Walls With Interior Rigid Foam

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Walls With Interior Rigid Foam

When remodeling an older home, it sometimes makes sense to install rigid foam on the interior side of exterior walls

Posted on Mar 27 2015 by Martin Holladay

There are two main ways of reducing 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 studs: you can build a double-stud wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation., or you can install a continuous layer of rigid insulation on one side of the wall.

Most builders who install a continuous layer of rigid insulation use rigid foam (polyisocyanurate, expanded polystyrene, or extruded polystyrene); a small minority of builders use semi-rigid panels of mineral wool.

Builders who install rigid foam on the walls of a new building usually install the foam on the exterior side of the wall. There are several reasons for this:

  • The exterior side of the wall has fewer interruptions (for example, electrical boxes and partition intersections), so it’s much easier to install an uninterrupted layer of insulation on the exterior than the interior.
  • Exterior rigid foam is a good way to insulate rim joists.
  • Exterior rigid foam helps keep 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. warm and dry.

Unlike new home builders, remodelers can’t start with a blank slate. If a remodeler wants to reduce thermal bridging through studs, the exterior foam option is usually prohibitively expensive — especially if the siding is in good shape. So many remodelers ask, “Why can’t I put the rigid foam on the interior side of the wall?”

The answer is, you can.

Back to the 1980s…

In the 1980s, most cold-climate builders were taught that walls needed an interior vapor retarder, and needed to be able to dry to the exterior. When a few pioneers started installing exterior rigid foam, some people thought the practice was risky.

“Rigid foam is a vapor retarder, so it belongs on the interior,” some old-time builders opined. “Exterior rigid foam is risky, unless you live in Florida. It’s a wrong-side vapor barrier.”

Eventually, researchers proved that these builders were wrong. (To learn more about the history of vapor barrier requirements and the origin of unjustified concerns about wintertime vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. , see “Do I Need a Vapor Retarder?”)

Exterior rigid foam works very well indeed, as long as the foam is thick enough to keep the interior face of the foam (or the wall sheathing) above the dew point during the winter. (For more information on this topic, see “Calculating the Minimum Thickness of Rigid Foam Sheathing.”)

In fact, research has shown that the type of wood-framed wall with the driest sheathing is a wall with an adequately thick layer of continuous exterior insulation.

Once builders understood the building science behind exterior foam, they readily adopted the practice — especially since exterior foam does such a good job of insulating rim joists and partition intersections. Moreover, installing exterior rigid foam is much easier and faster than installing interior rigid foam, because there are far fewer electrical boxes to work around.

These days, exterior rigid foam is much more common than interior rigid foam.

Do walls need to be able to dry to the interior?

Let’s say that you are a remodeler who wants to install interior rigid foam. The first question that you need to resolve is a building science question: Will there be any problems if your walls can’t dry to the interior?

The answer is no — as long as your walls can dry to the exterior. So if you’re planning to install interior rigid foam, you shouldn’t install any exterior rigid foam. Most other types of sheathing — including plywood, OSB, fiberboard, DensGlas Gold, and boards — are all vapor-permeable enough to work well on this type of wall.

Most wet-wall problems are caused by moisture that enters the wall from the exterior, not the interior. (In other words, the big problem is rain, not wintertime condensation — especially if the wall was built with attention to airtightness.) So walls that dry to the exterior work well, as long as the builder has installed adequate flashing. A ventilated rainscreen gap between the sheathing or WRB and the back of the siding isn’t mandatory, but it’s an excellent detail to encourage walls to dry to the exterior.

What about inward solar vapor drive during the summer? Interior rigid foam prevents any problems from inward solar vapor drive, because the exterior face of the rigid foam never gets cold enough to permit condensation. Even if the homeowners like to crank up their air conditioner until the indoor temperature is down to 66°F, the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the foam keeps the exterior face of the foam warm, so that the foam panel doesn't present a condensing surface to any moisture lurking between the studs.

What type of rigid foam is best?

Polyisocyanurate is the most environmentally friendly foam; it is manufactured with a benign blowing agent and it doesn’t include worrisome flame retardants. Its only drawback is that it doesn’t perform well at low temperatures — but this problem is irrelevant when the foam in installed on the interior side of a wall.

If you don’t want to use polyiso, choose expanded polystyrene (EPS), which is also manufactured with a benign blowing agent. The main environmental drawback to EPS is the fact that it includes worrisome flame retardants.

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.) is usually avoided by green builders, because XPS contains both worrisome flame retardants and blowing agents with a high global warming potential.

What do rigid foam manufacturers say?

Most rigid foam manufacturers are silent on the question of whether it’s a good idea to install their products on the interior side of walls. However, at least two manufacturers endorse the method.

