How to Install Rigid Foam Sheathing

What you need to know to install polyisocyanurate, XPS, or EPS on the outside of your walls

Posted on Sep 30 2011 by Martin Holladay, GBA Advisor

Exterior rigid foam improves air tightness, adds R-value, reduces the chance of condensation in wall cavities, and greatly reduces thermal bridging through studs. Not only that — foam sheathing will soon be mandated by code in some climate zones. The 2012 version of the International Residential Code will require builders in cold climates to include exterior foam sheathing or another comparable measure to reduce thermal bridging through studs.

What’s the best way to install foam insulation on the outside of a wall?

Although GBAGreenBuildingAdvisor.com has published many articles and videos on the topic, we continue to receive frequent questions from readers asking how to install rigid foam 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. on exterior walls.

My conclusion: it’s time to provide a primer on the topic.

Which type of foam should I use?

There are three major types of rigid foam: expanded polystyrene (EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest.), extruded polystyrene (XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.), and polyisocyanurate. All brands of EPS and XPS sold in the U.S. include a brominated flame retardant — hexabromocyclododecane (HBCD) — that many environmentalists find worrisome.

Moreover, there is another reason that most green builders try to avoid using XPS: it is manufactured with a blowing agent with a very high global warming potential.

That leaves polyiso, which enjoys a solid reputation as the most environmentally friendly type of rigid foam insulation. If you prefer not to use rigid foam, you can use mineral wool panels instead; for more information on this option, see Installing Mineral Wool Insulation Over Exterior Wall Sheathing.

Using rigid foam on the walls of a new home

If you are building a new home, there are two basic ways to install rigid foam on the exterior of a wall: The foam can either be attached directly to the studs, or the walls can be conventionally sheathed with OSB or plywood before the foam is installed.

If you decide to omit some or all of the OSB or plywood sheathing, you’ll need to come up with a plan to brace your walls. There are at least four ways to brace a foam-sheathed wall:

Include a few sheets of plywood or OSB at critical areas like corners.
Install 1x4 let-in bracing.
Install diagonal metal strapping.
Install inset shear panels.

For more information on wall bracing, see Four Options for Shear Bracing Foam-Sheathed Walls.

If you decide to install rigid foam insulation after your walls are conventionally sheathed with OSB or plywood, you don’t have to worry about special bracing details. However, you need to be sure that your foam is thick enough to keep the OSB or plywood above the dew point during the winter. To determine the minimum R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the rigid foam, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

Fastening foam to your wall

Rigid foam is usually attached to studs or wall sheathing with cap nails, which can be purchased in a variety of lengths up to 8 inches. If you intend to install vertical 1x4 furring strips on top of the foam to create a 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. drainage gap, then you only need to install a few fasteners — just enough to hold the foam in place until the strapping is screwed to the wall.

If you are installing more than one layer of rigid foam — for example, two layers of 2-inch-thick polyiso — remember to stagger the seams of the second layer to improve airtightness and to reduce the chance of thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. . Seams between foam sheets should be sealed with caulk, canned foam, or a compatible tape.

Most foam-sheathed walls include vertical furring strips to create a rainscreen gap. Use 1x4s, which are less likely to split than 1x3s. Vertical furring strips should be installed directly over the studs, which means they will usually be 16 inches or 24 inches on center. The furring strips are attached to the studs with long screws through the foam. Some suppliers of long screws include:

If you need more information on how many screws to use, see Fastening Furring Strips to a Foam-Sheathed Wall.

More details on installing rigid foam on a new house can be found in a Fine Homebuilding article, Save Energy With Rigid-Foam Insulation.

What do I use for a WRB?

The most popular material to use as a water-resistive barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material. (WRB) for a foam-sheathed wall is plastic housewrap. However, it's also possible to use the foam itself as a WRB.

If you want to use rigid foam as a WRB, you need to understand the code implications of your decision, and you need to have a good flashing plan. To learn more about this option, see Using Rigid Foam As a Water-Resistive Barrier.

If you are using housewrap as a WRB, you have to decide where to install it. The housewrap can either be installed under or over the foam. To learn more about these two approaches, see Where Does the Housewrap Go?

What about windows?

There are several ways to install windows in a foam-sheathed wall. If your foam is relatively thin, it’s possible to nail or screw the window flanges through the foam to the rough framing. If the foam is thick, you’ll probably want to install a “picture frame” around the rough opening to provide secure nailing, or to install a cantilevered window buck (usually made out of plywood) to hold your window. If you go the window-buck route, you have two options: the windows can be installed as “innies” or “outies.”

