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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 GBAGreenBuildingAdvisor.com 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.

Once you’ve visited the site, you can bookmark the page for future reference. 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.

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.

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

102.
Feb 18, 2014 9:52 AM ET

Response to Paul Fowler
by Martin Holladay

Paul,
No, you can't use the same minimum exterior foam thickness for a 2x8 wall as you would for a 2x6 wall. If the R-value of the insulation between the studs goes up, then you have to also increase the minimum R-value of the rigid foam, to make sure that your exterior sheathing stays above the dew point.

Here is a rough, back-of-the-envelope calculation: you have increased the thickness of your between-the-studs insulation by 32%, so you also have to increase the minimum R-value of your exterior foam by 32% -- in other words, from R-15 to R-20.

For a more precise calculation, follow the method described in this article: Are Dew-Point Calculations Really Necessary?


103.
Feb 19, 2014 6:55 AM ET

Foam
by Paul Fowler

Martain
Thanks for the reply. I went with zone 7 because I know that it is way colder their than where I live, bit over kill but that is okay. Spider is somewhere in between R 27 to 30 in a 2x8 cavity so that plus the R 20 foam should give me somewhere around R-48, is that right. Also how do you go about flashing the windows using foam? Plan on using one of those, not sure of the pronunciation, dudly box, windows are flange, nail on
Paul


104.
Feb 19, 2014 7:26 AM ET

Edited Feb 19, 2014 7:29 AM ET.

Response to Paul Fowler
by Martin Holladay

Paul,
Q. "How do you go about flashing the windows using foam? Plan on using one of those, not sure of the pronunciation, Dudley box."

A. For more information on how to flash a window in a wall with exterior rigid foam, see this article: How to Install Rigid Foam Sheathing.

For more information on the "Dudley box" method, see Window Installation Tips for a Deep Energy Retrofit.


105.
Feb 19, 2014 12:05 PM ET

Windows
by Paul Fowler

Martin
Excellent. Really good information there. Think that about answers all my questions for now. Thanks for your help.
Paul


106.
Jul 15, 2014 7:25 AM ET

unvented cathedral ceilings - foam/batt ratio
by Mark Fredericks

With unvented cathedral ceilings I'm curious about the interior/exterior insulation ratios and if there's a rule of thumb for a safe, condensation free assembly. I understand this article, but often see homes being built with thin foam or thick batts. For example, in the BSC Habitat Prototype house (link below) they used 4" of polyiso over the roof deck and R40 fiberglass batts in the rafter bays. I'm sure this house is doing fine, but this insulation ratio seems more heavily weighted to the interior which should give the exterior foam a harder time keeping the roof deck above the dew point. Is there a safe ratio, or is 4" of foam enough, regardless of how deep your rafter bays or stud bays are?

http://www.buildingscience.com/documents/case-studies/cs-ma-westford-hfh...


107.
Jul 15, 2014 7:39 AM ET

Response to Mark Fredericks
by Martin Holladay

Mark,
Your question is addressed in the article on this page. The relevant section of my article can be found under the heading, "Is there a similar chart for unvented cathedral ceilings?"

In the U.S., building codes require that any unvented cathedral ceiling with some rigid foam above the roof sheathing and fluffy insulation between the rafters meet minimum requirements for the R-value of the rigid foam layer. These requirements are found in section R806.4 of the 2009 IRC, as I explain in the article.

These code requirements aim to minimize risk. Homes have been built, of course, that ignore these requirements and use thinner foam. As long as the interior relative humidity is relatively low during the winter, these homes are likely to avoid problems. In homes with high levels of indoor humidity, however, the risk of sheathing rot rises dramatically.


108.
Jul 15, 2014 7:48 AM ET

Was still unsure
by Mark Fredericks

Thanks Martin, I read that section in the original article but was still unsure if meeting the prescribed R value meant anything about the ratio of the two insulating layers. It seems like that table should be adaptable to increases with greater rafter depths.

I guess the take home message is that it's always safer to have more insulation outside the structure than inside! Thanks again Martin.


109.
Jul 15, 2014 8:31 AM ET

Edited Jul 15, 2014 8:32 AM ET.

Second response to Mark Fredericks
by Martin Holladay

Mark,
The code apparently assumes the use of 2x10 rafters or "typical" R-30 or R-38 batts. Since codes aim for simplicity, they don't account for all possibilities. Codes make assumptions and try to reduce risk.


110.
Jul 30, 2014 6:57 PM ET

Be aware of R-value
by Tim Johnson

Be aware of R-value warranties on any insulation. XPS has an R-value warranty of 90%, isocyanate foam is less than that. A lot of foam thickness worries can go away by installing a smart vapor retarder on the inside of the wall along with rigorous airsealing and testing. If you don't test, you have no idea of what you have. A good alternative to foam is Roxul Comfortboard IS with ProClima products inside and out. ProClima also has some fantastic tapes for sealing things.


