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Vapor Barriers Are a Good Thing, Right?

Vapor barriers are supposed to stop vapor diffusion through roofs, walls, and floors. But they can also can trap moisture, causing rot and mold.

Posted on Aug 17 2010 by Scott Gibson

At the dawn of our current interest in building science, sheets of polyethylene were routinely stapled to interior framing before drywall was installed. The idea was to block the flow of water vapor into exterior walls. (Some builders tried to make their polyethylene seams airtight, so that the poly would do double duty — acting as an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. as well as a vapor barrier.)

Installing a vapor barrier (or more properly a vapor retarder) was considered cutting-edge.

But in time, builders began to see flaws in their approach. For one thing, it was virtually impossible not to damage or puncture the barrier during installation or after it was applied. The plastic sheeting was usually full of holes by the time the house was complete. As it turns out, these holes did not affect polyethylene's performance as a vapor retarder — a ripped, torn vapor retarder works very well. However, these holes undermined the poly's performance as an air barrier.

Since polyethylene reduces the ability of a wall to dry to the interior during the summer months, some builders began to worry the vapor barriers trapped moisture inside walls where it could do a lot of damage.

Thus began a muddying of the waters: when and where are vapor barriers a good idea?

Sandra Heiser wades into this debate with a Q&A post that's the subject of this week's Q&A Spotlight.

Leave the poly in, or take it out?

Heiser writes about a house near Buffalo, N.Y., that is ready for drywall. She describes the exterior wall assembly as follows: 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). siding, 1 in. of rigid polyisocyanurate foam, 1 in. of sprayed closed-cell foam, fiberglass batts, and, finally, clear plastic.

"I'm thinking [the plastic] has to go," she writes. "What are our options? Is it possible to vent the plastic with strategic slits in the plastic?"

Senior editor Martin Holladay has suggested before that exterior foam will prevent condensation, says one anonymous poster, so doesn't this mean the poly can stay?

"No, wrong," replies Dan Kolbert, a builder in Portland, Maine. "There's always moisture somewhere, and it has to escape either inward or outward or it will rot things out (sometimes with remarkable speed)."

And it's no little bit of moisture we're talking about, adds Robert Riversong.

"A typical 2,200 SF house, with 17,600 board feet of lumber, will release 300 gallons of water just from the lumber (never mind the concrete and other water-based materials like paint) in the first heating season as the wood acclimates from 19% moisture content (KD standard) to the typical 10% in a heated house," Riversong says.

Slitting the vapor barrier will only allow more air movement into the walls and ceilings, but it won't do much to prevent condensation, Riversong adds. No house in the lower 48 states should require an interior vapor barrier. Codes only require a 1-perm vapor retarder, such as a vapor retarding primer.

Does exterior foam help or hinder?

A related issue is the layer of rigid foam insulation on the exterior of the building. Polyiso foam is a vapor retarder, meaning the described wall assembly has a vapor barrier on both the inside and the outside.

If water gets in, how will it get out? And will water vapor condense on the inside of the 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. ?

Riversong is no fan of exterior foam and suggests a good rule of thumb is to keep the outer skin of the house at least five times as vapor permeable as the inside. "This means no exterior foam or ZIP wall or self-adhering flashings," he says.

"The more we slow down heat flux through a building assembly, the more we inhibit its ability to dry," he adds.

"Most of what passes for 'smart' building today has significant unintended consequences. Using relatively impermeable foam to insulate our homes will ultimately be understood to have caused more problems than it was intended to solve."

Sorry, writes Holladay, I disagree.

Holladay recommends omitting the interior polyethylene, noting that condensation occurs when it's cold, rot when it's warm. Condensation forming as frost on the back side of the sheathing won't damage the house because temperatures are too low. As long as the moisture can evaporate to the interior when the weather turns warm, the house will usually be fine.

"That said," he adds, "it's better to design walls that don't allow condensation to occur. The thicker the exterior foam the better...there is accumulating evidence that exterior foam helps protect wall sheathing from moisture problems.

