musingsheader image
Helpful? 2

Where Does the Housewrap Go?

If your walls have exterior rigid foam, does the housewrap go under the foam or over the foam?

Posted on Feb 18 2011 by Martin Holladay, GBA Advisor

Let’s say you’re building a house with plywood or OSB sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. . You plan to install 2 or 4 inches of rigid foam on the exterior of the wall sheathing, followed by vertical rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. strapping and siding. Where does the housewrap go?

Depending on who you talk to, you get two different answers:

  • It goes between the rigid foam and the vertical strapping, or
  • It goes between the sheathing and the rigid foam.

I’ve heard a variety of arguments in favor of each position. For example, I’ve heard that the housewrap belongs between the foam and the strapping, “because that way it’s easier to integrate with the flashing at the logical drainage planePath that water would take over the building envelope. Concealed drainage-plane materials, such as building paper or housewrap, are designed to shed water that penetrates the building’s cladding. Drainage planes are installed to overlap in shingle fashion (weatherlap) so that water flows downward and away from the building envelope..”

On the other hand, I’ve heard that the housewrap belongs between the sheathing and the foam, “because that way it protects the sheathing,” or “to prevent the housewrap from flapping in the wind,” or even “to protect the housewrap from extreme temperatures which might degrade the plastic.”

Both sides of this argument have merit. If you have a strong opinion favoring either position, it’s safe to say that either approach can work well, as long as the housewrap is properly integrated with all of the window flashing, door flashing, and the flashings protecting other penetrations.

Simplifying the decision

If this quandary has you discombobulated, though, here’s an easy way through the thicket:

  • If you are installing “innie” windows, your housewrap should go under the foam.
  • If you are installing “outie” windows, your housewrap should go over the foam.

If you’re unfamiliar with the innie/outie terminology, an innie window’s flanges are in the same plane as the OSB or plywood wall sheathing, while an outie window’s flanges are in the same plane as the back of the siding. (For more on this topic, see ‘Innie’ Windows or ‘Outie’ Windows?)

Integrating housewrap with “innie” windows

Innie windows are installed just like they are on a house without exterior foam. The housewrap covers the plywood or OSB sheathing, and the window is installed with your favorite flexible-flashing details. The foam is installed later, and there’s no reason to integrate the free-draining rainscreen gap with any of the other flashing details on the wall.

All you have to do is come up with exterior “jamb extensions” — including a sloped secondary sill tucked under the sill that comes with the window — to cover the edges of the foam at the window and door openings. It’s perfectly reasonable to argue that these exterior jamb extension details don’t really have to be watertight — just resistant to weathering, durable, and attractive.

Integrating housewrap with “outie” windows

Outie windows are usually installed with the window flanges in the same plane as the back of the siding. This usually requires the installation of a plywood box in each window rough opening; the box extends x inches beyond the plywood or OSB sheathing, with x = the thickness of the foam + the thickness of rainscreen strapping.

After the foam is installed but before the housewrap goes up, you need to install a “picture frame” of strapping lumber (installed flat to the foam) around each rough opening. The outer face of the strapping should be flush with the outer edge of the plywood frame. Then you install your window.

The housewrap goes up next, on top of the foam. At each window head, the housewrap is creased and extended out over the “picture frame,” and then down over the window flange or the flexible flashing at the window head. At the window sill, the flexible flashing protecting the rough sill extends over the housewrap. Then the rainscreen strapping is installed over the housewrap.

The method I’ve described is just one way to install housewrap over exterior foam. Of course, there are many variations to these installation and flashing details.

Can I just skip the housewrap?

Some builders argue that rigid foam is a perfectly good water-resistive barrier (WRB), so foam-sheathed walls don’t need any housewrap at all.