111.
Aug 7, 2014 3:19 PM ET

Rigid foam insulation over closed cell spray foam
by Eric Arey

House was built in 2008. I am told, by the builder, we have closed cell spray foam insulation with 2x4 construction. I like the idea of additional rigid foam insulation, plus rainscreen siding. Is there any types/brands of rigid foam insulation that could be used with this application?


112.
Aug 7, 2014 3:33 PM ET

Response to Eric Arey
by Martin Holladay

Eric,
It's not a great idea to sandwich OSB or plywood sheathing between two layers of vapor-impermeable foam, but people do it. According to some authorities, the use of a wrinkled housewrap between the sheathing and the exterior foam allows for some "hygric redistribution" which can lower the risk.

EPS is the most vapor-permeable type of rigid foam, so that's probably the type of foam to use. Choose EPS without any foil facing.


113.
Oct 17, 2014 8:23 AM ET

zone 4 double stud
by Timothy Godshall

I'm designing a house in Virginia - in climate zone 4, but very close to zone 5. I am debating between
1) a 10" or 12" thick double stud wall packed with cellulose, sheathed with plywood or
2) a 2x6 wall packed with cellulose and sheathed with Zip R6 panels.
3) a 10" thick double stud wall packed with cellulose, sheathed with regular Zip or Zip R6 panels

In all three cases, the sheathing would be taped to create an air barrier and I'd install housewrap over the sheathing. I'd either install vinyl siding directly over the housewrap or lapped fiber cement siding over a rain screen.

In our climate zone, is there any reason for concern with wall assemblies #1 and #2? Am I correct that vinyl siding would provide adequate drying to the exterior?

I am more concerned about wall assembly #3. I like the ease of taping the zip system that I have read about in various GBA discussion threads. But it would be nice to have the R-value of a double stud wall. In our climate zone, would it be inadvisable to sheath a 10" thick double stud wall with Zip panels due to the poor drying of that material? Would using the Zip R6 panels add enough exterior insulation to keep the dew point outside of a 10" thick cellulose-insulated wall?


114.
Oct 17, 2014 8:38 AM ET

Response to Timothy Godshall
by Martin Holladay

Timothy,
Q. "In our climate zone, is there any reason for concern with wall assemblies #1 and #2?"

A. No.

Q. "Am I correct that vinyl siding would provide adequate drying to the exterior?"

A. Yes.

Q. "In our climate zone, would it be inadvisable to sheath a 10-inch-thick double stud wall with Zip panels due to the poor drying of that material?"

A. In your climate zone, almost any type of sheathing should be fine, including plywood, OSB, or Zip sheathing -- especially if you include either a ventilated rainscreen or vinyl siding.

Q. "Would using the Zip R6 panels add enough exterior insulation to keep the dew point outside of a 10" thick cellulose-insulated wall?"

A. I don't think you have to worry about this issue, but if you want to perform the dew point calculation, here is a link to an article that tells you everything you need to know to perform the calculation: Are Dew-Point Calculations Really Necessary?


115.
Oct 20, 2014 7:42 PM ET

Roxul with Stucco
by Stephen Onstad

Any up date on the use of Roxul with a cementitious stucco finish.
In an article in 2011 Straube indicated more testing need to be done.


116.
Oct 21, 2014 3:16 AM ET

Response to Stephen Onstad
by Martin Holladay

Stephen,
I haven't heard anything about recent testing of wall assemblies with mineral wool and stucco.

That said, it's worth pointing out that mineral wool insulation is vapor-permeable, so wall assemblies with exterior mineral wool can dry to the exterior. If you can come up with details for fastening the mineral wool to the wall, installing co-planar furring strips, and installing stucco, then there is no reason to believe that this type of wall would have any moisture issues.


117.
Jun 18, 2015 1:45 PM ET

2x6 wall with ZIP R6
by David Phillips

Martin,
We are building high performance homes in Colorado, Zone 5, and your article was referenced regarding condensation point in walls. We are using ZIP R6 continuous sheathing over 2x6- 24" OC with dense pack fiberglass.
We added a 1" flash coat of closed cell foam in wall cavities on to meet the R 7.5 r value, and condensation concerns in the next house be built. There is a big debate here whether the flash coat is a necessary (big) cost. , These homes are very tight - around 1 air change per hour, have an ERV.
Any input would be appreciated. Thanks.


118.
Jun 18, 2015 2:26 PM ET

Edited Jun 18, 2015 2:31 PM ET.

Response to David Phillips
by Martin Holladay

David,
The recommendations in this article were developed to cover a range of indoor humidity levels. In theory, if there were any way to ensure the indoor humidity would always stay low, condensation (moisture accumulation) risks would drop. That would allow you to cheat on the foam thickness.

