musingsheader image
13 Helpful?

Calculating the Minimum Thickness of Rigid Foam Sheathing

In this case, the code is your friend — just follow the IRC’s foam thickness table

Posted on Oct 15 2010 by Martin Holladay

UPDATED on February 26, 2016 with a new table (Image #3)

If you plan to install exterior rigid foam on the walls of your house, how thick should the foam be? Although the Web site has addressed this question several times in our Q&A column and various blogs, the question continues to perplex readers. New questions along these lines come our way regularly.

The last time I answered the question was at the end of a long, very technical blog. In this blog, I'll cut to the chase.

Keeping walls dry

When it comes to 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. , thick foam is better than thin foam. Thin foam is dangerous, because it reduces the ability of the wall to dry to the exterior without warming the sheathing enough to prevent moisture accumulation (a phenomenon that is usually but incorrectly called “condensation”).

Fortunately, building scientists have calculated the minimum foam thickness required for different wall thicknesses and different climates. By following their recommendations, your wall sheathing (or the interior face of the rigid foam) will stay warm enough to prevent moisture accumulation during the winter.

Because foam sheathing reduces the ability of a wall to dry to the exterior, all foam-sheathed walls must be able to dry to the interior. That means you don’t want any materials with a very low permeance on the interior of a foam-sheathed wall or between the studs. If you are building this type of wall, you should not include interior polyethylene or vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). wallpaper, nor should you install any closed-cell spray foam between the studs. It's perfectly acceptable to fill the stud bays with open-cell spray foam, however, since open-cell foam is vapor-permeable.

Install thick foam and no interior poly

To sum up, there are two important points to remember about foam-sheathed walls:

  • The rigid foam must be thick enough to prevent moisture accumulation (“condensation”) in your sheathing or framing; and
  • This type of wall must be able to dry inward, so it's important to avoid low-permeance layers like polyethylene, vinyl wallpaper, or closed-cell spray foam on the interior.

Of course, foam-sheathed walls must comply with existing building codes. Until recently, that was difficult, because some building inspectors insisted on the need for interior polyethylene — even on foam-sheathed walls, where poly definitely does not belong.

Fortunately, the 2007 Supplement to the International Residential Code (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.) came to the rescue. Since that Supplement was adopted, the IRC has allowed certain cold-climate walls to dry to the interior. The code now includes a table, Table N1102.5.1, listing which types of wall assemblies have minimal requirements for an interior vapor retarder. (In the 2009 IRC, these provisions can be found in section R601.3; the new designation for the table is Table R601.3.1. In the 2012 IRC, the relevant provisions can be found in section R702.7; the new designation for the table is Table R702.7.1.)

The relevant table serves two purposes:

  • It gives permission to builders of foam-sheathed walls to use a minimal interior vapor retarder — one with the highest permeance values, known as a Class III vapor retarder. (Ordinary latex paint is all you need.)
  • It spells out the minimum R-values for exterior foam to be sure that moisture won’t accumulate in a wall.

All you need to know

Here is the essential information from Table N1102.5.1 that applies to foam-sheathed walls:

    Climate Zone     Minimum R-Value of Foam Sheathing
  Marine Zone 4   R-2.5 for 2x4 walls; R-3.75 for 2x6 walls
  Zone 5   R-5 for 2x4 walls; R-7.5 for 2x6 walls
  Zone 6   R-7.5 for 2x4 walls; R-11.25 for 2x6 walls
  Zones 7 and 8   R-10 for 2x4 walls; R-15 for 2x6 walls

Once you know the minimum required R-value for your foam sheathing, you can determine your foam thickness. To do that, you need to know the R-value per inch of your foam. The most common type of 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.) has an R-value of about R-3.6 per inch, while 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.) has an R-value of R-5 per inch.

These days, the R-value shown on polyisocyanurate labels is usually equivalent to R-5.7 to R-6.0 per inch. However, the actual performance of polyiso decreases at cold temperatures. Concerns about the cold-temperature performance of polyiso are real, so GBA recommends that cold-climate builders use caution when choosing a rigid foam designed to keep wall sheathing above the dew point during the winter. Either EPS or XPS is probably a safer choice for this purpose than polyiso, unless you derate the performance of the outermost layer of polyiso to about R-4 or R-5 per inch. For more information on this issue, see In Cold Climates, R-5 Foam Beats R-6 and Cold-Weather Performance of Polyisocyanurate.

What’s my climate zone?

If you’re not sure what climate zone you live in, you can look it up on the Department of Energy’s climate zone map. The map is posted here on the GBA website; click here to see it.

I have also included the climate zone map on this page (Image 2 at the bottom of the blog); just click the image to enlarge it.

Here is a link to a web page with climate zone information for Canada.

What if I live in one of the warmer climate zones?

If you are building a house in one of the warmer climate zones — zone 1, 2, 3, or 4 (except for 4 Marine) — you don't have to worry about the thickness of your foam. Any foam thickness will work, because your sheathing will never get cold enough for “condensation” (moisture accumulation) to be a problem.