"Needless to say, walls need to be able to dry out, which is precisely why I advised Sandra to get rid of the interior poly."

Beware of bulk moisture problems

Water vapor is but one threat to the house.

"The main concern for Sandra now and in the future is bulk water leaks from the exterior that gets past exterior foam board to OSB [sheathing]," says an anonymous poster.

"The OSB will then rot easily because any water getting in her foam sandwich will not evaporate fast enough to avoid decay starting. Once decay starts it takes very little moisture to keep the process going. Window and door flashing must be done very well. "

Leaks, ice dams, mechanical systems leakage, wind pressure, stack-effect pressures and water intrusion via capillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules. action all are potential sources of water, Riversong says, all the more reason to avoid trapping moisture with well intentioned but ultimately misguided wall assemblies.

Quoting building scientist Joseph Lstiburek, he adds: "Things get wet from the inside, the outside, and they start out wet."

"When the rate of wetting exceeds the rate of drying, accumulation occurs."

"When the quantity of accumulated moisture exceeds the storage capacity of the material, problems occur."

"Ideally, building assemblies should be designed to dry to both the interior and exterior. In heating climates, the primary drying potential is to the exterior."

"The drying potential of an assembly decreases with the level of insulation and increases with the rate of air flow."

"As such, energy conservation has the potential to destroy more buildings than architects."

We asked GBA advisor Peter Yost for an expert opinion:

You are right to be concerned when you have two vapor impermeable (or nearly so) layers on opposite sides of the assembly.

1. Get rid of the poly

Given how you will be "warming" the air and vapor permeable components of the assembly, you won't have trouble avoiding dewpoint temperatures during the coldest months of the year, unless you are running very high (above 50% interior relative humidity) during that same period. And remember that vapor pressure or vapor diffusion is a field effect; if you have 90% coverage of a plane, you have a 90% effective vapor barrier or retarder.

2. Think about the assembly, not the parts

Don't worry about the vapor permeability of just one designated layer. Instead, think about the vapor permeability of all layers. And be just as focused on drying potential as you are about wetting. Take a look at my blog on vapor profiles as a way of addressing assemblies in this manner.

Image Credits:

  1. Don Mannes, Fine Homebuilding # 204

Aug 17, 2010 6:37 PM ET

by Garth Sproule

95 % of the homes built in the last twenty or even thirty years, at least here in western Canada, where built with stick frames, fiberglass bats, and poly on the inside to act as a vapor barrier and in a lot of cases, as the air barrier as well. They are still allowed to build this way which is a bit ridiculous considering our climate.

My question is, is there a safe way to add insulation to the exterior of these homes??

Aug 17, 2010 11:28 PM ET

Vapor Barrier
by Raymond Pruban

This is a great article and in Minnesota builders are still required to use the vapor barrier unless the install a minimum of 2.5" of closed cell insulation. Our wall system we have been using for the last few years is the DOW SIS exterior structural insulated sheathing and a minimum of 3" of closed cell insulation. We use 2 x 6 walls 24" on center, but have had to learn all the extra backing required in order for siding, corner trim, etc having something to attach to.

Aug 19, 2010 12:19 PM ET

wall construction
by Ken ZiegelbauerAnonymous

Just a couple of weeks ago we were being told that there is now evidence that during the air conditioning mode a house is subject to moisture problems on the outside face of the interior vapor barrier, especially if the house has wood siding.

Who are we to believe ?

Aug 19, 2010 12:26 PM ET

Exterior Insulation
by Geoff Briggs

Mr. Riversong quotes Joe Lstiburek in support of his argument against exterior insulation. But Mr. Lstiburek, and the Building America approach that he and his colleagues helped develop, advocates just that. Here's a quote from the Building America Best Practices Series: Volume 5:

"Putting the vapor diffusion retarder roughly in the “middle” of the assembly by installing impermeable or semi-permeable insulating sheathing (such as unfaced, rigid, extruded polystyrene foam insulation) on the exterior of a frame cavity wall filled with permeable insulation. This is the system recommended by Building America teams."