To learn more about installing windows in a foam-sheathed wall, see Nailing Window Flanges Through Foam and ‘Innie’ Windows or ‘Outie’ Windows?

If you're installing new windows as part of a deep-energy retrofit that includes rigid foam sheathing, you might want to use “Dudley boxes.” For more information on the Dudley box approach to window installation, see Window Installation Tips for a Deep Energy Retrofit.

A great resource containing lot of specific recommendations for installing windows in foam-sheathed walls is REMOTE: A Manual from the Cold Climate Housing Research Center.

How do I flash the windows?

There are almost as many ways to flash windows in a foam-sheathed wall as there are window brands. The most important point: window flashings need to be integrated with your WRB. (If you aren’t sure whether your rigid foam or your housewrap is your WRB, that’s a sign of trouble.) Flashings should direct water to drain toward the exterior, usually to the rainscreen gap between the siding and the foam.

As a conceptual framework for your flashing plan, it's a good idea to remember this motto: “Flash the rough opening, not the window.” If the rough opening is waterproof, and if the rough sill directs rain to the exterior face of your WRB, you've done a good job.

Here are some GBA details for flashing “innie” windows:

Here are some GBA details for flashing “outie” windows:

More details showing flashing methods for both “innie” and “outie” windows can be found in a Journal of Light Construction article by Thorsten Chlupp, “Installing Exterior Insulation in Cold Climates.”

Installing rigid foam on the exterior of an existing house

If your house needs new siding, you have a rare opportunity to improve the thermal performance of your walls. Once your old siding has been removed, you can inspect the wall sheathing for rot or other problems, and these problems can be corrected. If necessary, dense-packed cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection. can be installed in your wall cavities from the exterior.

Then you can install a layer of housewrap, followed by one or two layers of rigid foam and vertical furring strips. Of course, these new materials will add thickness to your walls. If you are installing new windows at the same time, you may want to install them as “outies” to simplify water management details.

If you keep your existing windows, they will end up being “innies,” and you’ll need to spend a lot of time detailing the flashing, the exterior jamb extensions, and the new window sill required for such an approach.

Here are some articles that can guide your plans to install rigid foam on an existing home:

All of this sounds awfully complicated

The decisions outlined in this article make exterior foam sheathing sound more complicated than it really is. To get an overview of the steps involved in an easy-to-absorb way, check out these GBA videos:

There’s always more to say on this topic

This introductory article is far from exhaustive. If you have comments, corrections, or questions, please post them below.

Last week’s blog: “Air Sealing With Sprayable Caulk.”

Tags: EPS, expanded polystyrene, extruded polystyrene, foam sheathing, polyiso, polyisocyanurate, rigid foam, XPS, deep energy retrofit, Insulation, Board Insulation

Image Credits:

Rob Wotzak

1.
Fri, 09/30/2011 - 09:28

Where are the safety glasses?
by Doug McEvers

I urge all tradesmen to use safety glasses when on the job and don't forget hearing protection.

2.
Wed, 10/05/2011 - 02:31

Martin - Having done several
by David Northup

Martin - Having done several of these projects I think your summary is excellent and will be appreciated by many. I wish we had such a resource for our first job many years back...

3.
Thu, 10/06/2011 - 16:55

All rigid foam insulations provide net environmental benefit
by Walter Reiter

Thank you for the primer on installing rigid foam insulation. Great retrofit option for many structures.
EPS, XPS and Polyiso all deliver net environmental benefits when used to create the type of high performance structures needed if humans continue to demand a level of comfort from homes and buildings. If we need to condition our living and working spaces we must do so as efficiently as possible and rigid foam insulation is a great way to deliver those efficiencies in new construction or retrofit. Over 50% of our exisiting building stock will be in use as of 2050 so retrofit options are critical.
Although I recognize that the point of the article was not to make an environmental comparison between the different types of rigid foam, the environmental choice between these products and other insulation types is much more complicated than simply saying one enjoys a better reputation, warranted or not.
Distinctions can be made based upon an analysis of single attributes, but decisions as to which product to select should be based on an ana;ysis of facts and science, not reputation. All of these products deliver performance and all forms of rigid foam save energy over the useful life of the building. Another common threadbetween the different types of rigid foam is continual improvement of the environmental profile of these products. Every responsible manufactuer of resin and rigid foam continues to work hard to develop products with higher recycled content, increased end-of-life recycling opportunities, safer blowing agents and fire retardants and lower embodied energy. To report that a generic insulation type enjoys a better reputation is not helpful. That reputation certainly would not be warranted if the polyiso was manufactured with an outdated blowing agent, loaded up with tris and shipped halfway around the world compared to a high recycled content, recyclable EPS molded down the street in a factory with state-of-the-art recapture technology.
Keep up the good work.