If you decided to go this route, be sure to do your research before proceeding. It’s important to note:

  • Not all brands of rigid foam have been approved for use as a WRB.
  • Rigid foam can only be used as a WRB if you follow the fastening and seam-sealing details listed in the ICC-ESThis is the International Code Council Evaluation Service. ICC-ES is a non-profit public benefit corporation that evaluates building products, issuing final reports on code compliance of building products and materials. These reports on then made available at no charge to the building community at large. report used to obtain acceptance for your brand of foam to be used as a WRB.
  • Some building experts note that over a period of years, rigid foam may shrink, raising the question of whether an installation of taped foam will remain waterproof over the long term.

For more information on this option, see Using Rigid Foam As a Water-Resistive Barrier.

Advice from the Building Science Corporation

To read more information on this question, see the Building Science Corporation’s Guide to Insulating Sheathing.

Last week’s blog: “Superinsulated House Specs.”


Tags: , , , , , , , , , , , ,

Image Credits:

  1. GreenBuildingAdvisor

1.
Sat, 02/19/2011 - 13:12

Edited Sat, 02/19/2011 - 13:12.

polyiso
by j chesnut

Helpful? 0

It's difficult for me to distinguish EPS and XPS. Both are fairly dense and appear to not sponge up liquid water. Polyiso on the other hand appears more fragile and porous.
Have you heard any concerns with locating polyiso outboard the WRB or are all the rigid foam products considered identical in this regard?


2.
Sat, 02/19/2011 - 14:42

Response to J Chesnut
by Martin Holladay, GBA Advisor

Helpful? 0

J,
If polyiso makes you nervous, there's nothing wrong with installing EPS on the outside of your wall sheathing. (I did that on my own house.)

However, I don't think your worries are justified. Commercial roofers have decades of experience using polyiso, which holds up well on roofs. If polyiso is installed on a wall under a rainscreen, it's hard to imagine how it could accumulate moisture.


3.
Sun, 02/20/2011 - 07:19

Stucco
by Allan Edwards

Helpful? 0

Martin, what are your thoughts about where the WRB should be located in an application with stucco. Wall>sheathing>foam board>lath>3 coat stucco. I know you advocate an air space between stucco and foam, but assume there isn't one.

Allan


4.
Sun, 02/20/2011 - 07:54

Response to Allan
by Martin Holladay, GBA Advisor

Helpful? 0

Allan,
I stand by the recommendation I made in my earlier blog, To Install Stucco Right, Include an Air Gap:

"If you’re installing stucco over foam, here’s how to create a ventilated rainscreen gap:
* Install 1x3 or 1x4 vertical strapping on top of the foam, screwed through the foam into the studs.
* Fasten paper-backed metal lath to the strapping, and proceed with a standard installation of three-coat stucco."

If you want further protection, there's nothing wrong with including a WRB between the foam and the vertical strapping.


5.
Sun, 02/20/2011 - 08:17

Air Gap for Stucco
by Allan Edwards

Helpful? 0

Martin, I agree with you that it is obvious that an air gap between stucco and sheathing or foam makes sense. However, I would guess the % of homes with that detail is .00001%. That doesn't make it right or wrong, but the conventional method of not using an air gap seems to be working. Almost all problems with stucco are due to poor flashing, inadequate WRB, poor mixes of the stucco, or not painting properly with elastomeric paint.

Another problem is code requires fasteners to be 6" on center, so vertical stripping would be problematic.

If I had unlimited money I would use 3/4" or even 1-1/*8 plywood sheathing, foam, vertical 1x4's (treated #1 SYP), with 1x4 fastening into plywood. I would also creat a brickledge to accommodate the sheathing, foam, stripping, and stucco.

Allan


6.
Sun, 02/20/2011 - 09:17

Response to Allan
by Martin Holladay, GBA Advisor

Helpful? 0

Allan,
In some parts of the country, it's possible that few stucco homes have wet-wall problems. But in other parts of the country, the failure rates have been very high. As I wrote in the earlier blog, "According to Ron Glubka, the chief building official in Woodbury, Minnesota, the city’s wet walls represent ‘the largest construction defect problem in local history.’ … Glubka [reports that] the building department has issued permits for wall repair work for 344 out of the 670 stucco-clad homes built in Woodbury in the 1990s — a failure rate of 51%."