The manufacturer of Iso R Plus asserts that their graphite-infused EPS panels can be used as “interior insulation for walls and ceilings.” This product is available in three thicknesses: 1/2 inch, 3/4 inch, and 1 inch.

One manufacturer of polyisocyanurate sheathing, Atlas Roofing, asserts that their polyiso can be “used to insulate the interior side of metal and wood frame walls on retrofits, upgrades and renovations. In addition, many new construction projects using poured concrete or CMUConcrete masonry unit. Precast concrete block used to build walls. CMUs have hollow cores that can be filled with concrete onsite for additional reinforcement. The use of stronger, more lightweight types of concrete such as autoclaved aerated concrete (AAC) is becoming increasingly popular in CMU manufacture. block walls use polyiso board fastened or glued directly to the masonry wall.”

Is horizontal strapping necessary?

Many builders have installed gypsum drywall directly over rigid foam that is 1 or 2 inches thick. The drywall is secured with long screws that extend through the foam to the studs.

This method works, although it may result in more screw pops (small defects in the drywall compound that need to be repaired) than drywall that is attached directly to studs or strapping. Moreover, many drywall contractors hate installing drywall over rigid foam. The foam can be a little bit squishy — not enough to cause major problems, but enough to make contractors grumpy — and it’s harder to hit the studs when using 3 1/2-inch screws than when using 1 5/8-inch screws.

If you want to keep your drywall contractor happy, you may want to install horizontal 1x3 or 1x4 strapping (usually installed 16 inches on center) on the interior side of your rigid foam. This makes your wall thicker, of course, so you’ll lose some interior area. However, if you specified foil-faced foam, the 3/4-inch-deep air space will give you a small R-value boost (about R-2) — and that fact softens the blow of the lost interior area.

Electrical boxes

Horizontal strapping has another advantage: it can be used to help secure electrical boxes or wall cabinets.

If you are adding rigid foam as part of a remodeling project, you’ll need to make sure that the electrical boxes are installed at the correct depth. You have three choices:

  • Install new boxes (ideally, special airtight boxes) at the required depth (attaching the boxes to specially installed blocking if necessary),
  • Install electrical box extenders (sometimes called “goof rings” or “add-a-depth rings”) to the existing boxes (see Image #3, below), or
  • Install adjustable-depth electrical boxes.

How thick should the foam be?

Although some builders install interior rigid foam that is only 1/2 inch or 3/4 inch thick, these thin foam products have such a low R-value that they are hardly worth installing. (Most types of rigid foam have an R-value ranging from R-3.5 per inch to R-6 per inch, so 1/2 inch of foam has an R-value somewhere between R-1.75 and R-3.)

Most builders who install interior rigid foam choose foam that is 1", 1½", or 2" thick.

Pay attention to airtightness

Rigid foam is usually attached to studs with cap nails.

Whatever type of insulation you choose to install, it’s important to pay attention to airtightness. Since this basic principle applies to all insulation materials, it obviously applies to the installation of interior rigid foam. When installing the foam, it makes sense to use caulk or foam-compatible adhesive at the perimeter of each sheet of foam, and to seal foam seams with a high-quality tape.

Most housewrap tapes do a good job of sealing the seams of foil-faced polyiso. To seal EPS or XPS seams, use Siga Sicrall or Siga Wigluv tape; these tapes are available from 475 High Performance Building Supply.

Remember to address airtightness at electrical boxes. If you haven't invested in airtight electrical boxes, do your best to caulk the holes at the back of the box where the cable enters the box, and seal the crack between the rigid foam and the exterior of the box.

Are there any other drawbacks to interior rigid foam?

Interior rigid foam has a few more drawbacks in addition to those already mentioned:

  • Interior rigid foam complicates the installation of shelves, wall cabinets, and trim.
  • Remodelers who use this approach need to think about air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. continuity, especially at the perimeter of the wall in each room.
  • Remodelers who use this approach need to make sure that rim joists are properly air sealed and insulated.
  • Unless details are thought through carefully, partition intersections can represent thermal bridges.

Martin Holladay’s previous blog: “Solar Hot Air Collectors.”

Click here to follow Martin Holladay on Twitter.

Tags: , , ,

Image Credits:

  1. Image #1: Iso R Plus
  2. Image #2: William Lucrisia
  3. Image #3: Fine Homebuilding

Mar 27, 2015 9:45 AM ET

by Mark Fredericks

Thanks Martin for yet again summarizing another green building question so well.

You've got the following typo under What type of rigid foam is best?
"because XPS contains contains both"

Interior foam board seems like a great solution for remodeling dilemmas where exterior foam isn't a practical option, but I'm still unsure how the exterior should look, for this to be a safe assembly. I can think of my 1950's house with solid board sheathing, tar paper, and original wood shingles. This would seem safe enough, but this was all covered with 1/2" EPS, and wide plank color-lock siding. No rain screen gaps to encourage drying.

