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Can Exterior Foam Insulation Cause Mold and Moisture Problems?

A thick layer of rigid foam insulation amounts to a vapor barrier on the outside of a house. Won't this trap moisture and rot the OSB?

Posted on Jun 23 2010 by Scott Gibson

Many builders add one or more layers of rigid foam insulation to the outside of a house to lower heat losses. Rigid insulation has an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of up to 6.5 per inch, but it also can be an effective vapor retarder.

Ed Welch touched off an extended discussion in the Green Building Advisor's Q&A section when he asked whether the foam would trap moisture inside walls, creating mold as well as the potential for structural decay.

His concerns seem well placed. In a cold climate during the heating season, moisture vapor inside a building is driven outward into exterior walls. When it reaches a surface that's below the dew point, the vapor condenses into a liquid. That surface is typically the back side of the exterior 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. .

Rigid foam board, especially foil-faced polyisocyanurate, creates a vapor-impermeable barrier, so the wall would have limited drying potential. Even more permeable types of insulation, such as expanded polystyrene, are vapor barriers when the installation is thick enough.

No one argues this point of view more forcefully than Robert Riversong of Vermont, who has been building high-performance houses for many years. Riversong notes the exterior skin of a house should be at least five times as vapor-permeable as the interior. Yet 2 in. of extruded polystyrene insulation -- the amount it would take to keep sheathing above the dew point in a cold climate -- create an effective vapor retarder.

Further, Riversong argues that long-term studies have shown foam-clad walls have no way of drying out in the event of even minor rain penetration. He suggests there are better ways of increasing R-values and reducing thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. , such as double-framed walls or using foam board on the interior.

He also favors cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection. and other natural materials over petrochemical plastics because they are more forgiving of the inevitable moisture problems a house will encounter over its lifetime.

A common practice, but get the details right

The practice of using exterior rigid insulation is increasingly common as builders look for ways to increase overall R-values and reduce thermal bridging, the transfer of heat through the house framing. R-values of rigid foam range from 3.2 to 6.5 per inch, making it possible to boost energy performance substantially.

Building scientists such as Joseph Lstiburek of Building Science Corp. in Boston argue that if the foam is thick enough for the climate, the back side of the sheathing never gets below the dew point. Hence, no condensation and no moisture problems. Additionally, the insulation raises the overall R-value of the wall.

And Riversong's reservations notwithstanding, adding rigid foam insulation to the outside of exterior walls is a building practice that is probably here to stay.

Plastic vapor barriers sabotage this system

There are caveats. As senior editor Martin Holladay points out, it's important to get the details right. That means controlling the intrusion of water (rain and snow) from the outside and skipping an interior polyethylene vapor retarder so any moisture inside wall cavities can dry to the building's interior. "If your details are done right, a foam-sheathed wall will stay dryer than a conventional wall," he says.

Holladay provides an overview of the issues that builders need to focus on in a useful article, How to Install Rigid Foam Sheathing.

More insulation is (almost always) better

A key question is how much rigid insulation is enough, and the answer depends on what kind of foam is used and where the house is built. Holladay provides a table to help builders choose the right foam thickness in his article, Calculating the Minimum Thickness of Rigid Foam Sheathing.

There's some additional guidance on the question in Robust Walls, an article in Coastal Contractor by Ted Cushman.

Cushman leans heavily on the research that Lstiburek's company conducted, including years of computer simulations aimed at predicting moisture levels in exterior walls. In the end, Lstiburek's team came up with a fairly simple formula in which the average temperatures of the three coldest months are averaged to create a winter design temperatureReasonably expected minimum (or maximum) temperature for a particular area; used to size heating and cooling equipment. Often, design temperatures are further defined as the X% temperature, meaning that it is the temperature that is exceeded X% of the time (for example, the 1% design temperature is that temperature that is exceeded, on average, 1% of the time, or 87.6 hours of the year).. The thickness of foam is then calculated to keep the condensing surface above the dew point.

It's not foolproof, Lstiburek admits, "but it's a very good approximation -- it gets us 98% accuracy with one easy calculation."

In the examples that Lstiburek uses, for a house in Boston, Massachusetts, several combinations of wall cavity insulation and exterior foam would allow the house to meet current energy codes while preventing condensation inside the wall. Both 1 in. of polyisocyanurate and 1 1/2 in. of extruded polystyrene would work. In general, the colder the climate, the thicker the layer of rigid foam board must be.