What about walls with above-code levels of air-permeable insulation?

If you plan to install a thicker-than-usual layer of fluffy insulation, you'll also need to install a thicker-than-usual layer of rigid foam (to make sure that the proper ratio of rigid foam to fluffy insulation is maintained). The table reproduced as Image #3, below, includes the relevant percentages that need to be observed.

For more information on this issue, see Combining Exterior Rigid Foam With Fluffy Insulation.

What about flash-and-batt jobs?

Builders following the flash-and-batt method — that is, a hybrid insulation system using a thin layer of closed-cell spray polyurethane foam against the interior side of the wall sheathing, with the balance of the stud bay filled with fiberglass batts or cellulose — can follow the recommendations in the table above for the minimum thickness of the spray foam. Closed-cell spray polyurethane foam has an R-value ranging from R-6.5 to about R-6.8 per inch.

The 2012 IRC specifically endorses this approach to flash-and-batt calculations in Footnote a to Table R702.7.1. The relevant footnote reads, "Spray foam with a minimum density of 2 lb/ft3 applied to the interior cavity side of wood structural panels, fiberboard, insulating sheathing or gypsum is deemed to meet the insulating sheathing requirement where the spray foam R-value meets or exceeds the specified insulating sheathing R-value."

The table can also be used as a minimum foam thickness guide when following the cut-and-cobble method (insulating between studs by combining a layer of rigid foam installed against the interior side of the wall sheathing with fiberglass batts in the rest of the stud cavity).

Although the fiberglass batts in a flash-and-batt stud bay will be thinner than the fiberglass batts in a wall with exterior foam sheathing, thinner batts move the wall in the direction of more safety rather than more risk, since thinner fiberglass keeps the interior surface of the cured foam warmer (and therefore less likely to collect condensation).

If you want to sharpen your pencil, you can get away with thinner foam for a flash-and-batt job than an exterior-foam job. As long as you retain the ratio of foam R-value to fluffy-insulation R-value shown in the table, you should be OK. For example, the table recommends R-5 foam for a 2x4 wall filled with R-13 fiberglass insulation in Climate Zone 5 (38% foam and 62% fiberglass). For a flash and batt job, you could get away with R-3.6 foam and R-9.5 fiberglass insulation. However, in most cases you don't really have to sharpen your pencil quite this much.

Why doesn’t every cold-climate wall have rotten sheathing?

Since most homes don't have foam sheathing, what keeps the cold sheathing on a typical home from developing moisture problems?

Good question; the answer can be found in another blog, How Risky Is Cold OSB Wall Sheathing?

Is there a similar chart for unvented cathedral ceilings?

The same logic used to calculate the minimum thickness of foam wall sheathing can also be applied to unvented cathedral ceilings.

Recent versions of the IRC allow unvented roof assemblies insulated with a combination of rigid foam insulation above the roof sheathing and air-permeable insulation in the rafter bays. The relevant code provisions can be found in section R806.4 of the 2009 IRC and in section R806.5 of the 2012 IRC. (The IRC defines air-impermeable insulation as “an insulation having an air permeance equal to or less than 0.02 L/s-m² at 75 Pa pressure differential tested according to ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. E 2178 or E 283.” Although spray foam insulation and rigid foam insulation can meet this standard, dense-packed cellulose cannot.)

The code requires that “rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.4 for condensation control.” These values are:

  • Climate Zones 1-3 — R-5
  • Climate Zone 4C — R-10
  • Climate Zones 4A and 4B — R-15
  • Climate Zone 5 — R-20
  • Climate Zone 6 — R-25
  • Climate Zone 7 — R-30
  • Climate Zone 8 — R-35

For more information on this topic, see How to Install Rigid Foam On Top of Roof Sheathing.

For more information

More information on Table N1102.5.1 can be found in a useful article posted on the Building Science Corporation Web site, Insulating Sheathing Vapor Retarder Requirements.

If you are a masochist, and want to delve deeper into the intricacies of dew-point calculations, you can check out my earlier blog on this topic, Are Dew-Point Calculations Really Necessary?

For instructions on installing rigid foam on the exterior side of wall sheathing, see How to Install Rigid Foam Sheathing.

Finally, for an assessment of the risks associated with too-thin foam, see Rethinking the Rules on Minimum Foam Thickness.

Last week’s blog: “Solar Versus Superinsulation: A 30-Year-Old Debate.”

Tags: , , , , , ,

Image Credits:

  1. Image #1: Ty Keltner, Cold Climate Housing Research Center
  2. Image #2: U.S. Department of Energy
  3. Image #3: Martin Holladay

Mar 21, 2016 10:30 AM ET

Response to Michael Prisco
by Martin Holladay

The batts should be in direct contact with the exterior sheathing. You might want to install some steel insulation supports (like these) to keep the batts in place and prevent them from slumping.

Mar 21, 2016 11:16 AM ET

Response to Martin Holladay
by Michael Prisco

Thank you. This website and community is such an incredible resource. I learn so much every time i visit.