This is not uncommon for the older homes in my area to have several layers of siding accumulate over the years. Can you comment more on when interior foam shouldn't be used? When does the exterior stack up create too much risk?

Mar 27, 2015 10:18 AM ET

Response to Mark Fredericks
by Martin Holladay

Thanks for catching the typo.

There is no simple answer to your question about exterior layers.

Unfaced EPS that is 1/2 inch thick is fairly vapor-permeable (about 4 to 12 perms), so that's not much of an issue. If the EPS has a plastic or foil facing, of course, the permeance could be much lower.

Multiple layers of wood-fiber products, even when painted, are also (usually) permeable enough not to cause problems.

If you install interior rigid foam, the interior foam layer reduces air leakage, and just about stops outward diffusion in the winter. That helps keep the wall assembly dryer.

As long as the wall isn't getting regularly soaked by wind-driven rain, and as long as the wall has good flashing, I wouldn't worry too much. If the sun hits the wall, the wall will dry. North walls (and walls shaded by foliage) tend to stay a little damp, however.

Mar 27, 2015 10:20 AM ET

foam for furring out studs
by David Hicks

Martin -- it's a little off the topic of your blog, but I wonder if you had an opinon on the method described by Stephen Bonfiglioli in the April issue of FHB, whereby he furred out the studs to the interior with foam to create a thermal break.

Mar 27, 2015 10:36 AM ET

Response to David Hicks
by Martin Holladay

My first reaction to the idea of "Bonfiglioli strapping" is that this method seems quite labor-intensive.

But one thing I've learned over the years is: there is no way to discourage a builder who has found a technique that he or she is comfortable with. If Stephen Bonfiglioli likes his system, that's great. It works for him. That makes it a good system.

There are many, many ways to build a wall: the Mooney wall (2x6 wall with interior horizontal 2x4 strapping, all filled with cellulose), the double-stud wall, the Larsen truss wall, the Klingenberg wall, the wall with exterior foam, and the wall with interior foam. Now we can all consider the idea of Bonfiglioli strapping.

Choose a system you like, and build it.


Bonfiglioli strapping.jpg

Mar 27, 2015 6:51 PM ET

by R Miller

Martin, did you notice the vent stack on the exterior wall in the FHB article? That was surprising.

Mar 28, 2015 5:27 AM ET

Response to R. Miller
by Martin Holladay

I agree. Locating a DWV pipe in an exterior wall is usually a bad idea -- especially if you are an energy-conscious builder, and especially if the wall has no exterior rigid foam.


DWV pipe in exterior wall.jpg

Mar 28, 2015 4:46 PM ET

by Malcolm Taylor

A few companies make thermally broken studs - rstud and Nordic Industries come to mind. They don't seem to have caught on. It would be interesting to compare their prices once you factor in Mr. Bonfiglioli's time to make his homemade versions.

Apr 2, 2015 10:02 AM ET

Vapor barriers revisited
by Steve Robertson

At first I thought this might be a cold climate only technique. But you mentioned that this technique is fine even in climates with inward solar vapor drive during the summer. After reading your articles on vapor barriers as well as Joe Lstiburek’s, I am slightly confused.

I’m assuming that all poly-iso panels are vapor barriers (Class I). In Joe's BSD-106 paper, figure 6 shows a brick veneer and typical 2x4 wall construction. My understanding is that the vapor barrier was eliminated from this wall by design. Are you saying that it is OK to add the poly-iso panels under the drywall in this type of wall?

Would this rigid foam technique be allowed for an unvented cathedral ceiling? That is could the poly-iso be placed between the drywall and the rafters with batt insulation between the rafters?

Thanks for this blog. Hopefully I will understand vapor barriers after this.

Links to Martin’s blogs and Joe’s paper:
Do I need a vapor retarder?
Insulated cathedral ceiling
Cut and cobble insulation
BSD-106 Understanding vapor barriers

BSD-106 figure 6.jpg

Apr 2, 2015 10:22 AM ET

Edited Apr 4, 2015 6:02 AM ET.

Response to Steve Robertson
by Martin Holladay

I don't see any reason why you couldn't install rigid foam on the interior of the wall shown in Lstiburek's sketch. Because rigid foam has R-value, it isn't as dangerous as polyethylene. Air-conditioned poly can be a condensing surface, but the outward-facing layer of rigid foam wouldn't get as cold as the poly.

No, you can't install interior rigid foam on an unvented cathedral ceiling (unless you are following the cut-and-cobble technique, which I don't recommend). However, interior rigid foam is often used on vented cathedral ceilings.

For more information, see How to Build an Insulated Cathedral Ceiling.