After extensive interviews with building scientists and stucco experts, I concluded that these problems have multiple causes. You have listed some of the possible causes in your last post. I would also add several more, including the use of OSB, the use of interior polyethylene, high indoor humidity, and inadequate clearance between the bottom of the stucco and grade.

Every contractor needs to decide (in consultation with an insurance company) what level of risk to live with. If you want to install stucco without an air gap, it's a risk that you are apparently comfortable with. I wouldn't do it.


7.
Tue, 02/22/2011 - 02:26

Edited Tue, 02/22/2011 - 02:30.

General Response
by Sam Marsico

Helpful? 0

What about permeability? It is important to know the permeability of the foam being used, and whether or not to include an interior vapor barrier. The dewpoint within the wall becomes as important as when installing 'flash and batt', especially with foil polyiso (0 perms). To me, this is the real question to ask.
(But thanks for the discussion anyhow, I just asked J H on facebook and was referred to the following link: http://www.jameshardie.com/pdf/USTB_Increased-Thermal-Performance-with-J... )


8.
Tue, 02/22/2011 - 03:34

Edited Tue, 02/22/2011 - 03:36.

Response to Sam Marsico
by Martin Holladay, GBA Advisor

Helpful? 0

Sam,
Q. "What about permeability?"

A. The permeability of the foam or the housewrap doesn't matter much, because such walls are designed to dry to the interior, not the exterior.

Q. "It is important to know the permeability of the foam being used."

A. I disagree. When installed at a thickness of 2 to 4 inches, most rigid foams are low permeance anyway.

Q. "It is important to know whether or not to include an interior vapor barrier."

A. You are right about that. You do not want an interior vapor barrier.

Q. "The dew point within the wall becomes as important as when installing 'flash and batt', especially with foil polyiso (0 perms)."

A. No, it is not necessary to perform a dew-point calculation, whether or not you use foil-faced polyiso. All you have to do is be sure that the foam is installed with the minimum R-value required for your climate and wall thickness. To learn more, see these two articles:

Calculating the Minimum Thickness of Rigid Foam Sheathing

Are Dew-Point Calculations Really Necessary?


9.
Tue, 02/22/2011 - 05:38

Air Gaps & Stucco
by Allan Edwards

Helpful? 0

Martin, I am just not convinced an air gap is necessary when using stucco, especially as you propose (furring strips). For one thing, how do you get around the nailing pattern requirement for lath (6”OC). I really think it is an overkill solution that is expensive and adds a design detail that has to be addressed: the added thickness of the furring strips in relation to foundation, windows, and doors. With sheathing, foam, furring strips, and stucco you could be talking about 3” of material past the frame. I would not want that much hanging out past the frame, you almost need a brick ledge like detail, and of course allow for this in your design.

Now if you advocated drainage mattes, I might agree. I also would not use a city in Minnesota as my sample data, I just don’t think of northern cold climates areas as hot beds for stucco use. It would be like using Houston as a test area for basement construction, we just don’t do enough to be proficient with them. If they are having 50% failure rate, they just don’t know what they’re doing in regards to stucco and flashing. Don't get me wrong, there are a lot of bad stucco (and mostly flashing) jobs out there, I just don't think adding furring strips is the solution. Although I will admit it is a bit idiot proof.

For years my stucco contractor has talked about using a “rich mix” in regards to stucco, I found this article interesting. http://www.coastalcontractor.net/article/94.html I think this is another factor, the mix of the stucco. Also, as an extra precaution, I have always used 3 layers of WRB, and pay close attention to flashing detail. And of course I use nothing but copper flashing, I would never use galvanized. I also keep the stucco of the roof 3”-4” on rake walls, and as you mentioned keep the stucco minimum 6” above grade.