Mar 21, 2016 6:23 PM ET

Follow up question to Martin Holladay
by Michael Prisco

I wanted to follow up with your response from this morning. I would really like to get the highest R value I can out of the walls. I had originally planned on using mineral wool in the 2x6 walls which is more like r-23. Using r-23 I would likely put the dew point somewhere in the mineral wool. Has the MemBrain proved effective in a case like this? Or would it be safer to go with the R-15?

Mar 21, 2016 6:59 PM ET

Response to Michael Prisco
by Martin Holladay

It's hard to gauge the risk, frankly. I know what building scientists recommend -- and I tend to be conservative. I also know that people sometimes break the rules, and that many rule-breakers don't suffer problems.

The families that have problems with sheathing rot are probably the families that maintain high indoor humidity levels. Interior MemBrain will probably keep your wall safe, but nothing is guaranteed. If you install R-23 mineral wool and MemBrain, it wouldn't hurt to keep an eye on your interior humidity. Whatever you do, don't install a humidifier.

I'm sorry that my answer can't be more definitive.

Mar 23, 2016 2:27 PM ET

Another question for Martin Holladay
by Michael Prisco

Thanks again for you quick responses. They are very helpful. I keep going back and forth on a decision. I like the idea of being conservative. If I went with r15 insulation in a 2x6 wall what would the effect of the 2" air gap between the drywall and the fluffy insulation? Would it create a place for convection and greatly reduce the r 15 value or does the air gap add R value?

Mar 23, 2016 6:02 PM ET

Response to Michael Prisco
by Martin Holladay

Q. "Would it create a place for convection and greatly reduce the R-15 value or does the air gap add R-value?"

A. It would potentially create a place for convection, but the reduction in the R-value of the insulation would probably be minor. You're going to have to let that worry go, Michael, because what you are doing is deliberately making sure that the stud bays are poorly insulated. The reason you are doing that is because you didn't install enough exterior foam. You've boxed yourself into a corner, frankly -- so stop trying to improve the R-value between the studs. You want the R-value to be low.

Mar 24, 2016 4:31 PM ET

Response to Martin Holladay
by Michael Prisco

Thank you for your frankness. I did box myself into a corner and know what I have to do. Cheers.

Apr 23, 2016 10:41 AM ET

Edited Apr 23, 2016 10:44 AM ET.

New Building Wall Section
by John McFarland

Hello Martin-

Great article and blog! We are trying to finalize a wall section for a new project. We are in zone 5 (Chicago). We have 2 x 6 metal frame walls 24" on center. We are considering the following section outside to inside:

-4 inch face brick
- 1.5 inches Rmax / EcomaxCI with a stated R value of 10.
- 5/8 Dense Glass Gold
- R13 fiberglass batt insulation
- 5/8 interior drywall

Do you see any issues with this section? Should we use faced or unfaced batt insulation? Do the aluminum facers on both sides of the EcomaxCI present any problems? If we went to R-19 batt insulation do you think we should increase the EcomaxCI to 2 inches?

Thank you

Apr 23, 2016 4:47 PM ET

Response to John McFarland
by Martin Holladay

Steel-stud walls don't perform like wood-frame walls. Thermal bridging through the steel studs completely dominates the thermal performance of the stud wall; for all intents and purposes, and insulation installed between steel studs is a waste. It's money down the drain.

With a steel stud wall, all of the insulation has to be installed on the exterior side of the steel studs. In Climate Zone 5, the code calls for a minimum of R-20 wall insulation. If you want to use polyiso, remember to de-rate the polyiso R-value because polyiso performs poorly at cold temperatures.

I'd say that your wall needs at least 4 inches of polyiso -- and 5 inches would be better.

Apr 25, 2016 5:14 PM ET

Thanks for the reply Martin.
by John McFarland

Thanks for the reply Martin. We thought about four to five inches of exterior insulation but were concerned about window installation and how masonry ties could be adhered by having to go through that much insulation. Any thoughts on that? Thanks again

Apr 26, 2016 6:40 AM ET

Edited Apr 26, 2016 6:41 AM ET.

Response to John McFarland
by Martin Holladay

This question has come up on GBAi a few times before. One GBA reader recommended Simpson Heli-Tie Helical Wall Ties. Contact Simpson for more information.

Another company you might want to contact is Rodenhouse. This video shows one of their fastening and brick tie systems. I'm not sure of the maximum thickness of rigid foam that this system can accommodate, but it might be what you are looking for.


Apr 26, 2016 9:15 AM ET

Is the DensGlass structural?
by D Dorsett

Positioned between the foam and the steel studs what function is the exterior side gypsum serving?

Code min for residential construction is 2x6/R20 between wood studs or 2x4/R13+ R5 continuous insulation, either of which performs about the same as R15 continuous insulation. The maximum U-factor is U0.057, which R17.5 whole wall, after adding in the performance of the brick, the interior gypsum and the air films, etc.

