--What is your take on the first part of Bob's comment? Why can't/shouldn't you use foam thicker than 1" with Hardie Panels? Why can't/shouldn't you use screws rather than nails?
    Thank you.

  30. GBA Editor
    Martin Holladay | | #30

    Response to Steve Patterson
    Bob is apparently referring to this James Hardie document (installation instructions for HardiePanel):

    In my opinion, you should be able to install HardiePanel on vertical strapping, as long as the strapping is at least 7/8-inch thick -- even if the strapping is over 2 inches of foam -- but that's my opinion, not necessarily the opinion of James Hardie Co.

    As an installer or homeowner, you have to satisfy (1) yourself, and possibly (2) your building inspector, and in some cases -- but not all cases -- (3) the product manufacturer.

    It's true that most codes require all materials to be installed according to the published instructions of product manufacturers, so you should deviate from their recommendations only if you have thought through the consequences.

  31. cody_fischer | | #31

    Can we get an update to this article that a list of any EPS manufacturers that have passed the AC71 test?

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