From a builder's perspective, however, the risk isn't worth it. Future indoor conditions can't be controlled by the builder. You want to build a robust wall, and you don't want any callbacks, headaches, or wet wall disasters that land you in court (or land your company's name in the newspaper). So follow the rules.


119.
Aug 25, 2015 11:15 AM ET

Flat Roof Issues
by Barry Evan

I am dealing with a 110 year old stucco single family with a flat roof. Insulation was modified about 20 years ago with blown in cellulose. Decking, and likely some roof joists, have failed. I suspect moisture and rock ballast may have played a role in the failure.
The roof configuration consists of a low pitch roof (probably 1:12) a parapet wall on three sides of the roof deck, and three structures rising above the roof deck (1) a chimney, (2) a doghouse (2’ x 2’) and (3) a hexagon shaped “turret” (approximately 4’ diameter).
Based upon your article I would like to close off the ventilation, leave or supplement the cellulose to pack it under the roof deck and install foam insulation board on the deck.
I was considering XPS on the roof deck because I read in a Fine Homebuilding article that it had superior vapor barrier properties and minimal moisture retention. But I have now read and considered some other issues, such as, polyiso has a higher R value per inch, and XPS requires a thermal barrier cover board such as high-density wood fiber when installed under a dark membrane.
Do you see a benefit either way?
Will polyisocyanate and XPS both act as a vapor barrier?
Can EPDM be direct glued to polyiso? Is there a preferred type of polyiso such as SecurShield or HP-H ? Is either of these suitable for direct glue? Is a thermal barrier cover board required?
If I were to use XPS, there are different varieties with different PSI ratings X= 15 psi, IV = 25 psi and V= 100 psi. Is there a preferred choice?
A roofer told me that using XPS would create a second vapor barrier, the EPDM being the first. This makes no sense, does it?
On the high side of the roof there may not be enough space to install 4.3 inches to achieve an R-25 (Minneapolis: Zone 6A). How much of a curb on the parapet wall would be required for EPDM? Perhaps we’ll need to raise the parapet wall to allow for a sufficient curb.
Is there suitable insulation board with a higher R rating to reduce the needed thickness?
If I install the R-25 board on the roof deck I assume I’ll need to open up the doghouse, turret and parapet wall and spray CC foam to create a continuous vapor barrier and R25 insulation.
Does the GBA have a list of roofers qualified to install flat residential roofs in Minneapolis?


120.
Aug 25, 2015 11:37 AM ET

Response to Barry Evan
by Martin Holladay

Barry,
First of all, I'm surprised that you posted your questions here. There is a more relevant article that your probably want to read; here is the link: Insulating Low-Slope Residential Roofs.

Q. "I was considering XPS on the roof deck ...But I have now read and considered some other issues ...., polyiso ...Do you see a benefit either way?"

A. From an environmental and performance standpoint, your best bet is EPS. Choose one of the denser types of EPS that will hold up to foot traffic. Polyiso performs poorly in cold weather, and XPS is manufactured with a blowing agent that has a very high global warming potential.

Q. "Will polyisocyanate and XPS both act as a vapor barrier?"

A. Yes.

Q. "Can EPDM be direct glued to polyiso?"

A. The best way to answer that question is to ask the manufacturer of the EPDM roofing product that you intend to use.

Q. "Is there a preferred type of polyiso such as SecurShield or HP-H?"

A. I don't recommend the use of polyiso in Minneapolis, which is a cold climate.

Q. "If I were to use XPS, there are different varieties with different PSI ratings X= 15 psi, IV = 25 psi and V= 100 psi. Is there a preferred choice?"

A. As far as I know, any of those would work. But check with the roofing manufacturer about the need for a cover board.

Q. "A roofer told me that using XPS would create a second vapor barrier, the EPDM being the first. This makes no sense, does it?"

A. The roofer is right. But having two vapor barriers won't cause any problems.

Q. "On the high side of the roof there may not be enough space to install 4.3 inches to achieve an R-25 (Minneapolis: Zone 6A). How much of a curb on the parapet wall would be required for EPDM? Perhaps we’ll need to raise the parapet wall to allow for a sufficient curb."

A. Perhaps you will.

Q. "Is there suitable insulation board with a higher R rating to reduce the needed thickness?"

A. Assume R-4 per inch for EPS, R-5 per inch for XPS, and something like R-4.5 to R-5.5 for polyiso, depending on the temperature.

Q. "If I install the R-25 board on the roof deck I assume I’ll need to open up the doghouse, turret and parapet wall and spray CC foam to create a continuous vapor barrier and R25 insulation."