The R10c.i. + R13 between steel studs with the brick and both gypsum layers meets that performance level but only barely. At a typical 15% framing fraction R13 in steel studs delivers about the same performance as R5 continuous insulation:

So, from a thermal performance point of view, at the very same wall thickness you'd be better off replacing the 5/8" of gypsum with more polyiso and using steel bracing for structural rigidity, but keeping the R13s in steel, which would add another ~R3 to the whole wall R (after subtracting the performance of 5/8" gypsum and adding in the performance of 5/8" polyiso.) Without the R13s in steel studs you'd have to go with at least 3" polyiso to hit code min performance, when properly derated for temperature/climate.

Do the math on whether R13s in studs is more expensive than another inch of polyiso, or whether it's cheaper or better to go with gypsum vs. cut in bracing if wall thickness is a concern.

Jun 10, 2016 10:18 AM ET

by Tony Tibbar

Is the 27% of the wall value the real wall value including the framing factor or just simply adding R values?

What is the percentage for cathedral ceilings? Zone 5 specifies R20 that would mean a total of 20/27x100=R74. That looks a bit high.

Jun 10, 2016 11:31 AM ET

Response to Tony Tibbar
by Martin Holladay

The answer to your questions can be found in this article: Combining Exterior Rigid Foam With Fluffy Insulation.

Concerning your question about the "27%" figure in my table on walls, the explanation in my article notes, "The tables are based on the nominal value of the insulation layers, without taking into account the effects of thermal bridging 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."

The same article also discusses cathedral ceilings. In Zone 5, 41% of the roof assembly's total R-value needs to come from the rigid foam layer (assuming we are talking about a roof with a combination of exterior rigid foam and interior fluffy insulation).

More information on cathedral ceilings can be found in these two articles:

How to Build an Insulated Cathedral Ceiling

How to Install Rigid Foam On Top of Roof Sheathing

Below is the table with information on walls.


Table showing minimum R-values for rigid foam installed on exterior of wall sheathing.jpg

Jun 10, 2016 12:25 PM ET

Thanks Martin
by Tony Tibbar

Totally clear now. I overlooked the 41%

Sep 6, 2016 1:16 AM ET

Will 2 inches of Polyiso be adequate in Zone 6
by Steve Wolfe

I've been pouring through this website but I can't seem to find an answer to this. Looking at the chart, I need a minimum of R 11.5 for exterior rigid foam sheathing. A lot of the 2" Polyiso foam board I see advertised is R 13. But I've read you have to degrade the R factor in colder temperatures. So will 2" Polyiso be OK? I am building in North Idaho and will be right on the border between Zone 5 and 6, but am in Zone 6. I ask as the cost of sheets of 2" Polyiso appear to be 60% of the cost of going with 2.5 inch XPS.

Sep 6, 2016 9:49 AM ET

Response to Steve Wolfe
by Martin Holladay

The answer can be found in the article on this page, which states, "GBA recommends that cold-climate builders use caution when choosing a rigid foam designed to keep wall sheathing above the dew point during the winter. Either EPS or XPS is probably a safer choice for this purpose than polyiso, unless you derate the performance of the outermost layer of polyiso to about R-4 or R-5 per inch."

So, if you feel safe assuming that polyiso has a value of R-5 per inch in cold weather (rather than R-4 per inch, which is more conservative), then 2 inches of polyiso (R-10) is insufficient. You should go with 2.5 or 3 inches, or switch to a different type of rigid foam.

Apr 5, 2017 10:21 AM ET

Edited Apr 5, 2017 10:36 AM ET.

Residing Project
by user-6808940

Our home in Maine was built in the late 80s, has 2x4 construction and was poorly insulated, probably R10 unfaced. We are replacing the roofing and siding (cedar shakes) and would like to add exterior rigid foam to the walls. I have read exhaustively through this website and others and it seems we need a minimum of R7.5 so I'm thinking of 1.5 inches of XPS followed by another layer of 1/2 sheathing, draining housewrap, then the shakes. Unfortunately, the interior walls do have a layer of poly behind the drywall. I have read that is a potential issue as it would impead drying to the inside. Would we be making a big mistake to proceed with the foam anyway? Wondering if the risk is just too much and we should give up on the foam.

Apr 5, 2017 10:30 AM ET

Adding exterior rigid foam to a wall with interior polyethylene
by Martin Holladay

Many energy experts worry that it may not be a good idea to install exterior foam on a house with an interior vapor retarder. Although it would be better if the vapor retarder weren't there, the fact is that tens of thousands of Canadian homes with interior polyethylene have been retrofitted with exterior rigid foam, and there haven't been any reports of widespread problems. According to building scientist John Straube, all indications show that these retrofits are "not so risky as most people think. These homes will probably be fine."

That said, the installation of exterior foam is not advised on any home that has suffered wet-wall problems like leaking windows or condensation in stud cavities. If you plan to install exterior foam during a siding replacement job, keep an eye out for any signs of moisture problems when stripping the old siding from the walls. Investigate any water stains on housewrap or sheathing to determine whether the existing flashing was adequate.