Apr 3, 2015 10:34 PM ET

polyiso board facing
by Roman Stankus

Martin - can one use a fiber faced polyiso board rather than foil faced board - the fiber faced board is easier to obtain - (standard low slope roofing insulation)? Is this an air sealing issue?

Apr 4, 2015 6:00 AM ET

Response to Roman Stankus
by Martin Holladay

Yes, you can use almost any type of rigid foam in this location, including polyiso with fiber facing. I don't have advice about the best tape to use, since I haven't ever had to tape this product. In my experience, however, you can tape almost anything with Siga Wigluv tape.

Feb 20, 2017 1:56 PM ET

Edited Feb 20, 2017 1:57 PM ET.

Moisture concerns about sandwiching interior plywood
by Chris Stratton

Hi Martin -

I'm retrofitting a house in southern California with uninsulated stucco walls and no sheathing. My plan is to use the "sticks" in the stud bay corners method you describe to create an air gap behind the stucco/building paper and then cut and cobble polyiso in the 2x4 stud bays. So far so good I think. (Yes I am aware of the labor intensity of cut and cobble.)

But the structural engineer has specified that these need to now be shear walls since there is no shear bracing otherwise. So inboard of the studs will go 1/2" plywood. I'm concerned about thermal bridging through the studs and through the plywood, and some simulations I've done seem to substantiate these concerns. So I am considering adding another 1" of polyiso inboard of the plywood, then drywall, and affixing the drywall by running screws through the polyiso into the plywood/studs. (Do I need furring between the polyiso and drywall?)

Should I be concerned about sandwiching the plywood in between the interior continuous polyiso and the cut and cobbled polyiso in the stud bays? Will the studs provide sufficient pathway for any moisture in the plywood to get out? Thanks for your help.

Feb 20, 2017 2:41 PM ET

Response to Chris Stratton
by Martin Holladay

I assume that you have read this article: Insulating Walls in an Old House With No Sheathing.

Q. "Should I be concerned about sandwiching the plywood in between the interior continuous polyiso and the cut and cobbled polyiso in the stud bays?"

A. I don't think there is much reason for concern. There shouldn't be much vapor drive through this wall assembly -- either inward vapor drive or outward vapor drive -- that might lead to moisture accumulation in the interior plywood sheathing.

That said, there is no reason to choose a very low-perm material if it isn't necessary. What type of polyiso are you talking about -- foil-faced polyiso or a fiber facer? For the interior layer of rigid foam, the best choice would be either polyiso with a fiber facer (not a foil facer) or EPS. These types of rigid foam are somewhat vapor-permeable.

-- Martin Holladay

Feb 20, 2017 9:13 PM ET

response to Martin
by Chris Stratton


Yes I have read your article about insulating a house with no sheathing. It was very helpful. Thank you for writing and for referring me to it. I'll go with fiber-faced polyiso for the wall insulation, per your recommendation.

Thanks again,

Sep 12, 2017 10:37 AM ET

Edited Sep 12, 2017 10:48 AM ET.

interior rigid foam
by Christopher Merino

Does installing rigid foam insulation over interior wall studs negate the need for insulation in the wall cavities themselves? May they be disregarded?

Sep 12, 2017 10:47 AM ET

Response to Christopher Merino
by Martin Holladay

Q. "Does one need insulation in the wall cavity if installing foam board on the inside?"

A. No, there is no need for insulation in the wall cavity, as long as the insulation plan meets local code requirements. (In most cases, retrofitting insulation in an older home will not require compliance with modern energy codes, but regulations vary from town to town, so it's usually wise to call your local building office if you are uncertain of your local regulations.)

Nov 12, 2017 6:55 PM ET

interior foam plan and a service cavity?
by Ian DeGraaf

Thanks for this excellent resource. I am currently remodeling an 1890s house in climate zone 6. My wall from exterior looks like this:
8" lap steel siding
15lb felt / original
original 1" x 6" horizontal sheathing
true 2x4 studs / balloon framing

Unfortunately, I did not consider installing insulation on the exterior when I resided a few years ago, so now my plan is to add this to the interior wall based on what I have read in multiple articles on this site.

air seal the interior of the walls in the joist bays
add insulation in the joist bays likely fiberglass or rock wool. R 13 likely
2" of polyiso board on the interior of the studs = R 12 total
2x strapping for both holding polyiso boards in place, attachment for interior wall coverings and to create an electrical chase.

Another option would be to simply build out the studs by 2" vertically and I could use the Roxul 2x6
R23 bats and then another 1 or 2" of polyiso on top.
By my math this would only give me an additional R2 (23 in wall + 2" rigid @ 12 total)

I am open to any feedback on this.

Also has anyone built an electrical service chase on top of the interior rigid foam? It seems that the strapping for wallboard could work perfectly as the framing for this service cavity as well.

Thanks in advance

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