Another thing I’ve noticed is that I have never seen water or signs of water draining via the weep screeds. We build on foundations and you would think that there would be some kind of water staining after years of water weeping down thru stucco. Makes me wonder how much water really gets thru stucco.

Allan


10.
Tue, 02/22/2011 - 05:59

Edited Tue, 02/22/2011 - 06:05.

Response to Allan
by Martin Holladay, GBA Advisor

Helpful? 0

Allan,
I have never installed stucco, so I'm happy to defer to the stucco experts when it comes to attachment details. If any GBA readers want to share their preferred methods for attaching wire-backed lath to a foam-sheathed wall, ideally with an air gap behind the stucco, please share them. What's the maximum thickness of foam you have ever installed?

As I wrote before, Allan, you seem to have found a stucco installation method that works for you, at a level of risk that you feel comfortable with. I'm not trying to convince you to change your techniques.

A few more comments:

1. Integrating thick foam sheathing with stucco is difficult, especially if you want an air gap between the stucco and the foam. Anyone who insists on stucco cladding might prefer a wall construction method that doesn't require exterior foam -- for example, a double-stud wall or a wall with interior horizontal strapping to address thermal bridging.

2. It's up to you to decide: what's more important, the air gap or the foam?

3. Not every house needs stucco! Unless it's installed directly onto concrete blocks, mortared stone, or poured concrete -- the way it was originally installed in Europe -- it's a relatively risky siding.

4. You could always consider water-managed EIFS.


11.
Tue, 02/22/2011 - 07:01

More Stucco
by Allan Edwards

Helpful? 0

I don’t want to beat this to death, I think your blogs on stucco and WRB placement are very well written and good advice. It’s just that over the years I’ve heard some negative things about stucco (Atlanta in particular had a lot of problems). Since I started using stucco in the mid-late 80’s, maybe built 75-100 homes with stucco, I’ve had less callbacks, warranty, and problems with stucco than any other aspect of construction. In fact, amazingly I don’t remember any callbacks on stucco.

By the way, EIFS has a terrible reputation and not even sure it is allowed on residential.

In my climate (hot and humid), I really like the idea of foam board (even ¾”-1”) on the exterior. One difficult thing regarding double walls and the such, I feel I have to work within the confines of what local architects design and “traditional” building methods. It is very difficult to implement methods too much out of the mainstream, smaller incremental changes are a bit easier.

Allan


12.
Tue, 02/22/2011 - 10:54

Martin, If you follow the
by Sam Marsico

Helpful? 0

Martin,
If you follow the link in my first post, scroll down to the table and read 'special considerations'. JH is recommending interior vapor barriers with foam sheathing in cold climates. What is your opinion?
I see your point about not needing to do dp analysis if you install at least the minimum foam thickness for the respective climates, but I'm still cautious about relying on drying to the interior. Osmosis occurs from high to low concentrations, so wont vapor always drive toward the exterior during a cold dry winter?
About Stucco:
I recently used Benjamin Obdyke homeslicker behind a stucco installation. I installed an air barrier, then the homeslicker, and had the subcontractor install 60lb jumbo tex with cap nails, so the stapler wouldn't rip the paper as the Obdyke product compressed. My stucco sub absolutely refused to use paper backed lath, claiming it is for bottom feeders.
Thanks for the links.


13.
Tue, 02/22/2011 - 11:00

Edited Tue, 02/22/2011 - 11:03.

Response to Sam Marsico
by Martin Holladay, GBA Advisor

Helpful? 0

Sam,
You wrote, "James Hardie is recommending interior vapor barriers with foam sheathing in cold climates."

No -- read it more carefully. They are recommending vapor retarders, not vapor barriers. The best vapor retarder for these applications would be vapor-retarder paint.

Q. "Won't vapor always drive toward the exterior during a cold dry winter?"

A. Yes, that will be the direction of the vapor drive. But not much vapor can drive through rigid foam. Moreover, if the entire wall assembly is warm, what harm is there? There aren't any cold surfaces that allow condensation or moisture accumulation.