Moving insulation outside the box

Ironically, the use of insulation in wall cavities along with rigid foam on the building exterior can actually increase the risk of condensation if the wall system is improperly designed. The reason is that cavity insulation slows the flow of heat outward and has the effect of keeping the back side of exterior sheathing cooler, thereby making condensation more likely.

This has ledLight-emitting diode. Illumination technology that produces light by running electrical current through a semiconductor diode. LED lamps are much longer lasting and much more energy efficient than incandescent lamps; unlike fluorescent lamps, LED lamps do not contain mercury and can be readily dimmed. some builders to skip cavity insulation altogether and put all of the insulation on the outside of the walls, what's been dubbed "outsulation." Holladay discusses a technique called PERSIST (short for Pressure-Equalized Rain-Screen Insulated Structure Technique) that was developed in the 1960s by the National Research Council of Canada as an example of just how far it's possible to take the exterior insulation route.

PERSIST walls are framed with 2x4s, even in very cold climates, sheathed with plywood or OSB and then completely covered with a peel-and-stick membrane. The walls and roof are covered with at least two layers of foam insulation, up to 8 in. thick, before vertical strapping and siding is added. The peel-and-stick membrane is an air barrier, vapor barrier and water-resistant membrane all in one, and the framing stays dry in all seasons and climates.

Although the technique is expensive, it's highly effective. You can read Holladay's summary of the PERSIST superinsulation system in one of his blogs, “Getting Insulation Out of Your Walls and Ceilings.”

Riversong has been building energy efficient houses for a long time, and he makes a strong case for a non-foam approach. He advocates "building envelopes that are tolerant of occasional moisture," as well as natural building materials like wood, cellulose insulation, straw bales, earthen plasters and natural paints.

Still, foam insulation added to a building exterior can do just what it's supposed to — improve thermal performance without introducing moisture problems. The key is designing the house as a system, meaning the foam board is part of the overall design and not an idle afterthought.

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  1. Daniel Morrison

Jun 23, 2010 9:40 PM ET

Exterior foam issues...
by Armando Cobo

The key is designing the house as a system, meaning the foam board is part of the overall design and not an idle afterthought.

Your last statement is the key to it all. You can have almost any building system, whether Persist, Remote, Larsen, SIPs, ICF or stick frame, in any climate zone and as long as is designed, detailed, planned and executed correctly. You should not have problems with moisture and/or mold as long as your humidity and ventilation are controled as part of the overall system of the house. Many builders prove it every day around the country. Thank God for Building Democracy.

Jun 24, 2010 3:34 AM ET

by mike

we ran wufi models for seattle w/ a rear ventilated rainscreen over 'outsulation' and there were no moisture issues in the wall assembly.

when that assembly was moved to anchorage, moisture build-up in the wall was greater than seattle, but still very minimal and shows to dry out.

Jun 24, 2010 9:43 AM ET

retrofits vs. new construction
by j chesnut

Thank you for this excellent summary of the related issues of thermal and hygrothermal performance including the subtle differences in viewpoint from the people whose opinions we all follow closely.
The principles for low risk, high performing wall assembles, for new construction at least, seems to have been firmly established for all climate regions.
I work in a cold climate more frequently with retrofits then new construction where the opportunities to add insulation to the thermal envelope are typically from the outside in tandem with re-cladding the home. Properly detailing the walls according to the principles outlined above can quickly become cost prohibitive if we need to remove insulation from the stud cavity and remove an interior vapor barrier (which for my jurisdiction may be in violation of code).
I'd like to see a discussion focused on the challenges posed by retrofits per era of construction and per climate region.

Jun 24, 2010 8:24 PM ET

Market penetration of exterior foam sheathed walls
by Mike Guertin

A representative of a major building material company shared some independent research he commissioned regarding the numbers of builders using rigid foam on exterior walls. Custom, spec and small scale home builders had the lowest use of rigid foam on exterior walls at between 5% and 10%. Production builder use is in the 10% to 15% range. And generally when production builders incorporated rigid foam, it was part of a division-wide practice implementation.

While I think as Scott notes, more builders (and renovators doing exterior work) are incorporating rigid foam, it's still not a widespread practice. It will be interesting to see how fast and deep the move to incorporating rigid foam is over the next 5 years.

Jun 27, 2010 4:53 PM ET

documented cases?
by Paul Eldrenkamp

Does anyone have any actual documentation (photos, moisture meter readings) of water problems caused by continuous exterior rigid foam insulation--not including the well-documented issues with faulty EIFS installations--that they could post, along with their hypothesis regarding the cause of the problem?