4.
Thu, 10/06/2011 - 19:02

Response to Walter
by Martin Holladay, GBA Advisor

Walter,
Although your comments are accurate, many of your points are irrelevant to the question at hand. The fact remains that right now, polyiso is the most environmentally benign foam. That's why I recommend polyiso to anyone asking which foam to use on an above-grade wall. If manufacturers change the way they make their products, trust me -- I'll report that fact here. Until then, choose polyiso.

5.
Thu, 10/06/2011 - 23:42

Attaching a Deck?
by Dan Clark

Martin,

I'm hoping to build / have built a new home in northeastern North Carolina. Part of the plans call for a walking-width deck (approx. 5 feet wide) on both sides of the house and for a large (12' x 24') screened porch on the back of the house overlooking a creek. I can't seem to find any discussion of how to attach the deck to the house when there is a layer of foam on the outside of the house other than supporting the decks/porch on separate posts. I've come up with two ideas: (1) Build the walls of the first floor a few inches in from the outer edge of the band board then bring the exterior foam down to the flooring so that the outside of the foam aligns with the outside of the band board; or (2) build the basement concrete block walls with exterior buttresses (see the illustrations at " http://en.wikipedia.org/wiki/Buttress ") or quoins being 8" (one block) thick with the buttresses/quoins supporting the weight of the deck with bolts going through the band boards to hold everything in place horizontally but not bearing any shear weight. In either case the WRB would run down the wall to the concrete blocks.

Do you have any comments on these approaches?

Thanks for all the articles you've written over the years.

6.
Fri, 10/07/2011 - 05:31

Response to Dan Clark
by Martin Holladay, GBA Advisor

Dan,
I don't recommend attaching a deck to the rim joist of a home, even when there is no foam sheathing. It's just a bad idea. Although a variety of flashing methods have been developed to try to keep rim joists dry, many of these details are poorly executed, and rim joist rot is the usual result.

I'm strongly in favor of supporting decks with independent footings. It's not that hard.

7.
Sun, 10/09/2011 - 02:16

Deck attachments
by David Northup

Dan - Martin's link to the CCRHC provides a deck detail that I have used more than once.....

Martin is correct that most decks that fail, fail at the ledger / rim attachment. That said - with attention many decks can be just fine with being attached to the house.

Here it is quite expensive to unattached decks and unnecessary IMO....

8.
Sun, 10/09/2011 - 06:43

Edited Sun, 10/09/2011 - 06:46.

Response to David Northup
by Martin Holladay, GBA Advisor

David,
Thanks for the reminder about the deck ledger attachment detail in the CCHRC document (REMOTE: A Manual); I'm glad you pointed it out. The detail (copyright CCHCR) is reproduced below.

Deck ledger attachment detail - CCHRC.jpg

9.
Mon, 04/02/2012 - 16:33

Is polyiso between studs and plywood sheathing appropriate?
by Carolyn mayo-brown

Zip system R board has an inch of polyiso bonded to a structural OSB type sheathing with an attached green air/vapor barrier of sorts. It is not available yet in Vermont and I'm about to build. I was wondering if instead I can use an unbonded version by attaching one inch poly iso on the outside of a 2x6 stud wall, then cover with plywood on the exterior, then homeslicker +typar. This would provide the firm nailing base for cedar shingles siding needed. The popular and preferred wall option of 2x6 studs, sheathing, polyiso, then furring strips is not an option as the individual cedar shingles I'm using as siding have to match an existing pattern of curves, arches, etc and not regular horizontal rows attached to horizontal strapping. So would 2x6 wall with closed cell foam in the cavities, 1" polyiso on outside of studs, 1/2" plywood, homeslicker +typar, be the best option to attach the required individual cedar shingle siding that includes arches, curves and swirls ? If so what is the maximum thickness of the polyiso that could be used between the studs and sheathing?

10.
Mon, 04/02/2012 - 16:38

Response to Carolyn
by Martin Holladay, GBA Advisor

Carolyn,
Your plan might work, but I don't recommend it. For one thing, the OSB or plywood sheathing won't provide proper shear bracing unless it is installed directly to the studs.

You could overcome this problem by providing bracing with let-in 1x4s or steel L-profile bracing, I suppose. But you still have the problem that the polyiso will feel a little squishy when it is only supported every 16 in. or 24 in.

The standard solutions to your dilemma are to install horizontal furring strips or a second layer of OSB on top of the rigid foam.