If there is any sheathing rot, determine the cause -- the most common cause is a flashing problem, but condensation of interior moisture is not impossible -- and correct the problem if possible. If you are unsure of the source of the moisture, hire a home performance contractor to help you solve the mystery.

If your sheathing is dry and sound, I don't think you need to worry about adding exterior foam. Adding a rainscreen gap will certainly go a long way toward avoiding future moisture problems. Of course, it's important to be meticulous with your details when you are installing your new WRB and window flashing. It's also important to keep your interior relative humidity within reasonable levels during the winter. Never use a humidifier.

To summarize, here are four caveats:

1. Be sure that your foam is thick enough to keep the wall sheathing above the dew point in winter. Read more on this topic here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

2. When the siding is being removed, inspect the existing sheathing carefully for any signs of water intrusion, and correct any flashing or housewrap problems.

3. Install rainscreen strapping so that there is a ventilated gap between the new exterior foam and the siding.

4. Keep your interior humidity under control during the winter; if the interior humidity gets too high, operate your ventilation fan more frequently.

-- Martin Holladay

Apr 5, 2017 12:43 PM ET

by Charlie Sullivan

I would also advise using something other than XPS, because the R-value of XPS decays over time as the gas it's made with diffuses out. That gas also is a potent greenhouse gas. EPS or neopor does not have that problem, and is also more vapor permeable so you can have at least a tiny bit of drying to the outside. And 2" of EPS is probably cheaper than 1.5" of XPS.


Apr 5, 2017 1:46 PM ET

What Charlie said.
by Dana Dorsett

Using 2" of unfaced 1.5lbs density "Type-II" EPS delivers more dew point control, but also allows a higher drying rate toward the exterior, since it's vapor permeance is about twice that of 1.5" XPS.

XPS performance eventually decays to that of EPS of similar density, whereas the thermal performance of EPS is stable over decades.

Using reclaimed EPS from salvage companies or foam reclaimers is cheaper still. EPS is a popular roof insulation in flat commercial roofs, and 2-4" thick reclaimed EPS is usually pretty easy to find at 1/4- 1/3 the cost of virgin stock goods. In southern ME you're in reasonable trucking distance from some of the larger recliamers in MA (Green Insulation Group in Worcester, or Nationwide Foam in Framingham), and even with freight going with 2-3" EPS is going to be quite a bit cheaper than virgin stock 1.5" XPS at building materials distributors.

Apr 10, 2017 3:53 PM ET

by user-6808940

Thank you Martin, Charlie, and Dana. After getting some quotes for adding 1.5 to 2 inches of either XPS or EPS followed by another layer of 1/2 plywood sheathing, I found the labor of the additional layer of sheathing stresses our budget too far. However, I'd still like to add some insulation value, thermal break, and air sealing during our siding project. We are going to side with cedar shakes. Would 1/2" EPS be permeable enough to avoid moisture due to condensation? The plan would be to apply the EPS to the existing sheathing, followed by drainable housewrap, then the shakes.

Apr 10, 2017 4:07 PM ET

1/2-inch EPS?
by Martin Holladay

You should contact the manufacturer of the rigid foam to find out the vapor permeance of the EPS in question -- some brands of EPS have a vapor-impermeable facing. If there is no facing on the EPS, its vapor permeance may be as high as 4 perms or 5 perms.

If I were you, I wouldn't install a product like EPS that slows outward drying of the wall sheathing. You really want damp wall sheathing to dry quickly, not slowly.

According to my article, if you install rigid foam on the exterior side of 2x4 walls in Maine (Zone 6),the rigid foam needs to have a minimum R-value of R-7.5. I stand by that advice.

-- Martin Holladay

Apr 10, 2017 5:01 PM ET

Shake installation tool and mineral wool
by Charlie Sullivan

I agree with Martin--even though EPS can have some vapor permeability, I'd be disinclined to do anything less than the recommended thickness.

Here are some other suggestions that might help:

1) There's no minimum thickness requirement for mineral wood insulation boards, because they have high vapor permeability. You could install whatever thickness you can afford.

2) To reduce labor cost on your project, and have some spare change to put into insulation, consider this installation tool for the shakes.

I'm not sure how many installers are already using those, but the carpenter who installed our shingles hadn't seen one before and though it was a big improvement.

Apr 10, 2017 5:37 PM ET

3/8" fan-fold perforated siding underlayment would be OK
by Dana Dorsett

The fan-fold XPS siding underlayment runs about 5 perms and has a reasonable history in retrofits. It's only about R1.9 when new, dropping to ~R1.5 slowly over time. On typical 2x4 wall that would be about a 13% improvement in wall performance on a U-factor basis.

Half inch Type-I EPS would be about R1.9 on day 1, and on day 100,001, but the facers create a moisture trap.