Jun 28, 2010 7:16 AM ET

by Richard Bill

I have been thinking for years about how to add insulation on the OUTSIDE of stone walls of my house here in SW France. What problems can I anticipate that are different to those for standard housing in the US & Canada please? It is so easy to add insulation outside of thick and uninsulated stone and occasional cement block that is coated with standard crepis skin. There is no insulation in the tiled floor and only standard fibre-glass in the roof. But ..?

Jun 28, 2010 8:10 AM ET

Response to Richard Bill
by Martin Holladay

Many European contractors are experienced at adding exterior foam insulation to existing masonry buildings. These systems usually employ synthetic stucco (EIFS).

Instead of installing rigid foam to the exterior masonry, it's also possible to install metal or wood studs, hanging from the roof overhang or attached (with standoffs) to the masonry. Then closed-cell spray polyurethane foam can be installed against the exterior of the masonry and between the studs.

It's very hard to retrofit insulation under an existing slab-on-grade floor. However, it's certainly possible to retrofit vertical rigid foam board at the perimeter of the slab. The foam should extend below grade.

Jun 28, 2010 11:36 AM ET

by NewHammer

Foam below grade creates space tor termites, does it not?

Jun 28, 2010 11:43 AM ET

Response to NewHammer
by Martin Holladay

New Hammer,
Whether or not below-grade foam creates problems with termites depends on your climate. Where I live, in northern Vermont, there are no termites, so foam is routinely used to insulate the exterior of foundations. In other regions of the country, codes limit the use of exterior foundation foam.

Richard Bill is writing from southwest France. I don't know if southwest France has a termite problem.

Jun 28, 2010 12:06 PM ET

by Richard Bill

Yes. We do have a termite problem in the region; considerable, together with other cellulouse eating insects. Something else to think about, even on a stone building but with oak and other older wood interior and roof beams.


Jun 28, 2010 2:04 PM ET

Rigid board on interior?
by Doug

Riversong suggests using foam board on the interior. I've got a case now where we won't be changing the siding, but are down to the 2x4 studs. Would interior foamboard be the way to go? Portland Oregon area.

Jun 28, 2010 10:40 PM ET

Rigid foam on INTERIOR?
by John

I too would be highly interested in this technique. I am in the process of desiging a new home with rainscreen walls, wet blown cellulose and air tight drywall (no poly). In order to eliminate or greatly reduce thermal bridging I am contemplating INTERIOR foam board over the studs.

A variation would be to install strips of foam board over the studs and have the insulation contractor blow wet cellulose flush with these strips. This option would alieviate concerns about covering the entire interior surface with a low perm membrane.

Vancouver Canada.

Jun 30, 2010 2:48 PM ET

What about a house wrap over the foam board?
by B Mac

Just curious why no mention of using a house wrap over the foam board was not considered. Wouldn't a permeable house wrap help the wall system dry out and prevent moisture issues?

Jun 30, 2010 2:52 PM ET

Specialist architecture engineer
by hind abdel moneim

It's a very important report. I live in Riyadh City. I notice in many building that architects use outdoor or exterior wallpapers with special textures. I think by experience that's a weak solution. It's better to use natural building materials -- texture like stones or marble -- or eco green products in painting or any new facade building materials in new technologies world.

Jun 30, 2010 3:12 PM ET

Interior foam
by T.C. Feick

My only concern with interior foam sheathing is that the entire wall cavity would potentially cool to a dew point, thus making condensation a larger problem than with outside foam sheathing, no? I understand Mr Riversong recommends a much higher permeance to the outside, but with a well exceuted and sealed exterior foam, is there really a concern if the proper thickness is correctly installed and the interior humidity levels are adequately controlled in the winter months?

Jun 30, 2010 3:27 PM ET

Response to B Mac
by Martin Holladay

B Mac,
Adding a permeable housewrap on top of exterior foam insulation will not improve a wall's drying ability. The reduction in the wall's drying ability is due to the low permeance of the foam layer. Adding a higher permeance layer on top of the foam has no effect on the permeance of the foam below the housewrap.

Jun 30, 2010 3:29 PM ET

Foam, details, Lstiburek, and dew point
by Ed Voytovich

There is no argument that there can and probably will be problems with exterior foam if the details are not right, but that is not a persuasive reason not to use this highly effective strategy.

Look at it this way: you can fall in love, get married, and have children. There are a whole lot of details in that process that can can cause problems, but that doesn't mean that people shouldn't fall in love, get married, and have kids.

It's a good rule of thumb to follow Lstiburek's recommendations. He's the first to tell you so, of course, but the kicker is that he's usually right. I wish my financial advisor was half as reliable.

