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Community and Q&A

Housewrap under Roxul Comfortboard IS?

user-6504396 | Posted in Green Building Techniques on

We are designing our new house in the PNW in a Marine 4 zone. The walls will be 2×6 with Rock wool batt insulation. We are planning to use plywood CDX sheathing taped or otherwise sealed as the air barrier. 2″ Comfortboard IS would go on the outside of the CDX with furring strips for a rain screen under the siding. Siding will either be clapboard or cedar shingles if I can figure out how to do the attachment efficiently to the furring strips (may have to have horizontal over vertical furring strips).

My first question is: do I need a house wrap type WRB between the plywood and the Comfortboard? Roxul insulation guide recommends it, but I don’t understand why. The vented rain screen and Comfortboad will keep the wind driven rain off of the plywood. The Comfortboard is supposed to be water resistant with a high vapor permanence to allow for drying of any liquid that would get behind the siding.

My second question is: what is the best technique for sealing around the windows and doors since one cannot tape to the Comfortboard? Perhaps this is ultimately the reason for the WRB, but then perhaps it should go on the outside of the Comfortboard.


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  1. Andrew_C | | #1

    More authoritative types will likely chime in soon, but to answer your questions, I'd suggest reading BSI-085 at the BSC website.
    Yes, you need a water resistive barrier behind the mineral wool.
    And you need to incorporate your flashing into the WRB.
    The BSC document has illustrations of this, with the correct flashing sequence when using exterior mineral wool insulation.

    I like your wall, just make sure you've thoroughly gone thru the flashing details prior to proceeding.

  2. Jon_Lawrence | | #2

    Check R703 of your state's version of IRC 2015. I would be surprised to see a state in the PNW not adopting the WRB requirement. Code aside, most people would recommend it - if for no other reason than cheap insurance. Yes, Roxul is very permeable and with a rain screen you can get up to 500 ACH which will dry the wall out very quickly. But that assumes you get a dry day to air it out. If you get months of continuous damp weather then it won't dry so well. If you are going the plywood route, Tyvex/Typar is one of the more economical WRB routes to go.

    I am currently designing a house with a similar wall assembly to your's and we are building window bucks. The windows are then set inside the buck and taped to it on both sides to seal it, vapor closed on the inside, vapor open on the outside.

  3. LucyF | | #3

    We built a very similar wall only with blown-in cellulose between the 2 x 6 studs. We had to use polyiso around the windows and doors so we could tape the flashing. I'll see if I can find a picture.

    We didn't use window bucks because I thought I would have more trouble describing how I wanted it done to the carpenters.

    We did use house wrap under the Roxul. I really didn't understand the reason either, but when you are doing everything that you can reasonably can to build a robust, long-lasting energy efficient home, it doesn't make sense to push the envelope (so to speak) by leaving out the house wrap which is relatively easy to install and not very expensive in the scheme of things.

  4. ERIC WHETZEL | | #4

    Here is a YouTube video showing a similar wall assembly by Hammer and Hand: (Madrona Passive House - Ventilated Rainscreen System)

    Hopefully the link works.

    They have another one that I found extremely helpful: Passive House Wall Assembly Mock

    It's a mock-up of their Madrona Passive House wall assembly. It's really helped us figure out all of the small details of our own wall assembly. You'll see in the videos that they take care of air sealing, waterproofing, and flashing their openings before the Roxul and furring strips go on.

    For a WRB over your plywood sheathing you could use a liquid applied membrane like Prosoco's Cat 5 (roll it on or spray it on). You could also use their Joint and Seam Filler, Fast Flash, and Air Dam products to air seal and waterproof your window and door openings, as well as the seams of your plywood.

    We were going to use their products, but now it's too cold here in Chicago (they require 40 degrees and rising during and after application). Instead, we're now going to use the Pro Clima series of tapes and their Contega HF sealant for air sealing and waterproofing our openings --- available from 475 High Performance Supply ( They also have a peel-and-stick weather barrier that you could use over your plywood. Another peel-and-stick that I see increasingly used around here is Henry's Blueskin.

    If not for the weather, we'd definitely use the Prosoco series of products --- they just seem more straightforward and less fussy to install (more difficult to screw up, in other words).

