Combining Exterior Rigid Foam With Fluffy Insulation

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Combining Exterior Rigid Foam With Fluffy Insulation

Figuring out how thick your rigid foam layer needs to be can be tricky

Posted on Feb 26 2016 by Martin Holladay

It’s becoming increasingly common for builders to install one or more layers of rigid foam on the exterior side of wall 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. and roof sheathing. Typically, these walls and roofs also include some type of air-permeable insulation (fiberglass, cellulose, or mineral wool) between the studs or rafters.

These wall assemblies and roof assemblies perform extremely well, as long as the rigid foam is thick enough to keep the sheathing above the dew point during the winter. (Exterior foam reduces the ability of sheathing to dry to the exterior. Thin rigid foam is more dangerous than thick rigid foam because it isn't thick enough to prevent moisture accumulation in the sheathing during the winter; however, it's just thick enough to lower the rate of outward drying.)

Guidelines for determining the thickness of exterior rigid foam are discussed in several articles:

The thickness of the rigid foam varies by climate zone

To prevent moisture accumulation, cold-climate builders need to install thicker rigid foam than warm-climate builders. So 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 exterior rigid foam in these assemblies varies by climate zone.

Guidelines for roof assemblies aren’t the same as guidelines for wall assemblies. Why? Because (due to the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season.) roof sheathing is at greater risk for moisture accumulation than wall sheathing. Moreover, because of nighttime radiation cooling, roof sheathing gets colder at night than wall sheathing, increasing the chance of moisture accumulation in the roof sheathing. These facts lead to more conservative guidelines for roof assemblies than those used for wall assemblies.

What about walls with above-code levels of insulation?

If you plan to build a 2x4 or 2x6 wall, the guidelines for the minimum R-value of exterior rigid foam shown in Table R702.7.1 in the 2012 International Residential Code — guidelines that I summarized in my article, “Calculating the Minimum Thickness of Rigid Foam Sheathing” — are fairly easy to follow. (Watch out for a few traps, however, including the “R-20+5” trap in Climate Zones 6, 7, and 8. For more on the “R-20+5” trap, see The 2012 Code Encourages Risky Wall Strategies.)

Things get complicated, however, if you plan to build a wall system with above-code levels of insulation. If your framed wall will have more insulation than the R-20 assumed by code for 2x6 walls, you’ll need thicker rigid foam than the amount required by Table R702.7.1.

Similarly, if you plan to build a roof system with above-code levels of insulation, you won’t be able to depend on Section R806.5 of the 2012 IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code.. You’ll probably need thicker rigid foam.

In either of these cases, you need to figure out how much rigid foam you need to stay out of trouble. To make these calculations easier, I’ve created two tables. The tables are based on the nominal value of the insulation layers, without taking into account the effects 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. through the studs. The tables show the minimum percentage of the total R-value of the assembly that needs to come from the rigid foam layer.

First, the table for walls.

If you are planning to design a wall with above-code levels of insulation, use the percentage numbers in the last column. For example, let’s say you want to build a wall in Climate Zone 5 that will have 2x8 studs filled with fluffy insulation rated at R-26. How much rigid foam would that wall need? The table shows that 27% of the wall’s insulation needs to come from the rigid foam. That means that the remaining 73% of the wall’s insulation needs to come from the air-permeable insulation (which we know has a nominal R-value of R-26). If we divide R-26 by 0.73, we discover that the entire wall has an R-value of R-36. So the rigid foam needs to have a minimum R-value of 0.27 times R-36, or about R-10.

Next, I’ll present my table for roofs.

Again, I’ll walk through a typical example. Let’s say that you are planning to build an R-60 roof assembly in Climate Zone 6, using 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. and fiberglass. In that climate zone, you need to make sure that 51% of the total insulation comes from rigid foam. That amounts to about R-31 of rigid foam (about 8 inches of EPS). To complete the assembly, you would need to install about R-29 (about 8 inches) of fiberglass insulation under (and in direct contact with) the roof sheathing.

Thicker rigid foam is perfectly OK

It's important to emphasize that these calculations determine the minimum R-value for exterior rigid foam. Whether we're talking about walls or roofs, there is no harm in increasing the thickness of the rigid foam beyond these minimum requirements.

Remember: Thicker rigid foam makes the assembly safer, but thicker fluffy insulation makes the assembly more risky. Installing a higher R-value of rigid foam keeps the sheathing warmer during the winter, and that moves the wall assembly in the direction of lower risk. On the other hand, installing a higher R-value of air-permeable insulation makes the the sheathing cooler during the winter, moving the wall assembly in the direction of higher risk.

Martin Holladay’s previous blog: “How to Design a Wall.”

Click here to follow Martin Holladay on Twitter.

Image Credits:

  1. Fine Homebuilding

Feb 26, 2016 11:02 AM ET

Perhaps another error?
by David Foley

Thanks for an excellent article, Martin. These are helpful guidelines. I wonder if the bent aluminum at the bottom of the foam should end behind, instead of in front of, the vertical furring strips? Since that gap provides a vented rainscreen, do we really want water that gets past the siding and runs down the drainage plane to collect on the aluminum at the bottom? As drawn, the aluminum appears to be like a "gutter" in which water would pond.