The wall only has to dry to one side. I'd worry a whole lot more about vinyl wallpaper than about exterior foam.

Finally, if you want local help and support, track down your Dow rep. They have a lot of excellent information on this assembly.

Jun 30, 2010 3:39 PM ET

Leave the exterior foam's
by Anonymous

Leave the exterior foam's seams untaped in conjunction with a permeable housewrap. This allows you to breathe through the seams as well as get the advantages of having a permeable membrane. I've heard multiple building scientist say they like this method with the wrap under the foam so you can flash/detail the windows/doors properly.

Another disadvantage of the exterior foam is that you are completely reliant on tapes and flashing that is not technically properly shingled vs the traditional method of flashing below the WRB wrap and directly against the sheathing; ASTM D2112 flashing method I do believe. This should cause concern.

Jun 30, 2010 3:40 PM ET

basic prinicples
by Anonymous

My experience brings 2 major principles to follow and then almost any wall system will work in any climate. 1 - Keep the vapour barrier on the warm side of the insulation. Note- your building may shift so if you are using your spray foam insulation as a vapour barrier - you may want a back up when it cracks. 2 - Use an "air" barrier on the exterior of the insulation - not another vapour barrier like house wrap (not all house wraps -but do your research....) Walls need to breath!

Jun 30, 2010 5:30 PM ET

Response to Anonymous
by Martin Holladay

Dear Anonymous,
Your rule, "Keep the vapor barrier on the warm side of the insulation," is a poor one.

First of all, vapor barriers like polyethylene have largely been replaced by vapor retarders like kraft paper or paint.

Second, even if I follow your rule and install poly on the interior side of my insulation (assuming the work is performed in January) -- what happens in July? Surprise! My poly is now on the cold side of the insulation. Once inward solar vapor drive begins sending exterior moisture into my wall cavity, I suddenly realize that my poly is soaking wet.

It's a simple rule, but it is wrong. There's no substitute for a full understanding of the thermal and moisture performance of any suggested wall assembly. Your simple rule won't work.

Jun 30, 2010 6:54 PM ET

vapor barrier
by Kent Hicks

Has anyone used or seen research on the working longevity of the "Membrain" product?

Jul 1, 2010 11:10 AM ET

Foam strips on studs
by Laura T

John's idea from June 28th suggested attaching foam strips on the outside of the wall framing before the sheathing goes on. I would think this would lessen thermal bridging by creating a thermal break where the studs touch the sheathing and give more space for cellulose in the cavity plus allow the wall to breathe normally. I decided to abandon the exterior foam idea on our new house after getting sticker shock on the cost of the foam and the labor. I had read the Robust Walls article and calculated that we would need 1 1/2" of XPS foam to prevent 98% of condensation. Now, I wonder how this "strip" method would work and if it could be a more affordable method?

Would I have trouble with condensation around the foam? Will I not be able to keep the dew point from settling in the walls and therefore between the studs / foam / sheathing where I could get rot? what about using EPS foam which is able to "breathe" and therefore dry with the wall assembly when it does encounter condensation?

Thanks John - I think it's a great idea but I want to get more opinions before I do it!

Jul 1, 2010 11:12 AM ET

by Laura T

I stand corrected -- John asked about interior foam -- but I want to know what others think about the same principle but on the EXTERIOR side of the stud walls?

Jul 1, 2010 12:49 PM ET

Laura T be careful
by T.C. Feick

Putting foam on the exterior of the studs prior to sheathing may mean that you do not properly brace your walls, as it will be difficult to provide adequate lateral bracing as required by code, unless you employ another method of bracing.

Jul 1, 2010 2:15 PM ET

"outsulation" complexity
by Nathan Middleton

The theory seems great however some Issues to consider with outsulation:
a) The location of door and window openings with in the wall assembly are tyipically in line with the structural wall (ie studs). This can create diagonal thermal bridging issues and extensive flashing compexity.
b) Supporting exterior finishes on the outside surface of the insulation can be problematic.
c) Relying on the walls to dry from the inside by reducing or removing the vapour barrier, is highly problematic because wall finishes vary with their individual vapour resitance capacity. These finishes vary through out the life of the building and are beyond the control of the building designers. i.e. paint vs vapour barrier paint, Wall tile etc.. If we are relying on the wall to dry from the inside, at some point in the life of the building these walls may not opperate as designed simply because a tenant may - unknowingly - paint the walls with a vapour resistant paint of decide to tile a wall.