    It seems like the best tapes are either the Pro Clima products or the Siga series of tapes (you can find those at

    If you haven't decided on fasteners for attaching your furring strips through the Roxul and plywood and into your framing members, this is what we're planning to use: Fastenmaster Headlok screws (they're available in my local Home Depot, but I also found them on Amazon).

    On their website they have a document to help you figure out length of fastener that's required (we needed to submit this with our construction drawings to get our permit).

    Good luck with your project!

  5. user-6504396 | | #5

    Andrew and Jonathan, thanks for the replies. The BSI-085 article was helpful. With using mineral wool board I can now see the need to have the water control layer at the sheathing. And, in the PNW it can go for months with high levels of outdoor humidity.

    I like the ZIP system sheathing also. It just seems to me like it is easier to seal correctly than house wrap. I always see the house wrap up on unfinished buildings being ripped apart by the wind, not being sealed up correctly around openings, etc. I am leaning towards plywood as I was planning to use pressure treated CDX around the first couple feet of the wall. The house is in a floodplain and I have seen where others have used pressure treated sheathing below the windows to protect for rot. Do either of you have an opinion on liquid applied water barrier for the sheathing?

    Jonathan, I'd be interested in hearing more about the bucks you plan to use. I'm not sure I can picture how everything seals up.

  6. user-6504396 | | #6

    Lucy, thank you for the reply. Why did you choose blown-in cellulose in the wall cavity versus the mineral wool batts since you were already using the mineral wool on the exterior? Did you have to use a vapor barrier on the interior with the blown-in-cellulose? I see a lot of people cellulose and want to understand more what the benefits are.

  7. user-6504396 | | #7

    You obviously read my mind as our two responses passed in cyber space. I was obviously looking for a way to seal the CDX plywood without having to use house wrap. Your suggestions are spot on and the Hammer and Hand videos are very helpful. Hammer and Hand folks very well respected builders / craftsmen in this area. I don't know if you noticed this or not, but if you go to the Prosoco's Cat 5 web page, the building they show is a Hammer and Hand building that they also show on their website.

    Have you used the Cat-5 membrane before or do you have a feel for the installed cost of that system versus something like a Zip system?

    I will definitely look into all of your suggestions including the fasteners. I really appreciate you taking the time to share the details and options. It's perfect for the stage of planning I am at now.

  8. Jon_Lawrence | | #8


    Just think of the window buck as a bunch of inside corners, so you would seal that just as you would any other exterior inside corner. If you go with Zip for your sheathing, you can also use if for the buck. You will still have to flash the inside of the buck and you can use liquid or tape for that too.

    You don't need a buck, but I like the extra nailing surfaces for the trim and to help keep the ComfortBoard IS from squishing out around the windows.

    The other reason I am using a buck is because I will have exterior shades on my southern and western exposures and I am creating a drapery pocket inside the buck to hide the shade completely when fully retracted.

    If you google passive house window buck you will find some good examples.

  9. LucyF | | #9

    We chose cellulose because the Roxul was a Home Depot special order that took a long time to arrive. Also we did dense pack cellulose in cathedral ceilings which can be a no-no, but we had 3 inches of polyiso on the roof which was built from 2 layers of 1.5 inch sheets that were staggered so the seams did not overlap. Also it was taped on each layer. I did a lot of the taping so I know the roof was very well air sealed.

  10. user-6504396 | | #10


    Thanks. I did take some time to look at the different examples of the window bucks online and do think it makes sense especially with the mineral wool board on the outside. I've now seen a couple examples of the external shades as well. I'm going to have to look into those more. The view side of our house is facing NW at 300 degrees. We also have some windows facing 210 degrees. I'm worried about all of the solar gain that we are going to get. Sometimes it will be wanted since normally we are in heat mode in the PNW but during the summer it will occasionally overheat the house. Have you decided on the specific shades you are going to use?

  11. user-6504396 | | #11


    I am learning a ton from you, so thank you for the long post. I didn't realize Hammer and Hand had the wealth of information that they have. Based on your tips, I have now gone through a lot of their videos. I hadn't seen the installation manual, but now will be using that as well. I have prior spent a lot of time looking at the building science website and obviously Green Building Advisor. Now I also will be spending time on your website. I really appreciate the time and effort you have put into documenting your learning for others like me!