A. I don't follow you. Where do you want a vapor barrier, and why?

Q. "Does the GBA have a list of roofers qualified to install flat residential roofs in Minneapolis?"

A. No.


121.
Aug 25, 2015 12:39 PM ET

Foam Board on Roof Deck
by Barry Evan

Thank you Martin for your fast response. Correct, I meant to enter my comments on your article: Insulating Low-Slope Residential Roofs. Can my comment be moved?

Regarding the roof penetrations:
Since the doghouse, turret and parapet walls are not covered by the insulating boards each would allow moist vapor to rise through them and moisture to accumulate on the cold surface at the top of the parapet wall, doghouse and metal capped hexagonal turret. I thought I would need to open up each of these structures and spray closed cell foam to at least R25 to be continuous with the R25 of the roof deck.
What is the required minimum number of inches of curb for an EPDM installation?
Thanks


122.
Aug 25, 2015 12:59 PM ET

Edited Aug 25, 2015 1:03 PM ET.

Response to Barry Evan
by Martin Holladay

Barry,
I don't know the purpose of the structures you describe as a "doghouse" and "turret." If these structures are intended to vent the roof, you don't need them for an unvented roof. The structures should be removed if they are unnecessary.

To achieve continuity of your insulation layer at the parapets is tricky; each building is different. Suffice it to say that you want a continuous, unbroken thermal barrier (insulation barrier) around the building. The wall insulation must be connected to the roof insulation, without any gaps or thermal bridges. Where necessary, spray foam or rigid foam boards can be used to maintain the continuity of the thermal barrier.

Q. "What is the required minimum number of inches of curb for an EPDM installation?"

A. The manufacturer of the EPDM roofing that you intend to use, or your roofing contractor, should be able to provide requirements for the roofing installation.


123.
Aug 26, 2015 11:49 AM ET

Foam Board
by Barry Evan

Hi Martin, I haven't found EPS or XPS in thicknesses above 4.5. To achieve 6 inches Is there any problem using a double layer of 3" and staggering the seams?
Should EPS or XPS seams be taped? With what?


124.
Aug 26, 2015 12:04 PM ET

Response to Barry Evan
by Martin Holladay

Barry,
Q. "To achieve 6 inches, is there any problem using a double layer of 3 in. and staggering the seams?"

A. No, there is no problem. In fact, two layers of 3 inch foam with staggered seams is preferable to one layer of 6-inch foam. For more information, see this article: How to Install Rigid Foam On Top of Roof Sheathing.

Q. "Should EPS or XPS seams be taped?"

A. Ideally, yes -- although some builders skip this step. It's a good idea to tape at least one of the two layers of rigid foam.

Q. "With what?"

A. Three tapes to consider are Siga Sicrall, Siga Wigluv, and 3M All Weather Flashing tape. For more information, see Return to the Backyard Tape Test.


125.
Sep 18, 2015 11:04 AM ET

exterior rigid foam- should i or shouldn't I
by Michael Miller

Hello Martin,
we are building a new house in north Ontario- zone 6 and have triple glazed windows. we want a tight house and very well insulated as we have extremely cold winters. our thoughts were to use a 2x6 wall filled with open cell spray foam insulation and plywood exterior sheathing. then we were going to use 3 or 4 inches of rigid EPS covered by Typar and then 1-1/2" drainage/ventilation space and wood siding. After reading your comments, I wonder if this could be a big mistake. should we move the wall sheathing to the outside of the rigid foam or possibly consider a type of structural rigid insulation like Tuff R. or maybe we should just eliminate the exterior rigid foam?
we really appreciate your help.
mike


126.
Sep 18, 2015 11:18 AM ET

Response to Michael Miller
by Martin Holladay

Michael,
I think that you need to be a little big more specific. I'm not sure why you are worried that you might be making "a big mistake."

In your climate zone (Zone 6), if you install 3 or 4 inches of EPS on the exterior side of your wall sheathing, you'll end up with between R-12 and R-16 of rigid foam. That's plenty of foam for a 2x6 wall; the interior face of your plywood sheathing will stay warm enough all winter that it won't accumulate moisture. Moreover, because you are specifying open-cell spray foam (which is vapor-permeable) between the studs, your wall sheathing will be able to dry to the interior if it every gets wet.

So, exactly why are you worried?


127.
Sep 18, 2015 1:09 PM ET

Response to Michael Miller
by Dana Dorsett

:Leave the structural sheathing where it is. The sheathing is a class-II vapor retarder, but the sheathing is also susceptible to moisture, which is the problem you're trying to solve.

As long as the average temp of the sheathing over the winter is above the average dew point of the interior air (about 4C), it won't adsorb much moisture via vapor diffusion through the open cell foam cavity fill. Putting the EPS on the exterior keeps it warm enough, as long as the ratio of the exterior foam's R value to the total center-cavity R-value is high enough. In zone 6 about 35% of the total R needs to be on the exterior to have any margin.