11.
Sun, 04/08/2012 - 10:21

Polyiso between studs and sheathing
by Carolyn mayo-brown

Martin,
So instead of 2x6 studs an inch of polyiso then plywood sheathing, home slicker plus typar and then the swirly individual cedar shingle pattern matching the house a better option ( since furring strips won't work out due to the irregular siding pattern) would be
2x6 studs with closed cell foam between them, then 1/2 in OSB or Plywood , 2" polyiso, another layer of 1/2 " OSB or plywood , then home slicker+typar then cedar shingles. Correct? Any problem with moisture retention in Southern Vermont with this?

Whew! that's a lot of labor for many times
around the house installing each layer.

12.
Sun, 04/08/2012 - 15:00

Response to Carolyn Mayo-Brown
by Martin Holladay, GBA Advisor

Carolyn,
The solution you describe -- OSB sheathing, followed by rigid foam, followed by a second layer of OSB sheathing -- is one I call "site-built SIPs."

It's a surprisingly popular detail in Maine; when I visited Maine recently, I saw two builders using that method. Dan Kolbert was doing it for a deep-energy retrofit, and Wright-Ryan Construction was doing it for a new-construction job. In both cases, the reason for the detail was the same: to accommodate cedar shingle siding.

You're right: the detail is labor-intensive and expensive. You could use SIPs instead (that's what GO Logic, another Maine contractor, does), or OSB plus nailbase.

13.
Mon, 05/07/2012 - 16:32

Rigid foam across foundation also?
by Kacey Zach

Our house is 2x6 construction built in 1984 near Milwaukee, WI. The cavity is stuffed with fiberglass then expanded foam sheathing followed by cedar board and batten siding. I recently had some of the cedar off to reveal huge gaps where the foam has shrunk, I guess that explains last winter's (our first in the house) high heating bills. So I was planning on pulling the siding to put up some polyiso foam, 2 one inch layers. I was kinda thinking on tearing out the current eps to make sure there are no issues with anything else. Also, I have exposed foundation (house built on slab with radiant floor heat) which I'd like to cover. Could I cover the foundation and wall with 1 layer of XPS foam (same layer) then wall only with polyiso? Should I use an adhesive on both the foundation and stud walls? Currently there is no air gap in the board and batten siding, I was planning on creating one with 1x4's set horizontally outside the 2 layers of foam. I assume a air gap would help? How do I address some sort of insect protection? I'm not sure what is there now, it seems like nothing with some of that foam tubing stuff (forgot what it's called) here and there. Do I need some sort of flashing around the bottom edge of the polyiso?

14.
Thu, 11/08/2012 - 18:12

Rigid foam over log walls?
by Jon Ackley-Jelinek

I'm in the process of renovating a kit log cabin home, built in the early 90's. My wife and I are not a big fan of the "Lincoln Log" look, so I'm looking into covering over the exterior of the milled logs with a conventional siding. In order to boost the energy efficiency of the wall system, I'm strongly leaning towards installing rigid foam on the exterior.

Currently, I'm planning on:
1. installing the rigid foam directly over the logs (since it's a kit home and the logs are milled, the exterior surface is largely in plane and free from excessive bumps, warping and inconsistencies);
2. then installing furring strips;
3. and then the siding.

The windows/doors all need to be replace, so I'll look at installing them with nailing flanges/flashing outside of the foam.

Is this method acceptable, or do I need to first install OSB/plywood over the logs and then housewrap before applying the rigid foam?

Thanks for the great content!

15.
Fri, 11/09/2012 - 09:38

Edited Fri, 11/09/2012 - 09:40.

Response to Jon Ackley-Jelinek
by Martin Holladay, GBA Advisor

Jon,
As you probably know, the two weaknesses of most log homes are the low R-value of the walls and the fact that the walls leak a lot of air.

It's very important that you establish an exterior air barrier on your log home. You can do this with plywood, with housewrap, or with the rigid foam layer. No matter which approach you take, you have to aim for an airtight installation.

I don't know how bumpy or out-of-plumb your walls are. But if they really are both plumb and co-planar, you are lucky. In that case, you may be able to achieve an air barrier with a layer of rigid foam. I would advise that you use polyisocyanurate. Here is the controversy: rigid foam may shrink, so some builders are wary of depending on the rigid foam to be your air barrier.

In any case, the foam needs to be thick enough to keep the interior of the foam above the dew point in your climate, and the seams need to be carefully taped with a high quality compatible tape.

If you are leery of the dimensional stability of the rigid foam, you should establish your exterior air barrier with taped housewrap or taped plywood before proceeding to install the rigid foam.