    Being in the PNW where moisture is a problem my favorite wall assembly now (if cost wasn't a factor) would be pressure treated CDX around the first 2-4 feet of the structure (elevated floor in a floodplain and earthquake zone) and under all windows, plywood sheathing everywhere else (partly because of the strength), liquid membrane, ComfortBoard IS on the exterior, and mineral wool batts inside the framing. I do though have other good choices for inside the framing. Cellulose scares me a little because of the moisture retention.

    Since the floor is elevated, we are using SIP panels for the first floor. They are strong, easy to air seal and easy to construct over beams with wider spacing. We can get them with pressure treated OSB so that they are more water resistant, although I don't know if I want that on the internal surface or not. I am thinking of using ComfortBoard IS or their commercial product, that has a poly liner on the outside, underneath the SIP panels between the beams. As I understand it, the SIP panels will transmit sound and I would like to have a fire proof layer under the floor since it is a higher risk area (fire underneath house)

    We still have to work out the ceiling and roof structure with the architect. I want pitched and higher ceilings wherever we can get them and where they look good. He now is planning on using manufactured trusses, but SIP roof panels or exterior insulation over the roof may be an option in some places. We do have a height restriction we have to stay under. If we do use trusses, I will have to do some more research on the best way to achieve the air seal. H&H may have something on their website also.

    I hadn't heard of the WUFI analysis before your post, so I will need to look into that more. I also need to start pricing a lot of these options, including windows and possibly external shades. What did you choose for your windows? Is that on your website?

    Thanks again for the help!

  12. Jon_Lawrence | | #12


    You will find that the overheating issue is actually more likely in the fall and spring when the angle of the sun is lower so the sun is radiating into the house as opposed to on the roof. You can reduce the impact with low SHGC windows, but that usually results in lower VT. I don't like dark windows, so I am specifying high SHGC windows - and shades. This gives me the added benefit of having free heat in the winter in my heating dominated climate. I have not decided on which shades I will use, but I have a couple of choices locally. One is In-Sync Solar who sources the majority of their products from Europe - where shades are the norm. I currently have Q-Motion shades (internal) on the due West facing windows of my current house and they make a huge difference. Q-Motion is now owned by Legrand who makes a lot lighting and low-voltage products.

    I am a big fan of H&H and have learned a lot from their videos and best practices manual. I had a chance to visit their shop in Portland and offices in Seattle this summer. If you are interested in a custom made PH door, they are a great option.

    I spent a lot of time working on my roof/ceiling assembly. I wanted high pitch (8/12+), but we also have height restrictions. I also wanted a conditioned attic and I don't mean just insulated knee walls. I decided on an unvented roof assembly without rafter tails. So the Zip wall sheathing is taped to the end of the Zip roof sheathing. The rafters are cut parallel to the wall sheathing, and they sit on top of the top plate and floor joists in what is best described as a reverse birdsmouth cut. Instead of hurricane ties, we are using Simpson SDWC screws that go through the top chord of the joist, top plate and into the rafter. The bottom chords of the joist receive an SDWC screw on each side of the chord into the plate of the 2nd floor wall. I found this assembly in an H&H video. I had to get the joist manufacturer and my structural engineer to sign off on it. Outbound of the Zip roof sheathing is a GAF ThermaCal nail base panel with 4" of polyiso, 1" vent channel and 5/8" OSB. So while the roof is unvented, the shingles are so if any water gets behind them it can drain/dry out.

    Now I am working on the HVAC design. The fun never stops.

  13. ERIC WHETZEL | | #13


    Glad I could help! Hope it saves you some time. It's pretty amazing how much info has been made available by "green" builders.

    If you're thinking about SIPS, you might want to check out Chris Corson's new company, Ecocor ( I think they're the first pre-fab company to receive Passive House certification. I think they even use the Pro Clima membranes and tapes that 475 sells. Not sure where they come in, in terms of cost.

    I don't know if you're going for any kind of certification, but the Pretty Good House concept may be an option if you're trying to reign in your budget (there are some great articles and posts about it here on GBA). It can get you most of the way to PH, but without some of the expenses (in particular official certification).