I've never seen half inch EPS of any density/type without facers. Maybe it exists (and maybe the tooth fairy gets around on a unicorn too.) It would be too fragile to survive much handling without knocking off corners, unless it's of a sufficiently high density to reduce

3/4" asphalted fiberboard is about 15 perms when the humidity is high- that's more vapor open than Typar housewrap, and it would add about R2 to the stackup. Half inch asphalted fiberboard runs about R1.3. Fiberboard has reasonable fastener retention for siding, and would add structural rigidity to the stackup. Whether it's "worth it" for that marginal improvement is up to you, but it's a better choice than way-too-thin foam. Half inch fiberboard is easier to find than 3/4", and under $10/sheet for 4'x 8' if you can find a local distributor stocking it.

A single layer of half-inch fiberboard would deliver about a 11.5% improvement in U-factor for a typical 2x4 wall.

For about the same material cost as a layer of half-inch fiberboard, single layer of 2" reclaimed polyiso would deliver a 55% improvement in U-factor. That's less than half the heat loss of the "before" picture, or more than double the whole-wall R value.

Apr 10, 2017 6:59 PM ET

See BSI-092 for discussion
by Jon R

See BSI-092 (How Come Double Vapor Barriers Work?) for a discussion about less-than-condensing amounts of somewhat vapor permeable foam and an interior-side vapor barrier working well. With 2x4 walls, 3/4" foam would have even better moisture performance.

Jun 14, 2017 1:10 AM ET

Adding poly-iso in climate zone 3 to 2x4 wall
by Mats Lundgren

Hi Martin, as part of my remodel to improve earthquake resistance (and support some larger window openings) most of my 2 story 2x4 wall has been updated with PSL beams and hardy frames. They fill the complete wall depth so I will loose insulation and I was thinking to add 1" poly iso. House in Woodside, CA (Climate zone 3).
Can I have the following setup from exterior to interior:
1. 5/8" cedar shiplap (vertical install)
3. 1" poly iso
4.OSB wall board with every gap sealed with siga-tape (OSB nailed to create needed shearing)
5. 2x4" wall with blown in cellulosa or cotton batt insulation
6. Interior shearing if needed, f not this is the final drywall.
7. If shearing also needed from the inside OSB in step 6 and drywall as step 7.

Any problem with this setup?

Also how to keep the poly iso in place? Can you use 1.5" nails to nail the house wrap and through the poly iso and into the OSB?
How to then hold the rain screen until you have the siding in place? Nail again into just the poly-iso (just lightweight plastic mesh)?
Finally the cedar with shiplap can be nailed through and into to the OSB?


Jun 14, 2017 6:29 AM ET

Edited Jun 14, 2017 6:35 AM ET.

Response to Mats Lundgren
by Martin Holladay

Q. "Any problem with this setup?"

A. No.

Q. "How to keep the polyiso in place?"

A. Polyiso is installed with cap nails. For more information on this issue, see How to Install Rigid Foam Sheathing.

Q. "Can you use 1.5 inch nails to nail the housewrap and through the polyiso and into the OSB?"

A. Housewrap is also installed with cap nails. As the article which I linked to explains, cap nails are available in a wide variety of lengths. You are correct: you need the nails to reach the OSB. Be sure to follow the installation instructions of the housewrap manufacturer.

Q. "How to then hold the rainscreen until you have the siding in place?"

A. Again, cap nails will work (as far as I know -- but it never hurts to read the installation instructions -- in this case, the instructions provided by Benjamin Obdyke). All you need for each of these layers is minimal attachment until the next layer is secured. You just need to prevent everything from being stripped away by the wind until the job is done.

Q. "Finally the cedar with shiplap can be nailed through and into to the OSB?"

A. In most cases, siding is fastened to the studs, not to the sheathing. That's why horizontal siding is easier to install than vertical siding. Many types of siding, including vertical shiplap, are best attached to horizontal blocking between the studs. If you haven't anticipated this problem by installing horizontal blocking, you need to either (a) switch to horizontal siding, or (b) attach your siding with screws rather than nails, and talk to an engineer (or the siding manufacturer) to verify that attaching to the sheathing is sufficient.

Aug 8, 2017 6:23 PM ET

Foam thickness zone 6a
by Zone 6

Hi Martin, we are residing a 1950 something ranch house. The inside walls are 3/8 drywall with plaster on top. Wall cavity is 2x4 with Rock Wool Batts (paper on both sides). Appears to be Shiplap sheathing.
I would like to put 2" Type 2 EPS foam on the sheathing and then vinyl siding. Could I put #30 felt on the sheathing followed by the foam and then use Tyvek? or should I skip the felt? Just foam and Tyvek. FYI new windows will be installed with the siding so they will be outies. How thick can the foam be before a Dudley is needed? Thank you!

Aug 8, 2017 7:19 PM ET

Response to Zone 6
by Martin Holladay

Q. "Could I put #30 felt on the sheathing followed by the foam and then use Tyvek?"

A. Yes.

Q. "Or should I skip the felt? Just foam and Tyvek?"

A. It's your choice. Every wall needs a water-resistive barrier (WRB). If you decide to install both asphalt felt and Tyvek, you need to decide which of these two layers will perform as your WRB. If you skip the asphalt felt, then clearly the Tyvek should be your WRB.