Jul 1, 2010 2:27 PM ET

by Nathan Middleton

We have completed a seniors care facility in Ucluelet, British Columbia, Canada that uses this system of outsulation. The additional coats were justified because of the cold, rainy and highly humid coastal enviroment. The project was compeleted in 2005 and is so far working well, however it is controlled in terms of the internal wall finishes.

Jul 1, 2010 2:33 PM ET

Response to Nathan
by Martin Holladay

Your concern that an occupant's decision to paint the interior of a wall with vapor-retarding paint may lead to wall failure is misguided.

Building scientists recommend the use of vapor-retarding paint precisely because it is forgiving. It satisfies ignorant building inspectors without unduly limiting a wall's ability to dry to the interior.

One interior finish that you didn't mention -- vinyl wallpaper -- is a much more serious problem than vapor-retarding paint. Hundreds of hotels in the U.S., especially in southern states, have mold-factory walls due to two factors: vinyl wallpaper and air conditioning. This problem can be extremely expensive to remedy, because of the extent of the required demolition.

Jul 1, 2010 2:46 PM ET

Outsulation - comment reply
by Nathan Middleton

Yes - good comment regarding VP paint - VP paint perhaps is not a good example to isllustrate the point. In our Ucluelet project we infact used vapour barrier primer. Your vinly wall paper example is better.

Jul 2, 2010 12:48 PM ET

Cavity insulation causing problems.....
by Ed Welch

As the first comment advised, I think we all need to remember the "house as a system" approach. Various building systems will work correctly in various climates, as long as someone is educated in building science. And, unfortunately, there are very few, if any, silver bullets that can be applied universally. And we need to stay very aware of relative humidity, dew points, and ventilation as we tighten homes, increase insulation levels, reduce thermal bridging, etc.

Scott, one question I have relates to this statement:

"Ironically, the use of insulation in wall cavities along with rigid foam on the building exterior can actually increase the risk of condensation if the wall system is improperly designed. The reason is that cavity insulation slows the flow of heat outward and has the effect of keeping the back side of exterior sheathing cooler, thereby making condensation more likely."

Makes sense and I have heard Martin mention it as well. But how can we calculate these wall thicknesses? Depth of stud cavities related to thickness of exterior outsulation, etc. I've just started utilizing the WUFI software from the Oak Ridge National Laboratory to analyze interior moisture potential, dew points, etc Maybe that is the best method?? Has anyone had much experience with that software? I wonder if Building Science Corporation has tested it and recommends it.

At the ACI conference recently, I asked John Straub about the PERSIST method (I'm still looking for that damn silver bullet!), and its application to various climates. He explained that in extreme climates (I assume, very cold and humid), PERSIST wall assemblies may be advised, but in more moderate climates, he did not know why you would give up the added insulation.....add sprayed cellulose to a 2X6, and you get an additional R-14. Systems and thicknesses of outsulation and insulation all change with latitudes and longitudes. Is WUFI the software silver bullet that will help me sleep better at night?

Jul 2, 2010 2:10 PM ET

Response to Ed
by Martin Holladay

There are various methods available to help ease your mind when specifying rigid foam sheathing over insulated cavity walls.

Your first resource should probably be Table N1102.5.1 in the 2007 Supplement to the International Residential Code (IRC). The table provides prescriptive advice; it lists minimum foam thickness for insulated 2x4 walls and 2x6 walls in a variety of climates.

A version of the table can be found here:

You may also be interested in a calculation method promoted by Joe Lstiburek and reported by Ted Cushman in his article, "Robust Walls":

Finally, there's nothing wrong with using good old WUFI.

Jul 5, 2010 9:31 PM ET

Great...Now What?
by Doug

I am in the middle of an exterior renovation and my architect spec'd 3/4" rigid foam insulation on the exterior of my house. The house was built in the 1850's and has major gaps in the board sheathing. I was on board with the exterior insulation and had my windows built out to accept the 3/4 foil faced board.

Now I am in a panic. Minnesota has major temperature fluctuations and this house is super drafty. We took all of the old siding off and replaced the windows. The foundation has been repaired.

Should I move forward with the 3/4" rigid insulation? I get a different answer from every architect I talk!

The goal is to lower my heat bill and make the house comfortable, NOT ruin the house or have mold issues.


Jul 6, 2010 6:57 AM ET

Doug, don't panic
by Martin Holladay

Don't panic. Exterior foam is a good idea. However, your architect probably hasn't spec'd enough of it.

Minnesota has two climate zones -- Zone 6 and Zone 7. To avoid condensation problems in your wall, your foam sheathing should have a minimum R-value of:
R-7.5 for a 2x4 wall or R-11.25 for a 2x6 wall in Zone 6; or
R-10 for a 2x4 wall or R-15 for a 2x6 wall in Zone 7.