Feb 26, 2016 11:21 AM ET

Edited Feb 26, 2016 11:35 AM ET.

Response to David Foley
by Martin Holladay

It certainly makes sense to provide an air inlet at the bottom of a rainscreen gap (to encourage ventilation drying). While large amounts of liquid water in a rainscreen gap are rare -- usually, a few drops of liquid water collect there, and the water evaporates quickly, whether it drains or not -- it also makes sense to allow drainage at the bottom of a rainscreen gap.

These issues are addressed in my article, All About Rainscreens. In many areas of the country, builders are using perforated J-channel to trim the bottom of their rigid foam (and to flash the bottom of their rainscreen gaps). A photo of perforated J-channel was included in my "All About Rainscreens" article; I will reproduce the photo below.

Concerning the errors in the drawing chosen to illustrate this article: I don't have a budget that allows me to commission original drawings for my weekly blogs. So I have to make do with illustrations from a variety of sources. Sometimes these illustrations are imperfect; I do the best I can, and (usually) I don't make the drawings myself.

Thanks for your comments. I appreciate this discussion, which is valuable.


Perforated J-channel.jpg

Feb 26, 2016 11:53 AM ET

by Malcolm Taylor

Another advantage of the flashing Martin included above is that it provides a level guide for the bottom of the rain screen furring. The installation is very quick.

Feb 26, 2016 12:44 PM ET

by David Foley

Excellent responses, Martin and Malcolm. The drainable J-channel is a good innovation. I have seen homes with vinyl siding and serious sheathing decay beneath due to poor detailing of J-channel, leading to water-entrapment behind the siding.

Understood about budgets and illustrations. An excellent, helpful post — thanks again!

Feb 26, 2016 7:50 PM ET

by David Martin

Zip-R is sold with one inch of foam between the studs and the sheathing and is adhered to the sheathing. In Zone 5 with 2x6 walls that would not be nearly enough foam if the foam were outside the sheathing. Is this a safe product? Would it work if the builder installed the foam inside the sheathing instead of buying the proprietary product?

Feb 26, 2016 10:29 PM ET

Edited Feb 26, 2016 10:34 PM ET.

Sheathing moisture content
by David Powers

I am currently wrapping up a home in northern Vermont (zone 6) with an exterior foam assembly similar to the detail shown. Four inches of polyiso over 2x6 wall with dense pack cellulose. I am finding the moisture content of the sheathing much higher than I expected. Generally it is 10 - 13% with some areas as high as 16%. Surface temperature of the sheathing is almost as warm as interior ambient temp.

Since the wall framing was panelized the building went up fast and stayed relatively dry. The slab did spend a lot of time in the rain. Any thoughts if these heightened moisture levels are a cause for concern or just a first year occurrence? I am thinking (and hoping) the latter.

The drainable J flashing is a fantastic idea. It could of simplified a lot of our rain screen detailing.

Feb 27, 2016 6:24 AM ET

Response to David Powers
by Martin Holladay

You report that "Generally [the sheathing moisture content] is 10 - 13% with some areas as high as 16%."

Congratulations. Your wall assembly is working well. Most wood experts don't get concerned until moisture contents rise above 20% -- and even if the MC rises above 20%, that's not necessarily a problem, as long as the sheathing can dry out later. What makes you think that 16% MC in winter is concerning?

If this is a new house, you are correct that your concrete slab is probably contributing moisture to the house. Moisture of construction can easily cause elevated indoor humidity levels for at least one year, and sometimes two years.

By the way, what type of equipment are you using to measure the MC of your sheathing? As you may know, these types of measurements are notoriously tricky and prone to error.

Feb 27, 2016 6:36 AM ET

Edited Feb 27, 2016 6:38 AM ET.

Response to David Martin (Comment #5)
by Martin Holladay

There are two versions of Zip-R sheathing: a thin version rated at R-3.6, and a thicker version rated at R-6.6.

I consider these building materials to be warm-climate materials. You could use the R-6.6 in Climate Zone 5, I suppose, but only for a 2x4 wall. Its R-value is too low for a 2x6 wall.

Even though the foam layer of Zip-R sheathing faces the interior, the same rules that I set out in this article still apply, because you don't want condensation on the interior surface of the rigid foam layer. If the interior relative humidity of the house is high enough, and the outdoor temperature is low enough, you can get that type of condensation on the interior of Zip-R sheathing -- leading to puddles on your bottom plate. It's not worth the risk.

Q. "Would it work if the builder installed the foam inside the sheathing instead of buying the proprietary product?"

A. Your question is confusing. Are you suggesting a type of wall assembly with rigid foam on the interior side of the studs? (If so, read this article: Walls With Interior Rigid Foam.) Or are you suggesting a type of wall assembly with rigid foam that is sandwiched between the studs and the exterior OSB sheathing? If the latter, you had better talk to an engineer before proceeding -- because OSB sheathing can't be depended on to brace a wall properly unless the OSB is nailed directly to the studs. If you have an alternative bracing plan (perhaps involving diagonal steel strapping or 1x4 let-in braces), that approach might work -- but check with an engineer.