    Doing WUFI seemed worthwhile, although it mostly confirmed what I learned here on GBA and from the reports by Building Science Corp (but there was still a palpable sense of relief when the results were good). Have you done PHPP? That too, might be worthwhile. You could look up a certified passive house consultant that's near you and see how much they would charge. They can help you decide on R-values, number and placement of windows, air sealing details, etc.

    Our window situation is a little unique/weird. My wife's cousin works for a "smart" glass supplier, Suntuitive (, and he already had a business relationship with Unilux, so that's who we're going with. They're beautiful windows and doors, and Unilux has been very gracious to help us keep the cost within our budget, but if not for the cousin's involvement they wouldn't have been an option for us because of price.

    We're going to use the "smart" glass for all the windows on our west facade to guard against afternoon summer sun (we won't have any windows on the east side, just a front door). The basic idea is the glass remains clear until the surface temperature rises above 70 F (don't quote me on this number, but it's close), at which point it begins to tint up. As a result, you get some solar heat gain in the winter, but more importantly you keep out the heat of summer. And it comes in a range of subtle color options (blues-grays-greens) when it tints up. It's been used mostly in commercial environments, but my wife's cousin says the price is coming down substantially, so it's becoming more realistic for use in residential high-performance homes. It's pretty cool to see it work in person (we've seen it at a local car dealership, where they had a two story wall of glass).

    The other brands of windows we considered included:

    It seems like there's more PH quality windows and doors becoming available all the time.

    As Jonathan points out, the "fun" never stops --- although sometimes it can get overwhelming in terms of options ("paralysis by analysis").

  14. ERIC WHETZEL | | #14


    Hammer and Hand seem to be big fans of the Prosoco products, so I'm not surprised to hear they're on the website --- especially since they do a lot of interesting work.

    The Hammer and Hand series of videos and their Best Practices Manual (available for free on their website --- huge amount of useful info) have all been invaluable to us as we planned out our wall assembly details. Even now we go back to them when we get stuck on how best to bring elements together.

    The Cat 5 is expensive. You might be able to justify the material cost when you consider ease of application (roll on or spray). Prosoco has a cheaper product, MVP (also roll on or spray). If I remember correctly, the Cat 5 is used more in commercial applications. Prosoco has a series of helpful videos for all of their R-Guard series of products. I also had good luck with getting questions answered by their technical assistance people via email.

    I've only experimented with the Prosoco and Pro Clima products on small, mock-up wall assemblies, just for practice, to get a feel for how the products work.

    In the end, we opted to go with Zip sheathing --- less expensive, and the WRB layer is already on. Maybe because I used to do house painting (caulking, drywall patching), but I found the liquid membranes easier to work with. The temptation with the tapes is to stretch them as you're putting them in place --- a big no-no based on what I've read. It's better to just gently lay them down without tension, that way they can expand and contract with changes in the structure (if you've already stretched them out, they have nowhere to go, hence more likely to fail).

    A lot of "green" or high-performance builders seem to prefer the tapes over the liquid applied membranes. For instance, Floris Keverling Buisman from 475 did our WUFI analysis, and he strongly recommended the tapes. Although my guess is personal preference plays a part in this as well. The consensus seems to be that either tapes or liquid membranes can work, just as long as they're applied correctly. If you're planning to do the air sealing yourself, you might want to buy a little of both kinds of products and then just experiment to find out which one you find easier to work with.

    The folks at 475 are recommending a combination of plywood with a peel-and-stick membrane rather than the Zip. They argue in a blog post on their site that the Zip (being OSB) will take longer to dry out if it gets wet. They, too, have a lot of useful information on their website.

    Any number of online resources have been helpful to us, especially GBA, Building Science Corp, and Hammer and Hand. I've collected the best bits on our own blog here: (Wall Assembly)

    Maybe it can help you save some time, even if you reach different conclusions. We also have a resources page with additional links that we hope others might find helpful. The willingness of so many to share their knowledge and experience has made our build possible, so we're trying to pay that back in some small way.