With 2x6 framing and open cell in the cavity, the cavity R is about R20, and with 3" of exterior EPS you'd have R12.5 on the exterior, for total of R32.5. The ratio R12.5/R32.5 is 38%, it's definitely enough.

At 4" of EPS you'd have about R16.8 on the exterior, for a center cavity total of R36.8. R16.8/R36.8 is 46%, which would be HUGE margin over the minimum necessary to protect the sheathing.

If you move the sheathing to the exterior side of the EPS it'll run colder, and it would dry toward the exterior into the ventilation gap. The amount of interior moisture arriving at the sheathing would be very small through 3-4" of EPS, which would have vapor permeance of about 1, maybe a bit less, so it won't load up over the winter. You could do that, but it would require using something else to provide the structural rigidity against racking forces, since the moment-arm of 6" fasteners for the sheathing-to-studs would be too great.

What WOULD be a big mistake is to install 6-mil polyethylene on the interior side of the assembly, since that would create something of a moisture trap between the low permeance EPS & plywood exterior, and the ultra-low permeance polyethylene.


128.
Sep 18, 2015 6:41 PM ET

exterior rigid foam
by Michael Miller

Martin,
This information is what I was hoping to hear, in terms of keeping the wall sheathing dry and avoiding future dew point concerns. From what Dana has indicated, 3" rigid will work well. And I can keep the sheathing against the wood framed wall assembly! I have decided on using the "outies" windows to help minimize any water infiltration potential and give us some decent size window sills for the cats to sit on. The windows will be needing fasteners that penetrate the 3/4" plywood window box, the 3" rigid insul, the 1/2" sheathing and anchor about 1-1/2" into the studs. Do you see any problems using long screws or will they compress the rigid insulation?

thank you for your help.


129.
Sep 19, 2015 4:13 AM ET

Response to Michael Miller
by Martin Holladay

Michael,
I'm glad you are reassured, but I'm surprised that the article on this page wasn't enough to reassure you. The information in the article (and the table that is the heart of the article) should have put your concerns to rest.

For the answers to your questions about window installation, I recommend that you read this article: Installing Windows In a Foam-Sheathed Wall.


130.
Oct 29, 2015 12:46 AM ET

"Value engineering" wall insulation options
by Timothy Oldfield

We are in the Boston area, Zone 5, I believe. The exterior wall section we designed for the renovation of our one-story house, built in 1950, was for existing and new 2x4 walls filled with dense-packed cellulose insulation in the stud cavities, 1/2" plywood sheathing, a fully adhered W. R. Grace Vycor enV-S weather restrictive barrier, covered by 2" of polyisocyanurate board insulation, furring strips, 3/4" wood siding, and "outie" fiberglass windows. The interior will have 1/2" GWB with veneer plaster and latex paint, no other vapor retarder or vapor barrier. We recently received final pricing updated from the preliminary pricing of a year ago, and the overall price has gone into the stratosphere. Apparently, this is fairly common these days in the Boston area where the construction industry is very hot. We are looking at ways to reduce costs in all aspects of the house design, the exterior walls included.

I would like opinions on making the new exterior walls (approx. 112 linear feet) of 2x6 studs with dense-packed cellulose, 1/2" plywood sheathing, a fully adhered W. R. Grace Vycor enV-S weather restrictive barrier, covered by 1.5" of XPS board insulation (R-7.5), furring strips, and 3/4" wood siding. Some have recommended the new walls of 2x6 studs with dense-packed cellulose, Huber Zip System R-Sheathing 1.5" panels (R-6.6), furring strips, and 3/4" wood siding. I am concerned that this Zip system is not suited for our Zone 5 because: the R-value is below 7.5 for a 2x6 wall, the polyisocyanurate insulation layer would be on the interior side of sheathing instead of the exterior side, and that sheathing is just impregnated OSB that would be screwed through 56 times times in every 4x8 panel, compromising its effectiveness as an air and water barrier. To economize on the existing 2x4 walls (approx. 77 linear feet), I am thinking of filling the stud cavities (which have 2-cell Infra radiant barrier aluminum foil insulation) with dense-packed cellulose or open-cell spray foam insulation, installed from the inside and not replacing the existing windows or adding the recommended R-5 sheathing for 2x4 stud walls and then needing to replace the siding. We also have approximately 62 linear feet of insulated window walls facing south, and approximately 58 linear feet of an 8" CMU wall facing east. Your thoughts? Can you recommend any other cost effective wall sections to achieve a similar R value (approx. R-26) in the exterior stud walls? Thanks for your input.