Feb 27, 2016 2:09 PM ET

Response to Martin
by David Powers

Thanks Martin for your insight. I was concerned with the 16% MC because the interior RH is what I would consider dry. The house is still uninhabited and the ERV system has been running for several months keeping the interior RH below 35%. My paranoia is that it could approach 20% once the house is occupied and moisture loads increase. Most likely it will decrease over time.

I am currently working on house with a DP cellulose double stud wall and interior "smart" membrane. It will be interesting to compare the sheathing of the two.

The equipment I am using is a Delmhorst meter with custom made long probes. I was not aware of potential inaccuracy of the equipment. I have always been comfortable with the accuracy, at least for my needs. It is a great tool to find my own mistakes. Seven years ago I built a roof assembly without enough exterior foam (2x10 DP Cellulose w/ R15 exterior foam). This time of year the sheathing is well over 30% MC. I have opened the cavity up for inspection. The sheathing is definitely wet but no sign of mold or even raised grain on the sheathing. We will see if it lasts another 7 years without damage.

Feb 27, 2016 5:16 PM ET

Minor point regarding Zip-R sheathing
by Douglas Horgan

Zip-R combines a layer each of OSB and polyiso insulation.
It is designed to install foam-inward, with the OSB facing out.
The foam is between the OSB and the studs and it does have some impact on the structural properties of the OSB. Huber's engineering is embodied in the ICC-ES Evaluation Report and the instructions for the material. Among other things, you have to fully sheathe all walls (can't use the product for just corners with foam board infill, as is sometimes done with regular OSB), and use nails spaced more closely.
It would be smart to consult an engineer if you used your own foam board & OSB combo, but Huber has done the engineering for some basic installations of Zip-R.
General directions:

Feb 28, 2016 7:59 AM ET

Response to Douglas Horgan
by Martin Holladay

Thanks for providing further information on the implications of the studs / rigid foam / OSB sheathing approach.

Feb 29, 2016 1:42 PM ET

Mineral Wool Instead of Foam
by C. B.

I would like to suggest that the exterior foam discussed might be better to be mineral wool instead. The Roxul ComfortBoard IS is what I used in my house last year since not only does it provide insulating R-value, but it it is breathable and dries so there isn't any trapped moisture. Additional benefits are that it is isn't petroleum-based and doesn't have chemical flame retardants (as it isn't flammable).

I did two layers: a 1.5" layer and a 1.25" layer for a total R-value of 11.

Feb 29, 2016 2:17 PM ET

Response to C.B.
by Martin Holladay

The subject of this article is a specific type of wall -- one that includes exterior rigid foam.

GBA has many articles that discuss other types of walls, including walls with exterior mineral wool. For example, see this recent article: How to Design a Wall.

That article has a section describing walls with exterior mineral wool, and in that section I include links to three more GBA articles on the topic.

Feb 29, 2016 3:26 PM ET

Edited Feb 29, 2016 3:33 PM ET.

Response to Martin
by C. B.

Sorry, I took the article to be wall design with exterior insulation in general.

A fair number of people in the industry only think of rigid foam for exterior insulation, or say "rigid foam" but mean any sort of exterior insulation. I should've known better for an article writen by a GBA advisor. ;-)

Feb 29, 2016 3:39 PM ET

Response to C.B.
by Martin Holladay

No problem. To respond to your original comment: You're right that there are many advantages to installing mineral wool insulation on the exterior side of wall sheathing (as I pointed out in the "How to Design a Wall" article).

It's just hard to include all information about walls in every wall article... so in this article I narrowed my focus.

Mar 4, 2016 7:43 PM ET

WRB location
by Thomas Nedelsky

In a 3C zone, 2 X 6 wall, cellulose cavity insulation, 1/2 OSB or ply, 1" rigid foam exterior, my question is about the the placement of WRB. If it were a product like Henry Blue skin, could it go on the exterior face of the foam? This would be the air barrier as well.

Mar 5, 2016 6:14 AM ET

Response to Thomas Nedelsky
by Martin Holladay

Your WRB can be between the OSB and the rigid foam, or it can be between the rigid foam and the siding. The choice is yours.

Just remember that all of your wall flashing needs to be integrated with your WRB. Depending on your flashing plans and window location, you may prefer one approach over the other.

For more information on this issue, see Where Does the Housewrap Go?

Aug 25, 2016 4:44 PM ET

Preparing for exterior insulation after the fact
by Michael Brackett

I plan on installing exterior rigid insulation on our new house we are building. The only problem is I didn't learn how important this was until after we had the windows already framed, and not with window bucks. The windows are already ordered too. Is there any recommendations on preparing the window for exterior insulation after the framing is set without allowing for the 3/4" needed for the buck?

Aug 25, 2016 8:05 PM ET

Response to Michael Brackett
by Martin Holladay

There are many ways to proceed. You can install the windows as innies, or install a "picture frame" of furring strips around the window opening if your foam isn't too thick. For more information, see Installing Windows In a Foam-Sheathed Wall.

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