Q. "How thick can the foam be before a Dudley box is needed?"

A. The question has no easy answer, because there are so many variables when it comes to window installation. A lot of the relevant discussion and recommendations can be found in this article: Installing Windows In a Foam-Sheathed Wall.

Aug 22, 2017 5:16 PM ET

Follow up questions
by Zone 6

Hi Martin!
Thank you for the quick reply, however now it seems we have more questions. Our window installer will not install windows over foam thicker then 1 1/2". He suggests a wooden picture frame around the windows (like you did in the 80's) if we want 2" foam. Here is his suggestion for our Wisconsin home.
"As far as the comments on the foam thickness, its acknowledged in the building science community that 1.5" XPS foam will be sufficient to control the dew point to mitigate condensation. Frost is simply "flash" frozen condensation, so the same would apply. That thought more commonly is when there is no other substrate existing, so the fact that you have wood sheathing that you are going over only serves to add to that. The type and qty of insulation in the wall cavities, presence (or lack thereof) of a plastic vapor barrier on the interior side factor into that as well. If I recall, you have a plaster over sheetrock interior wall, so no plastic exists. If this were my home, I would use XPS foam of between 3/4 and 1 1/2" thickness. If additional insulation was desired, dense pack the wall cavities with cellulose. I have concerns about the siding installation over 2" of foam without any furring as well. That's a pretty large gap for those nails to bridge. Lastly, I'd also recommend a having a blower door test after these installs as you will be tightening the home up quite a bit in terms of air exchange. You will want to ensure that the ACH is sufficient to maintain good indoor air quality."

Would the picture frame (maybe 4' wide) be a source of Thermal bridging?
Would a 2" thick frame made with a 1" thick board and 1" of foam solve the Thermal bridging problem?
Should we just give up and use 1 1/2" XPS?

If so

We would put the 1 1/2" XPS foam on the sheathing leave the joint untaped and use Tyvek as the WRB.
Do you think this method be good for our zone 6A home?

FYI- Not sure how many walls have the Rock Wool, I have found some places that have paper backed fiberglass.

Thank you!

Aug 22, 2017 5:37 PM ET

It's hard to parse some of those comments... @ Zone 6
by Dana Dorsett

Even with mental filters wide open I have a hard time figuring out what your contractor really means.

But here is is in an nutshell:

In US climate zone 6 a 2x4 wall with cellulose / rock wool / fiberglass cavity fill and at least R7.5 on the exterior meets the code prescriptive for dew point control:

The labeled R value of 1.5" XPS is R7.5, but it will eventually drop to about R6.3 as it's climate damaging HFC blowing agents leak out over a few decades. If XPS, use at least 2".

Leaving the foam seams untaped does next to nothing for improving drying, but it does reduce performance. Tape it.

Polyisocyanurate at 1.5" will underperform it's R9-R10 (~ R6/inch) labeled R during the coldest days of winter, but in your stackup you can count on at least an R5/inch average over a winter. If you're really stuck at 1.5", use polyisocyanurate. Polyisocyanurate is also greener, blown primarily with (low environmental impact) pentane, and it's very long term performance won't take the hit that XPS does.

Long-nailing through even 1.5" foam isn't a great way to hang vinyl siding, and may violate the siding manufacturer's specs.

Aug 23, 2017 4:52 AM ET

Response to Zone 6 (Comment #182)
by Martin Holladay

According to Joe Lstiburek, it's possible to install vinyl siding through rigid foam as thick as 1.5 inch with long nails. I have never tried this approach, but Lstiburek says it works. If you try it, remember that you can't nail everything snug -- the vinyl siding still has to be able to slide right and left in the nailing slots.

If the rigid foam is any thicker than 1.5 inch, or if you are nervous about the "long nails" approach, the solution is to install the siding on vertical furring strips. There is only one catch: most vinyl siding manufacturers prohibit the installation of their siding on furring strips. Does this make sense? Probably not, but that's what the manufacturers say. For more information on this controversy, see Can Vinyl Siding be Applied Over Furring Strips?

One way out of this box is to switch to a different type of siding -- for example, fiber-cement lap siding.

Not all "picture frame" methods of window installation involve thermal bridging. You should read this article: Installing Windows In a Foam-Sheathed Wall. In that article, I describe a type of picture frame that is separated from the sheathing by a layer of rigid foam: "Install a picture frame of 1x4 lumber, installed on the flat, on the exterior surface of the rigid foam. The 1x4 picture frame is screwed through the rigid foam to the sheathing and framing."

Aug 24, 2017 11:22 PM ET

type of foam zone 6A
by Zone 6

Hi Martin,

What Dana said about using Polyisocyanurate sounds good but I am no expert on the mean Temp.
Our cooling cost is high but our heating cost is not bad, so the Polyisocyanurate would be good that way as well.
The Polyisocyanurate will be ok uncovered while siding, another plus.

Does Neopor® GPS need to be covered quickly (Tyvek) to prevent damage from the sun?