131.
Oct 29, 2015 6:07 AM ET

Response to Timothy Oldfield
by Martin Holladay

Timothy,
First of all, you will get more answers to your questions (including answers from the many intelligent members of the GBA community) if you post your question on GBA's Q&A page. Here is the link:
http://www.greenbuildingadvisor.com/qa

Second: If you want to lower the cost of your planned construction project, the best person to talk to is your builder. Your builder will have a much better idea of ways to lower the cost of the project than we will.

Third: Your wall contains some unusual features, including a radiant barrier (something that makes me go "Hunh?") and a fully-adhered WRB (a feature that probably performs very well, but which undoubtedly costs more than most WRBs).


132.
Oct 29, 2015 3:42 PM ET

Response to Martin Holladay
by Timothy Oldfield

Martin,
Thank you for your quick response. As you recommended, I posted my questions on the Q & A page. We are working with the builder on lowering costs, and at the same time I wanted to get the advice of the broader GBA community. The Infra radiant barriers were installed in the walls and rafter bays in 1950! It's interesting to read about Infra and its rise in popularity in the '50's, and then its demise. I decided to use a fully adhered WRB after reading Joe Lstiburek's essay on its benefits. Thanks again for your ongoing work on this website.


133.
Nov 22, 2015 10:02 PM ET

Edited Nov 22, 2015 10:05 PM ET.

Exterior Wall for Small House in Main
by Glen Berkowitz

Martin:

I need to ask for your help. ASAP. I'm building (together with a professional residential contractor) a cabin (small house) for myself in MidCoast Maine (04553). Climate Zone 6.

The cabin is 24' x 20', will be a single story with a 12/12 gable roof..

We've already:

1. Built an ICF Foundation.
2. Installed sill plates, floor joists, and floor sheathing (Advantec).
3. The sill plates were setback 1.5” from the outside of the ICF to accommodate 1.5” of PolyIso that was going to be installed outside of the exterior sheathing, to which the cladding (with spacer) was going to be attached.

We're now ready to build the walls.

4. Until two days ago, we were planning on installing from interior to exterior:

a. 5/8 gypsum
b. 2" x 6" studs filled with cellulose or such
c. Zip Sheathing with Taped Seams on outside of studs.
c. 1.5” of PolyIso per #3 above.
e. Some sort of product to create air space
f. Cladding (likely cedar shingles

5. But in reading your blog, we realize what we were planning to do is INADVISABLE.

Can you give us advice, asap, as to what wall system you'd recommend we use. We were to start framing the exterior walls at 9AM tomorrow (Monday) morning. But we are going to try and hold off and wait to first try and receive word back from you.

Thanks in advance for your help.


134.
Nov 23, 2015 8:56 AM ET

Edited Nov 23, 2015 8:57 AM ET.

Response to Glen Berkowitz
by Martin Holladay

Glen,
The answer is provided in this article: in your climate zone, the minimum R-value for exterior rigid foam over 2x6 walls is R-11.25. Because polyiso doesn't perform well in cold weather, it's best to consider the R-value of polyiso in this location as R-4.5 or R-5 per inch. So 1.5 inch of polyiso should be considered to have an R-value of R-6.75 to R-7.5. That's not enough.

If you want to use polyiso, you need at least 2.5 inches of polyiso for this wall. Thicker rigid foam would also work.


135.
Nov 24, 2015 11:54 PM ET

R-3 foam board
by Shara Howie

Hi Martin,
I am considering hardie board siding on my house in Colorado. It currently has wood siding and is being eaten by woodpeckers and generally not in good shape. A contractor today said he could add foam board insulation under the siding but that it can't be very thick otherwise the screws won't go in enough to meet hardie board standards. So he said he could use a "thin" foam board that has an R-3 rating which sounded very low to me. Plus I knew there were moisture risks associated with the use of foam board under siding. I found your article and from what I understand this guy is really steering me wrong? It seems like just using very long screws with thicker foam board that meets your criteria would be what is needed here? But it also sounds like we shouldn't use foam board at all if there are any moisture issues happening under the current siding. Since I have holes in my current siding I'm concerned about this. Other solutions to the insulation need?
Thanks so much for the article!
Shara


136.
Nov 25, 2015 3:16 AM ET

Edited Nov 25, 2015 3:17 AM ET.

Response to Shara Howie
by Martin Holladay

Shara,
The answers to your questions are in the article on this page.

You didn't tell us your climate zone. Colorado has four climate zones, so you will have to check the climate zone map if you haven't done that yet. Here is the link:
http://www.greenbuildingadvisor.com/sites/all/assets/htzones_with_cities...

Then you have to check whether you have 2x4 or 2x6 studs. If you aren't sure how to do this, you should ask a builder to help you determine your stud size.

One you have determined your climate zone and the details of your wall framing, you can calculate the minimum thickness of any exterior rigid foam. If you live in the warmest corner of Colorado (zone 4A), your contractor's plan would work.