You should have a discussion with your architect and refer to Table N1102.5.1 in the 2007 Supplement to the International Residential Code (IRC).

Ideally, you'll install thicker foam. However, the fact remains that even if you stick with the thin foam, it doesn't mean your wall will turn to oatmeal. You'll just need to be a little more careful about your indoor humidity levels in winter. If you keep your indoor humidity level low, you should be okay. Refer to WUFI for more information.

Jul 6, 2010 12:28 PM ET

3/4" rigid foam
by Jesse Thompson


Having been in that same place, I'm with Martin. 3/4" is a bad thickness of exterior foam for your climate. It's thick enough to stop the moisture movement through the sheathing that has been successfully keeping the sheathing dry for all these years, but not thick enough to prevent condensation on that sheathing on your cold days once that moisture movement stops.

Go thicker would be my recommendation. Once you are headed towards exterior foam there is a minimal cost difference in going thicker, your labor cost won't change much at all.

Jul 6, 2010 1:31 PM ET

3/4" Foam
by Doug

Okay....I talked to my architect and he said 3/4" foil is fine if I install 15# felt under the rigid foam. Is this good advise?

All of my windows are built out 3/4". How can I go thicker and not make the windows look silly? I hope that question makes sense. I'm worried about the details....

This house has been such a stressful project. My roofer messed up the roof and took off with our money (we took him to court an won), found a foundation "issue." and fixed that. Tore off all of the siding and hired an architect. I showed my architect a Fine Homebuilder article about exterior insulation and we rolled with it. Now I'm 2 days from sealing up the house and run across this information....

I want to do this project once and do it right. It looks like I am in for a change order of sorts. I'm not sure my contractor or architect have the knowledge to pull this off correctly. Again....everyone has conflicting information.

My city building inspector has never seen exterior insulation and told me 3/4" "should be a huge improvement." and to "let him know how it goes." Funny....the city has no clue wither.

Thanks for your help.


Jul 6, 2010 1:52 PM ET

"It's okay if there is asphalt felt underneath"?
by Martin Holladay

Either you misunderstood your architect, or your architect misunderstood you, or your architect is ignorant.

Asphalt felt provides virtutally no R-value. I'm not sure of your climate zone or your framing thickness, but you need between R-7.5 and R-15 of foam. Moreover, R-7.5 will work only if your house is in southern Minnesota and your walls are framed with 2x4s.

It's possible that your architect is ignorant about the building science issues surrounding foam sheathing.

As I said before, you won't be the first person who has broken the minimum R-value guidelines for foam sheathing. If you end up with thin foam, be sure to maintain a low indoor relative humidity during the winter. No humidifers!

Jul 7, 2010 12:10 AM ET

Thanks, Martin....
by Ed Welch

.....for the links. Very helpful.

Jul 7, 2010 1:35 PM ET

Felt Under Foam
by Doug


Thank you for the information. I did not misunderstand my architect in regards to the tar paper or rosin paper installed under the foam. My architect instructed me to have 15# felt under the foam in case moisture did form and the paper would help keep the sheathing dry.

I am just a homeowner and I hired an architect because they are the professionals. Seems like my architect needs to learn more about exterior insulation. I showed him this blog and he basically told me that the subject was over thought.

Needless to say, I am trying to get more foam on the exterior, but I am stalled by the fact my windows are built out to accommodate 3/4" foam. The fun begins...

I guess I could just stick with the 3/4" foam and keep the humidity levels low, but that seems like a silly compromise since I have a house with the entire exterior siding removed. My architect has lost all interest in this subject and he has been paid in full, so I'm on my own.

Thanks for all of the help.


Jul 7, 2010 1:48 PM ET

"The paper would help keep the sheathing dry"
by Martin Holladay

The source of the problematic moisture is the interior of the home. If you install asphalt felt under the foam, that's where the condensation will form -- on the back side of the asphalt felt. The condensation will put the existing sheathing at risk. Although installing asphalt felt is a good idea, it does nothing to address your potential condensation problem.

Jul 8, 2010 10:43 AM ET

Exterior foam
by Josh R

I am also planning a large scale renovation/addition to my currently uninsulated 1951 cottage/ranch house. All of the existing siding and sheathing will be removed and I am planning on re-sheathing walls with OSB/plywood covered with a WRB with 3" of polyiso over that with vertical strapping installed for siding. I was thinking of using closed cell spray foam on the interior side, but after reading the above, I'm concerned that the sheathing will not be alble to dry either way. I don't know if this is such a problem if there is no water penetration, but I realize that there is no such thing as a waterproof wall. Would using cellulose on the interior be prefereable? I live in Richmond, VA- pretty hot and humid in the summer (use AC for probably 2-3 months) and can get cold in the winter, but not for long periods.