    We also debated whether to use cellulose in our walls. We opted for the Roxul batts because of settling concerns with the cellulose (if you can fully trust your installer, these concerns are probably unfounded), but we also liked the fact that the Roxul won't hold on to moisture, it's very fire resistant, and it's Green Guard certified (if high indoor quality is a goal for you). Nevertheless, we're going to use cellulose in our attic as blown-in. We considered Roxul in the attic, but trying to get to R60 or higher (appropriate for high performance builds here in Chicago) gets expensive fast with the Roxul.

    I agree with Jonathan, bucks seem like a good way to go. We're using 3/4" plywood (I've even seen some builders use 2x material) for our bucks. They make for an interesting architectural detail, but if you go with "innies" in terms of window placement there are some energy benefits as well. If you go with "innies" you can also over-insulate the exterior frames for some extra R-value and a "frameless" look. The head of Zola windows has a YouTube video for Hammer and Hand that shows this detail.

    Sorry for the long post, but hopefully some of it helps.

  15. user-6504396 | | #15

    Thanks for the additional information. I looked at the your sources for the window shades and also at the EcoSmart ones that was in the "Insulating Window Shades" article on the Green Building Advisor site. Our windows are going to be only about 30 - 40 feet from a salt water bay that gets a lot of wind. So I'm leaning towards inside versus outside shades, although I don't think they do as good a job with reducing the unwanted heat gain. I think I'm going to have to break down and get someone to do the heat gain and loss calculations for each of the rooms so that I can really know what I'm dealing with. I want to go with higher heat gain windows because we are in a 5,200 heat degree day climate. We also have a bowed window set-up that is about 14 feet across that that I am going to have to figure out how to shade.

    I like the roof design, especially the drainage plane for the shingles. I'm going to have to do some more work with the architect to see exactly what he is thinking. The Simpson SDWC screws are a great idea especially when trying to create the air barrier at the wall / ceiling / roof. I found the H&H video you were talking about.

  16. user-6504396 | | #16


    We are not going for any kind of certification. I am just trying to balance comfort, low maintenance, low operating cost and initial investment. I have seen some of the Pretty Good House stuff and it makes a lot of sense to me. We are in a wet and windy location, so the air sealing and moisture management techniques of Passive House seem worth it. I do have the luxury to perhaps choose not to do some things like ERV's and opt for less costly solutions.

    The Suntuitive windows are cool. I saw something similar used in Asia, but they changed tint with electric current. Thank you for the other window supplier recommendations. That is a big cost item, so I expect that I will spend a lot of time evaluating all of the options.

    We do have a passive house trained person in our town. He is reasonably priced - one of the benefits of living in a small town. I am planning to reach out to him to do the energy analysis. I don't know if he will use the full PHPP or not. Actually I have looked before to find people to do the analysis and those folks are hard to find. So, I'm hoping the guy in this area is still doing that work.

  17. StollerB | | #17

    Kevin. Seems like you've answered most of your questions so far, but I wanted to make a suggestion. Roxul Comfortboard is not commodity pricing, meaning that it isn't the same price for a given volume regardless of the dimensions. So, you should confirm with your supplier, but I have found that for our jobs, buying two layers of their 1.25" material is FAR cheaper than one layer of 2" thick Comfortboard. Even with the extra labour of installing a double layer, the cost is very close, and you get an extra R-2 out of it. Just a suggestion!

  18. user-6504396 | | #18

    Burke, Great suggestion. I may not have caught that, especially since the Roxul distributors seem to be a little hard to find and/or carry a limited selection of inventory. The other thing I like about using 2 layers is that I can overlap the seams. Maybe it's not a big impact, but it may cut down on the wind washing since I don't plan to use house wrap on the exterior of the Roxul. Thanks for taking the time to comment.

  19. StollerB | | #19

    Since you're referencing the PNW, we are on Vancouver Island, and get out product through the major drywall and insulation supplier in the west, WINROC (they have lots of locations on the island as well as on the mainland in Vancouver, if that's close to you). The stuff does seem to be challenging for suppliers to get large quantities of right now though, so give yourself a good 2 months lead time to order before you need it!

  20. user-6504396 | | #20

    Burke, we are in Port Townsend, so not too far away as the crow flies. It looks like they have a office in Tukwila, WA close to the airport. Thanks for the tip. I'll contact them when we get closer.