If it turns out that you need thicker rigid foam, your contractor can install vertical furring strips (1x4s) on the exterior side of the rigid foam, and can attach the new siding to the furring strips.


137.
Feb 15, 2016 6:50 PM ET

Edmonton Canada
by Quinn Parrott

This is very timely for me.

We do a lot of 1.5" EPS over StoGuard waterproof air barrier with closed cell spray foam in a 2x6 wall. The HRV is standard equipment in these buildings and we will be putting vinyl wallpaper in a couple of places on one on-going project.

We're normally pretty dry up here; should I wave the client off the vinyl?


138.
Feb 16, 2016 7:08 AM ET

Response to Quinn Parrott
by Martin Holladay

Quinn,
Your methods raise a lot of red flags.

Edmonton, Alberta is on the border of Climate Zone 6 and Climate Zone 7. (Here is a link to a climate zone map for Canada.)

1.5 inch of EPS has an R-value of R-6. That is nowhere near enough rigid foam to keep your sheathing above the dew point during the winter. If you are building 2x6 walls in your climate zone, the exterior rigid foam needs to have an R-value of R-15 or more. If you are using EPS, that means that you need at least 4 inches of EPS, not 1.5 inch.

If your walls have exterior rigid foam, it's essential that the walls be able to dry to the interior. That means that you can't install any interior polyethylene or vinyl wallpaper.


139.
Feb 16, 2016 6:36 PM ET

Thanks for this - the timing
by Quinn Parrott

Thanks for this - the timing has been perfect.


140.
Feb 17, 2016 8:43 AM ET

Not a problem, exactly...
by D Dorsett

He's insulating the cavities '...with closed cell spray foam in a 2x6 wall...".

If there's at lease 2" of closed cell foam on the interior side of the sheathing there isn't a dew-point control rationale for more exterior EPS, since the closed cell foam is a class-II vapor retarder (and qualifies as a "vapour barrier" per Canadian code definitions.) There's probably more like 4-5" of closed cell foam, at abotu 0.2-0.25 perms, WELL into class-II vapor retardency. The sheathing dries toward the exterior, through 1.5" of EPS (about 1-1.5 perms.)

Bumping the EPS to 4" would make it less than 1 perm but still over 0.5 perms, which would be fine if it's all dry when assembled. It still dries toward the exterior, but very slowly.

Vinyl wallpaper has some potential for creating a moisture trap, since the drying rates through the 0.2 perm closed cell foam are glacial at best.


141.
Feb 17, 2016 8:52 AM ET

Response to Dana Dorsett and Quinn Parrott
by Martin Holladay

Dana,
Thanks for your comment. You're right, of course, about the closed-cell spray foam.

When I originally read Quinn's comment, several alarm bells went off -- which is why I responded, "Your methods raise a lot of red flags." I meant to get around to the issue of using closed-cell spray foam between the studs -- but by the time I addressed the other issues, I managed to forget about the problems raised by the use of closed-cell spray foam.

Quinn: In my article, I wrote, "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." So my vote is to stop using closed-cell spray foam between the studs.

Better choices include cellulose, mineral wool, or carefully installed fiberglass. If you insist on using spray foam, make sure that it is open-cell spray foam, which is vapor-permeable -- not closed-cell spray foam.


142.
Feb 17, 2016 11:29 AM ET

Noted. Thanks to you both.
by Quinn Parrott

Noted. Thanks to you both.


143.
Mar 2, 2016 2:31 AM ET

Cut-and-Cobble and Moisture
by Miriam Kraatz

Hello! I am new here - not a builder, but generally interested in energy saving solutions that make sense.

I have a two-story house in Zone 4-A with a first floor that now has 2x6 studs (2x4 plus a 2x2 addition). For various reasons, doing cut-and-cobble will be the least time and money consuming method to increase the insulation of this first floor. Currently, there is 2x4 fiberglass between the studs, against the outside wall, and I would like to add some rigid foam on top before installing dry-wall. I have searched this and various other articles, but cannot find a clear confirmation that my method will work well. Can I use 1 inch rigid foam? (To clarify: Outside - fiberglass - rigid foam - drywall.) Should or should I not use some additional vapor barrier?
Thanks so much!

Miriam


144.
Mar 2, 2016 4:17 AM ET

Response to Miriam Kraatz
by Martin Holladay

Miriam,
It's possible to insulate walls with interior rigid foam. Here is a link to an article on the topic: Walls With Interior Rigid Foam.

However, your situation sounds unusual. I'm not exactly sure what you mean when you write that you have a house with "a first floor that now has 2x6 studs (2x4 plus a 2x2 addition)." Did you extend the existing 2x4 studs with 2x2 strips (measuring 1.5" x 1.5") and nail those strips to the 2x4s so that the studs resemble 2x6s? If so, that was a mistake.