Jul 8, 2010 10:57 AM ET

Response to Josh
by Martin Holladay

Sandwiching OSB between two layers of vapor-impermeable insulation (polyisocyanurate and closed-cell spray polyurethane foam) is a bad idea.

It would be safer to install a vapor-permeable insulation (for example, cellulose or open-cell spray polyurethane foam) between the studs.

Jul 17, 2010 9:55 AM ET

How about old fashioned wall assembly?
by Todd

We are undergoing a renovation on our 1930 arts and crafts style home in Ottawa Canada. We have taken apart the entire second storey and are installing a shed dormer on one side of the house. The new section and the old section will be built out to a 2x6 wall thickness, and the combination of diagonal walls (the old slope with knee walls), the new section and the roof insulation will all require different strategies.

For the vertical walls:
The current plan was to use a traditional wall system of 6mil poly on the inside (diligently caulked, taped and sealed at every point), R22 Roxul (mineral fiber I believe) batts between the 2x6's, 5/8 plywood sheathing (or 1x12 shiplap boards on existing walls), Typar wrap (air barrier), and a cement fiber siding product (or brick on the existing walls).

For the diagonal walls:
This is a challenge because I want this area to bring up ventilation from the soffit up into the roof and there are currently only 2x6 rafters. I plan to add the 'raft-r-mate' foam vents up against the roof surface from the soffit area, up to and extending into the attic area. I would then either add spray foam insulation, or friction fit Roxul batts into the 24" cavity and then the 6mil poly on the inside. Could build out the cavity by adding some 2x2's on top of the existing rafters, this would allow me to get close to R40. The roof is directly on top of this area, and will have a 'peel and stick' membrane and steel shingles added on top (Decra type).

For the Roof:
The idea was to have the standard 'builder' type system, 6 mil poly on the inside, and blown cellulose (up to R50 if possible) on top. The attic will be fairly shallow and I am a bit concerned that air swirling from the ventilation will redistribute the cellulose. Not sure even if that is a problem that anyone has heard of before (if not, good!).

I haven't contracted out any of this work yet, however the windows I have ordered will prevent me from adding any insulation on the outside of the house. I would however like to hear some comments or feedback on the proposed strategy to see what some of you knowledgeable folks have to say about this.can

Having took Building Science in school, (from Straube as someone mentioned earlier), I do have some basic understanding of the science and engineering behind this. I can estimate here that there will be some condensation happening in the wall cavity (in the Roxul batts), however drying could occur to the outside for the vertical walls, and the overall R value would be sufficient for the purpose and 2-3 times better than before (some walls were not insulated!).


Jul 18, 2010 4:41 AM ET

Response to Todd
by Martin Holladay

The problem with an old-fashioned wall assembly is that you get old-fashioned performance.

Mineral-wool batts in a 2x6 wall give you a whole-wall R-value of about R-14 or so (after taking into consideration thermal bridging through the studs). That's not much. You should be aiming for R-40.

In Ottawa, you can probably get away with interior poly -- but the interior poly can be risky if your home is ever air conditioned. I would skip the poly and use vapor-retarder paint.

If your sections of insulated roof are framed with 2x6s, and if you include a ventilation channel, you have very little room for insulation. Roxul batts definitely won't work there. At the very least, these sections need closed-cell spray polyurethane foam. Or else you need to build down your framing to thicken your roof. If you're a student of John Straube, you should be aiming for R-60.

Jul 23, 2010 12:02 AM ET

Interior Foam Board over steel studs?
by Erik

I am currently building a house out of steel studs. The house is located in western Sonoma County, California. Because of the steel studs and thermal bridging, we are being required to add a R-4 1" foam board to the exterior of the house. Two problems I have is 1. Windows are ordered and the 1" was not taken into account. and 2. we are doing a traditional 7/8" stucco and the stucco contractors around here won't do the job if the 1" foam board is used because of possible sagging problems with the weight of the stucco.

So I have decided to go with Tuff-R (DOW) in the 5/8" thickness that would give a R-4.1 value on the interior between the steel studs and 5/8" drywall.

Is this a good idea? Or am I asking for mold/moisture problems?