Your final question is: what material will you use as your WRB? Your WRB may be the same as your air barrier, but in some cases you will have different materials for these two layers. Study up on GBA -- there are plenty of articles on these questions.

16.
Fri, 12/21/2012 - 15:55

Edited Fri, 12/21/2012 - 15:56.

Exterior Siding Rerofit / EIFS
by Nicolaas Wilkens

I am looking at the expensive proposition of replacing a well-installed and largely sound existing EIFS system on a residence in climate zone 5 (with 4 Marine attributes). This is a bit of a head scratcher in that there are several possible approaches. The EIFS is about 15 years old, looks to be a polymer based system, with 1" thick EPS and is glued directly to OSB which is installed over 2x4 insulated stud walls with interior painted drywall. I believe that a testing of the EIFS in place has yielded only a small area needing to be replaced/ fixed. The existing windows are in good shape, are aluminum clad wood windows, with insulated glazing. The EIFS is installed flush with the face frame of the window and door systems.

It is on the one hand a shame to remove a perfectly well-performing system - but it is not my choice. The home is comfortable and in removing the entire EIFS, installing new siding over a water control layer over the existing OSB, etc. (First Option) the thermal and acoustical resistance of this wall assembly would be dramatically decreased. To add to this, the removal of all of the EPS would be a mess and IMHO a wasteful proposition. Seems like a backwards step. Adding a thin layer of insulation is apparently not a good option as it reduces the ability of the wall to dry to the exterior - trapping moisture between layers.

It was suggested that keeping the polymer-based EIFS in place and simply applying a siding system would be an approach to consider. In doing so, the EIFS would be protected, the water control layer would be the polymer, and we might actually be capitalizing on an existing insulation system similar to those being promoted above for retrofit situations. The door and window conditions would become "innies" with the addition of a box frame (removal of perimeter EIFS, re-taping of window flanges, new insulation, etc.) to the existing windows.

There is another option, which would leave the existing EPS in place, removing only the outer mesh and PB plaster, allowing for a new water control layer, drain mat and siding.

Am I barking up the wrong tree here? My gut says that the absolute safest way would be to remove all EIFS, install housewrap, then come right back and install an R5 layer of rigid insulation, the new siding over rainscreen.

Any thoughts or examples on dealing with existing EIFS? A good retrofit example?

17.
Fri, 12/21/2012 - 16:35

Response to Nicolaas Wilkens
by Martin Holladay, GBA Advisor

Nicolaas,
You didn't explain why the EIFS is being replaced, but perhaps that factor is irrelevant to your questions.

I think it might be possible to leave the EIFS in place, adding vertical furring strips and new siding on top of the EIFS. There are two major challenges to this approach:

1. Defining your WRB. This would most easily be done by the addition of a new layer of housewrap on the exterior side of the EIFS, so that the location of the WRB was clear and unambiguous.

2. Making sure that every window opening, door opening, and penetration was properly flashed, and that the flashing was integrated with the new WRB. This is challenging but possible. If the window rough openings aren't carefully flashed, with the new window flashing integrated with the WRB, it's not worth taking this approach.

18.
Sun, 12/23/2012 - 17:42

Exterior Siding Retrofit / EIFS
by Nicolaas Wilkens

Thank you for your fast reply. In looking at either option 1 or 2, the existing windows would need to remain in place. Given this, and looking at Option 1 - the WRB in this scenario is to the outside of the rigid insulation plane, with Option 2 rethinking the assembly and having the WRB at the OSB plane? This would allow for that integration of the two. Thank you

19.
Mon, 12/24/2012 - 08:00

Edited Mon, 12/24/2012 - 08:02.

Response to Nicolaas Wilkens
by Martin Holladay, GBA Advisor

Nicolaas,
In my last response, I listed "two major challenges." These are not options. Both challenges must be met.

If you want to use the rigid foam sheathing as your WRB, you can. However, you still need to address the challenge of flashing each window opening so that the rough window opening has a sill pan under the window. This sill pan (or site-built sill flashing) must direct water to the WRB -- that is, the exterior of your rigid foam. The head of the window and the jambs of the rough opening must also be flashed in a way that is intergrated with your sill pan flashing and your choice of WRB.

Whether or not you can achieve these goals depends on the existing flashing details and your willingness to do a thorough job. In many houses, achieving this goal will require the removal of the windows.

You might want to read this article: Using Rigid Foam As a Water-Resistive Barrier.

20.
Fri, 01/04/2013 - 20:36

Response to Martin Holloday
by Nicolaas Wilkens

Martin - Many thanks for the great insights and follow-up.

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How to Install Rigid Foam Sheathing