  21. jberks | | #21

    I was reading this post interested in the wall assemblies discussed. I'm doing something similar with 2x4 studs with roxul batting, densglass sheathing, a WRB, 2" roxul comfortboard, vented cement board siding.

    I'm trying to pick my house wrap. I'd like to do a self adhered peel and stick for air tightness and performance. So far I'm finding products at about $1/sqft which hurts the budget relative to normal wraps.

    With your experiences so far, which peel n stick or liquid applied would you say is the easiest on the budget?


  22. Jon_R | | #22

    Blueskin VP100?

  23. jberks | | #23

    Hi Ron,

    I priced out Delta Vent SA & Henry/Bakor blueskin VP100. They both come in pretty much the same price $1 cad/sqft. Where non SA wraps around around $0.16/sqft

    I'm going to call vaproshield Tomorrow to see where I can get it in Toronto.

  24. user-6504396 | | #24


    Thanks for posting here. I have learned a lot from this thread. If you don't mind me asking, why did you decide to go with densglass sheathing? What's that price compared to cdx plywood? Does this create any shear wall issues for you?


  25. user-6504396 | | #25

    I had another thought related to the assemblies we have been discussing in this thread. As I investigated using comfortboard on the exterior, the biggest barrier other than cost was the trades here being unfamiliar with using exterior board insulation and uncomfortable with comfortboard not being more rigid. The concern was being able to get the wall coplaner.

    As I have investigated rain screen options more, it appears that only a very small passage way is needed to allow the water to drain away, 1/32" or so. So, would it be possible to use the comfortboard in between the battens for the rain screen? I am planning to use horizontal battens with shingles on the exterior so I would get most of the thermal bridging advantage. The battens could be made thicker than the comfortboard by say 1/2" or so and the rain barrier would go behind the comfortboard as is normally the case. The only part that seems like it would be painful would be cutting all of the insulation in strips to go between the battens, which in and of itself probably kills the idea.

    I'm sure there is a reason this is a bad idea, but just thought I'd throw it would for some discussion on this thread.


  26. jberks | | #26

    Hi Kevin,

    The framer has requested to use Exterior drywall for the sheathing to have a sufficient fire resistance rating. Its a row house so we're building on party walls and exterior walls on the lot line.

    He wanted to do Densglass Gold and then OSB at the front and rear faces where we won't need the fire rating. I did a calculation base on the exterior square footage, and the majority of the envelope will need Densglass, and a small minority OSB could be used (front and rear faces of the house). Doing a cost analysis I found that the increase in price of doing the entire envelope with Densglass was only an additional $175, so I opted to do the whole thing. From my local suppliers in Toronto, (in $CAD) 7/16 OSB is $16/sheet, Drywall sheathing is $24/Sheet,1/2" Plywood is $31/sheet.

    Now regarding sheer strength, to be honest I'm not sure how strong it is compared to OSB. looking at the Densglass brochure, its racking strength is >654 Lbs/Ft. Like hell if I can find online what it is for OSB Boards. Hopefuly someone on here can chime in.

    Looking at the Ontario Building code, Gypsum for sheathing is allowed as thin as 3/8" for 16oc studs, I'm doing 5/8" for the fire rating.

    I also am hoping it will help with soundproofing. The house is downtown, I get so much exterior noise from Fire trucks, Helicopters, crazy people shouting etc, I'm hoping it will help out with creating more mass and improving the STC rating. Provided my windows have a good STC rating as well (looking into getting Dissimilar glass panels instead of triple pane glass)

  27. user-6504396 | | #27


    Thanks for the reply. I live in the Pacific Northwest and everyone here uses plywood instead of OSB. I believe it is mainly due to the fact that it can get pretty wet here in the winter and things take a long time to dry out. I see Densglass getting used a lot here on commercial buildings.

    I'm surprised that you had to use the Densglass in addition to the roxul batts and comfortboard to get your fire rating. I thought the rock wool provided great fire protection.

    The Milgard folks at their main office here recommended the dissimilar glass thicknesses to me for STC rating. They said it outperformed the triple pane in that regard.


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