Jul 23, 2010 5:34 AM ET

Response to Erik
by Martin Holladay

From your description of your plan, I don't see any reason why you will have moisture problems in your walls. However, your house will have energy-use problems.

Thermal bridging through steel studs is such a serious problem that any cavity insulation is virtually worthless. All you are going to end up with is 5/8-inch insulation for your walls. That's nuts.

You probably have a mild climate, but I don't think you should build a steel-stud house with less than 4 inches of rigid foam. If you can't find a stucco contractor to stucco your house, choose a different siding!

And by the way, it's easy for finish carpenters to install jamb extensions on your windows (or to use drywall returns to your windows on the interior of your house).

I can't imagine that your proposed wall meets California's Title 24 requirements.

Jul 23, 2010 7:24 PM ET

Response to Martin
by Erik

First off Martin, thank you for such a quick response!

The house is in climate zone 2. When the energy calcs were originally done, they were based on 2x6 steel studs at 16 o.c. with 1/2" drywall. The specific requirements for the 2x6 walls was R-19 plus R-4 continuous insulation. So at this point we would meet the requirements of Title 24.

But I am kind of torn on what to do now. We will be doing exterior sheathing on the house next weekend, so obviously I've waited too long to start dealing with this. I need to satisfy Title 24, but I don't want to spend money on something that won't make a difference!!

Jul 24, 2010 5:28 AM ET

Response to Erik
by Martin Holladay

Trust me -- thick exterior foam over steel studs will definitely "make a difference." Try to find a way to include 2 or 4 inches of exterior rigid foam. You won't regret it.

Jul 25, 2010 3:15 PM ET

foam behind Hardie board
by Gary H

The above discussion raises a question for me. My circa 1947 house has 2x4 stud walls with blown-in cellulose in the walls. When the cellulose was being blown it started cracking the old brittle sheet rock, and the installers "turned down" their machine, so I expect that the cellulose is not packed tightly and may not completely fill the cavities. I badly need new siding, and am leaning toward Hardie plank. Am I correct in thinking that the simplest way to avoid moisture problems is to wrap the old wood plank sheathing with Tyvek, and to install the siding directly over the Tyvek, with NO fan-fold or foam insulation? With a hip roof, and very narrow venting soffits, I am not interested to add more thickness to my walls.

Jul 25, 2010 5:04 PM ET

Response to Gary H
by Martin Holladay

Gary H,
The simplest way to avoid moisture problems in your walls is to do a superb job of flashing and to install a rainscreen air gap between your siding and your WRB (Tyvek in your case). The cause of most moisture problems in walls is wind-driven rain that saturates the sheathing.

Jul 26, 2010 3:57 PM ET

Foam on Dormers
by David B

I am going to be replacing the windows on 2 dormers. I live in far north central Minnesota where temps in winter frequently hit -40. The house was built in the 1940's with no attention to thermal performance. The dormer is built with 2x4's with little to no cavity insulation. I was planning on adding 4 inches of PIR insulation around the dormer vertical walls and fill the areas above the ceiling with cellulose insulation.

I wanted to make sure my plan was sound. I obviously would be required to extend the roof of the dormer out further past the added 4" of insulation. What is the detailing though where the dormer wall meets the main roof deck? Do I cut the insulation to be flush with the roof deck then use regular metal roof flashing
like you normally would?

It also seems a bit tricky with the batten boards that would be required with 4" of insulation on the dormer. I assume one of the batten boards would need to be placed parallel to the roof deck angle and the roof flashing would be behind that batten board?

I included a link to the picture of the dormers I am talking about.

Jul 26, 2010 4:08 PM ET

Response to David B.
by Martin Holladay

I'm not sure what you mean by PIR insulation, but I'm going to assume you are talking about polyisocyanurate.

You'll need to strip the dormer cheeks (the walls) down to the wall sheathing, and you'll need to strip your roofing down to the roof sheathing in the vicinity of the dormer. You need to install your rigid foam with attention to air sealing. The perimeter of the rigid foam needs to be caulked before it is installed; it should extend down to the roof sheathing. The gap between the roof sheathing and the new wall foam should be caulked.

Foam seams should be taped with housewrap tape.

I would install peel-and-stick membrane between the new wall foam and the roof sheathing. Then I would install my vertical strapping to create a rainscreen. I would install step flashing when re-shingling the roof (repairing the roof), with the step flashing butting up to the outside of the vertical strapping. (I know, there could be water behind the step flashing. But trust me, it'll be fine. That's where you want the step flashing to go. And the peel-and-stick will handle wind-driven rain that gets past your siding.)

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