If I can find 1.5 inch Neopor and assuming the cost is equal to1.5 inch Polyisocyanurate, which would you recommend?

Thank You!

Aug 25, 2017 6:08 AM ET

Edited Aug 25, 2017 6:09 AM ET.

Neopor protection
by Martin Holladay

Q. "Does Neopor GPS need to be covered quickly (Tyvek) to prevent damage from the sun?"

A. The Neopor instructions read as follows: "Care should be taken to keep exposed foam protected from reflected sunlight or prolonged solar exposure. If deformation of the insulation product occurs due to excessive heat transferred from reflected and concentrated sunlight, remove the reflective surface or shield the insulation product. A secondary method to protect the foam from direct sunlight and heat is to install sunscreen or tarp on the outside of the scaffolding, much the same that is used on building construction that protects the public when it is necessary for them to pass by construction site underneath the scaffolding."

I'll admit that this information is a little vague. For more detailed information, you might want to contact BASF (973-245-6000 or 800-526-1072).

Aug 25, 2017 11:08 AM ET

by Zone 6

I called BASF tech support, They don't actually make the Neopor and can't answer any questions or ever tell me where to buy it. If I can't find any Neopor should I just use 1.5 inch Polyisocyanurate? The charts still confuse me but I believe our mean temp would be ok even on the -15 days (2 weeks or so) and definantly ok on the 15-20 degree days. It seems Dana similarly interprets the charts. And I can buy the Polyisocyanurate at my local Menards.

Thank you

Aug 25, 2017 12:33 PM ET

Response to Zone 6
by Martin Holladay

Yes, you can use 1 1/2 inch of polyiso (as Dana explained to you in Comment #183).

Apr 7, 2018 2:21 PM ET

Vapor Retarded Required
by Bryanw511

There's all this talk about eliminating the interior vapor retarder, but what to do when you live in an area where one is required by code? See:
If I'm required to use a vapor retarder on the warm side of the wall, how do I utilize foam sheathing without creating a second vapor retarder? Or is that no concern as long as the foam has a minimum level of vapor permeability to allow drying?

Apr 9, 2018 11:53 PM ET

Dew Point Chart
by Bruce Davis

Martin states, "If you are building a house in one of the warmer climate zones — zone 1, 2, 3, or 4 (except for 4 Marine) — you don't have to worry about the thickness of your foam. Any foam thickness will work, because your sheathing will never get cold enough for “condensation” (moisture accumulation) to be a problem."

Officially, I'm Zone 4, but right on the border of Zone 5 (27%+ foam), so I go with Zone 4.5. I'm looking at using Zip-R6 1" polyiso / 7/16" OSB. Based on the BSC study, I'm de-rating it to R5 in the cold. Maybe a few days here and there, the temps drop to around zero F. I suppose it could de-rate to R4.3 at 15 degrees occasionally and below R4 at zero rarely.

Based on the Zone 5, 27%+ foam, most of the house then will be 2x4 R-13 wall fill with some 2x6. I suppose the 2x6 should also be kept at R-13 to keep the dew point in check. Do you think the Zip-R6 is sufficient for Zone 4.5? Should I stick with R-13 batts or could I up them to R-15? Thanks.

Apr 10, 2018 9:52 AM ET

It's the seasonal averages that matter (not the peak)
by Dana Dorsett

Having the occasional cold snap where the sheathing takes on some moisture via vapor diffusion isn't a problem, as long as it dries before temperatures rise high enough to support rapid mold & fungus growth. It's really about the average.

Being in the cold edge of zone 4, near the warm edge of zone 5 isn't really a concern here. If the IRC prescriptives are good enough for the cold edge of zone 5 they're overkill for the cold edge of zone 4. ANY amount of insulating sheathing (or none at all) is sufficient for keeping the average temperature at the foam/fiber boundary of a ZIP-R wall sufficiently warm to limit the peak and average moisture levels in a 2x6 cavity (at any fiber-R) to tolerable level. Even R23 rock wool is fine.

If you're the type who would always worry about it, use cellulose instead of fiberglass or rock wool. Cellulose can take on a fairly large amount of moisture without damage or loss of function, and redistributes any moisture within the fiber.

In Marine zone 4C (which has a much longer heating season and a shorter drying season than zones 4A & 4B) the IRC prescribes R3.75 for 2x6 framing. Even with derating the ZIP-R to a presumptive R5 it would have margin. But in zones 4A & 4B it's not really necessary, since the outdoor temps rise well above the indoor dew point temperatures much earlier in the spring season than in 4C.

Apr 10, 2018 10:13 AM ET

@ Bryanw511, I believe
by Michael Maines

@ Bryanw511, I believe Wisconsin requires a class 1 vapor retarder. Variable-permeance membranes like Pro Clima Intello, Siga Majrex and Certainteed Membrain meet that requirement, but if moisture accumulates within the wall their permeance increases to allow drying to the interior. (Technically with Majrex the permeance is different depending on which direction it's facing, but the effect is similar.)

Register for a free account and join the conversation

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