If you want to install interior rigid foam, the foam should be installed in a continuous layer, so that the rigid foam interrupts the thermal bridging through the studs. It's always a bad idea to cut rigid foam into narrow strips and to insert the narrow strips of rigid foam between the studs.

So, to give you the best advice, you should describe your current situation, and how you got there.


145.
Mar 2, 2016 7:57 AM ET

Response to Miriam Kraatz
by Charlie Sullivan

Miriam, I'll suggest you go ahead and submit your question as a new question is the Q&A section. That will probably help more people see it and make it easier to keep track of.


146.
Mar 2, 2016 1:02 PM ET

How I got to where I am
by Miriam Kraatz

Hello Martin,

thank you so much for your reply! I know it is convoluted, and there is thermal bridging, and it's time consuming and messy, but yes, I added 2x2 (1.5x1.5) extensions to my 2x4 studs. Here is the reason: The original studs are sturdy, but all over the place, both in parallel to the wall (i.e., they have all kids of weird angles), and when considering the direction into the room (i.e., some parts stick out a little, other parts have indentations etc.). The original wall covering was some horrible cheap ugly veneer.

The builders agreed that the studs as they were would be a little annoying to attach drywall to and possibly even problematic (this is the same reason why installing rigid foam over the original studs was not considered). A very, very handy woodworker installed the 2.x. extensions so that they create a good base for drywall. Now I have the two (1.5) inch deep cavities between fiberglass and where the drywall will be. I have had extremely pleasant experiences with air/draft sealing in another house that I live in.

Even if it takes an extra second and is not the best possible solution (which would have been tearing out the studs and old insulation, but that was not an option), I feel confident that this will lead to improvement and just want to make sure that I am not going to create a moisture risk for the house.

Also, don't scold me too harshly! I am new to this stuff, just trying my best, and someone is waiting to move into that room on the first floor now. Lol. First house. I will keep learning from mistakes!

@Charlie: I'll put it in there later tonight. Thanks for the suggestion!
Miriam


147.
Mar 6, 2016 1:51 PM ET

Wall with interior rigid foam
by Miriam Kraatz

Hello Martin, I read the article you mentioned, and it seems like my scenario will not create the moisture issues I am worried about. Would you also like me to repost my question as a new Q&A?

Miriam


148.
Mar 7, 2016 5:48 AM ET

Response to Miriam Kraatz
by Martin Holladay

Miriam,
It sounds like you are all set. Good luck.


149.
Mar 21, 2016 8:40 AM ET

With 2 in Polyiso in Zone 6 Should I reduce interior insulation?
by Michael Prisco

I finished installing 2" Polyiso foam board on the exterior of my new house in Maine in zone 6. The stud walls are 2x6. I just found the articles that say the polyiso is not really going to give me r 13 at cold temps but more like r 10 or even less. If i account for the decrease in the rating of the polyiso I no longer have enough exterior insulation. Should I consider using 2x4 sized r 13 fiber glass or r 15 mineral wool over r 19 or 21 2x6 insulation to prevent condensation?


150.
Mar 21, 2016 9:02 AM ET

Response to Michael Prisco
by Martin Holladay

Michael,
As the table accompanying this article shows, if you want to install exterior rigid foam on a house with 2x6 walls in Climate Zone 6, the rigid foam needs to have a minimum R-value of R-11.25. Now that building scientists understand that polyiso performs poorly at cold temperatures, builders in your zone should really be installing 3 inches of polyiso, not 2 inches of polyiso, over 2x6 walls.

There are three ways you can reduce the risk of damp sheathing:

1. As you propose, you can downgrade the fluffy insulation to R-13.

2. You can keep an eye on your indoor relative humidity during the winter, and aim to keep it at 30% or lower. That will reduce your risk compared to homes with 35% or 40% interior RH. However, if you ever sell the house, the next homeowner may not be as conscientious as your are, so this isn't really a long-term solution.

3. You can use full-depth fluffy insulation, but cover the studs on the interior with a smart vapor retarder like MemBrain. The smart retarder will reduce outward vapor flow during the winter, helping to keep the sheathing dry. That should keep your walls safe enough to allow you to sleep at night.


151.
Mar 21, 2016 9:22 AM ET

Response to Martin Holladay
by Michael Prisco

Martin,
Thank you for the quick and detailed response. Glad to know I have options. If I went the R-13 route would you have a recommendation on the placement of the batt? ie pushed back in to the sheathing of pulled out to the face of the stud with a gap between the insulation and the sheathing?

[Editor's note: To read the answer to this question (and to continue reading other comments), click the number "4" below to proceed to page 4.]


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