The conditions in your wall will be closer to the indoor conditions in your living room than they would be in a house without foam sheathing.


14.
Tue, 02/22/2011 - 21:04

Thanks, I didn't realize
by Sam Marsico

Helpful? 0

Thanks,
I didn't realize there was a difference.
Off topic, but today I was wondering why the snow was deeper where all the rafters were ghosting through on a comp roof. It seems to contradict what I thought I knew about thermal bridging. Do you have anything about this phenomenon in your blog?


15.
Wed, 02/23/2011 - 04:14

Edited Wed, 02/23/2011 - 04:15.

Rafter ghosting
by Martin Holladay, GBA Advisor

Helpful? 0

Sam,
The ghosting patterns on snow-covered roofs are always revealing. Different roofs show different melt patterns.

It's certainly possible for the rafters of a roof to be the most insulated part of the roof assembly. I have often seen the pattern you are describing -- a roof where each rafter shows a stripe of deep snow, but the snow between the rafters is melting.

What that means is the rafter is the best insulated part of the roof assembly. This is typically the case in an uninsulated roof over an unconditioned attic, a garage, or an outbuilding. If a little bit of heat is leaking from the building below, or if solar gain through windows has heated up the interior of an outbuilding, then snow will melt between the rafters. The rafters (being 9 inches of solid wood) have a higher R-value than the roof sheathing (which is often only 1/2 inch of plywood).


16.
Thu, 02/24/2011 - 06:02

Edited Thu, 02/24/2011 - 11:08.

A comment from Kohta Ueno
by Martin Holladay, GBA Advisor

Helpful? 0

[Kohta Ueno sent me the following comment by e-mail]

I have attached a shot of an installation of stucco over furring strips from BSC/Gauvin Construction's Coquitlam Vancouver test hut.

A 3/4" air space is a code requirement in Vancouver, so this installation of stucco is pretty much their standard solution now. They use a product called Hal-Tex Rainscreen Breatherboard.

However, I don't think that the installation here was over foam--I believe it was structural sheathing, building paper, 3/4" ventilated airspace, and BreatherBoard. Some of the folks who are closer to the experiment, like Joe Lstiburek, John Straube, or Chris Schumacher, would be able to confirm that.

But there's no physical reason why you couldn't do it over foam with furring strips; there's a boatload of compressive strength available.

Kohta

Coquitlam test hut 2.jpg


17.
Thu, 02/24/2011 - 16:33

Santa Fe Stucco
by kim shanahan

Helpful? 0

Typical frame, rigid insulation, and stucco details here in Santa Fe, which is quickly becoming virtually everything built here, is rigid installed directly on full OSB sheathing with minimal usage of spiral capped nails. Then double layer jumbo-tex, again with minimal nails, (some even use short roofing nails just to hold the paper in place on the foam) then the stucco netting with a whole lot of spiral-capped nails to hold the entire, foam,paper, wire assembly firmly in place. The theory of the double paper is that the wet, three-coat cementious stucco wrinkles the exterior layer of paper enough to create drainage channels between the two layers of paper so that moisture that may get into the stucco by wind-driven moisture and and then travels along all the spiral nails won't make it to the OSB. Interior moisture is not presumed to get to the backside of the OSB because of the use of blown-in cellulose or fiberglass with no interior vapor barriers.

Our challenge is that our "innie" windows are REALLY "innie" with no jamb extensions because we like the look of deeply recessed windows with big rounded bullnoses on all four sides. Often we install an extra 2x4 "buck" inside a 2x6 rough opening to enhance the recess. Some of our fancier neighborhoods even have covenants that require the plane of the window glass to be 4" deeper than the plane of the stucco! The idea is to mimic the look of a classic thick-walled adobe. Flashing is our greatest challenge, but we strive to get any moisture inadvertantly coming in through the window/stucco interface to exit between the jumbo tex and the stucco and then down to the weep screeds.

We recognize that we are a unique climate with unique design details and that our experience does not readily transfer to very many markets, but what I describe seems to be working very well here. We are a cold, dry climate with most of our ambient moisture occurring during our "monsoon" season of July and August.


18.
Thu, 02/24/2011 - 17:38

Response to Kim Shanahan
by Martin Holladay, GBA Advisor

Helpful? 0

Kim,
What's the thickest layer of foam that can be used with the assembly you describe?

How long are the spiral capped nails?


19.
Thu, 02/24/2011 - 18:23

thickness
by kim shanahan

Helpful? 0

2" R-10 is the thickest and most typical. Even Habitat for Humanity does it here because they get free material from Dow. The spiral nails are typically 3" , which mean penetration through the 7/16 OSB and occasionally into the 2' O.C. studs. Most good lathers will make an effort to hit the studs and then also fill the field to get the netting to lay flat. Habitat also glues their 2" rigid with a gun-grade adhesive, which I have advised them is overkill, but it keeps the unskilled volunteers busy and keeps the rigid tight to the OSB until the stucco pros show-up, which can be days later.


20.
Fri, 02/25/2011 - 08:52

Photo in Comment 16:
by Allan Edwards

Helpful? 0

With furring strips as wide as they show, how do you attach lath and satisfy the 6" OC requirement for fasteners?


21.
Fri, 02/25/2011 - 09:03

Response to Allan Edwards
by Martin Holladay, GBA Advisor

Helpful? 0

Allan,
See this technical bulletin from HAL Industries on the product in question:
http://www.halind.com/assets/pdf/RBP500_Tech_Bltn_2009-04-13.pdf

See also the attached image, which reproduces the relevant paragraph:

Rainscreen Breatherboard technical bulletin.jpg


22.
Fri, 02/25/2011 - 10:00

Edited Fri, 02/25/2011 - 10:00.

Response to Martin
by Allan Edwards

Helpful? 0

The document you referred to seems to be in conflict with the code spacing requirement as I understand it. We actually have a City inspection of lath (to verify spacing) and weep screeds before we can apply stucco. I will confirm.

It also recommends 1x2 furring strips 8: on center, I assume these run vertically. If your framing members are 16", 19.2", or 24" OC, that means the 1x2 only attach to sheathing. If your plate is 12' high, not sure I feel comfortable with that detail from a structural viewpoint.


23.
Fri, 10/21/2011 - 11:43

Edited Tue, 10/25/2011 - 20:10.

still confused
by chris steiner

Helpful? 0

So I've got my 2 layers of foam installed and have the picture frames for the openings made. I'm ready to install my typar house wrap but I'm not sure the order of events. Should I install the the peel and stick flashing on the openings and then wrap the typar over it, or should I secure the house wrap in the jambs and head and just do the peel and stick flashing on the widow sills? Also how do I even secure the house wrap over so much foam, The typar website just recommends nails with plastic washers but those are all too short. I bought the nails anyway and intend to remove all the washers and use them with 3" galvanized drywall screws but it seem like there must be a better way. I used roofing tins and screws to secure the foam like my local building supply place recommended, but they're already rusting and I could see them eventually staining the vinyl siding, so I definitely want to go a different route on the house wrap. I can't seem to find this info anywhere on the site, please help. -Chris


24.
Fri, 10/21/2011 - 14:15

Response to Chris
by Martin Holladay, GBA Advisor

Helpful? 0

Chris,
First of all, several suppliers, including Maze Nails, makes cap nails up to 6 inches long. (Maze doesn't mention them on their website, but they do make them. Call Maze at 800-435-5949.)

Secondly, there are many ways to flash a window. If you want to be a student of the issue, you can order ASTM E2112, the industry's window installation standard: "Standard Practice for Installation of Exterior Windows, Doors and Skylights." It is 89 pages long.

Or you can follow the instructions provided by your window manufacturer.

Or you can watch an 8-part video series on the topic on the GBA website.


Register for a free account and join the conversation


Get a free account and join the conversation!
Become a GBA PRO!