This is the approach that we have adopted in our practice for wood frame construction. Air sealing at the sheathing plane allows the envelope to be pressure tested before insulation is installed. Exterior rigid foam is applied to a thickness that insures the sheathing remains above the dew point, based on climate. Wall cavities are filled with dense pack cellulose or similar semi-permeable insulation allowing drying to the interior. Exterior siding is applied over a vented rainscreen.

And of course interior humidity levels are managed with mechanical ventilation.

Aug 19, 2010 12:47 PM ET

Response to Ken Ziegelbauer
by Martin Holladay

Ken Ziegelbauer,
The advice given here at GBA has been consistent: in all but very cold climates (e.g., the Canadian prairie provinces, interior Alaska or Arctic Canada), interior polyethylene does more harm than good.

When GBA recommends the use of an interior vapor retarder, we are usually referring to kraft facing, MemBrain, or vapor-retarding paint.

Aug 19, 2010 1:07 PM ET

vapor barriers
by Ken Ziegelbauer

I should have qualified my resource. I did read in the last couple of weeks about support for more rigid insulation on the outside to offset the effect of the air conditioning season. I don't agree with that.

Aug 19, 2010 1:23 PM ET

Response to Ken
by Martin Holladay

Well, you may not agree with it, but it's an excellent way to insulate a home. Thick exterior foam stops thermal bridging through studs, warms the wall cavity (and thereby prevents any condensation from forming in the wall cavity), and stops inward solar vapor drive during the summer.

Aug 19, 2010 1:47 PM ET

What do you have against architects
by Lloyd Alter

I bristled at the last line in this article "As such, energy conservation has the potential to destroy more buildings than architects." There are lots of architects who care about this stuff, who are working to save buildings, and read Green Building Advisor.

Aug 19, 2010 2:06 PM ET

Response to Lloyd
by Martin Holladay

Don't paint GBA with such a broad brush, Lloyd. You are quoting a summary by Scott Gibson, who is in turn quoting a GBA reader named Robert Riversong, who posted a comment on a GBA Web page in which he quoted Joseph Lstiburek, a well-known building scientist who delights in being provocative.

I tracked down the Lsitburek quote to its source -- a Web page where Robert Riversong probably picked it up. (If you are interested, here it is: .)

This GBA page -- one of a series called Q&A Spotlight -- attempts to draw new readers into our Web site by summarizing a recent exchange of views on our Q&A page. The quoted statements are the opinions of the posters, not GBA.

By the way, Joe Lstiburek is married to an architect. When an architect recently accused him of architect-bashing, he responded, "You should hear what I say about engineers." (Joe is an engineer.)

Aug 19, 2010 3:14 PM ET

ice dam caused leaking and interior ceiling damage
by Bradleyman

We live in SW Saskatchewan and our winters are harsh along with the occasional "chinook" which can drive the temperature to change 100 degrees in just a few hours {-50 to + 50}. We live in a reconstructed 1914 house with double studded framing {2x4 interior, 2x6 exterior staggered }. There is insulation in both walls as well as 1" styro on the exterior sheathing and a 1" layer of stucco and a vapor retardant paint. Our roof is metal and above the ceiling is 2x8 with 8" pink insol bats.

I think that the avalanche flashing caused the snow to build up and then turned to ice an began to creep up till it backed up under the edge flashing for our turret .

How can I repair this without serious roofing removal and reconstruction ? How can I prevent this from happening again. Do I remove the avalanche flashing... the eavestrough ? Do I need to add more insulation ? Venting ? motorized venting ?
Please help advise !

Aug 19, 2010 3:24 PM ET

Response to Bradleyman
by Martin Holladay

I'm not sure what "insol bats" are; I'm guessing you mean fiberglass batts. If that's what you've got, then your ceiling is insulated to about R-25 (if the insulation is very well installed). In Saskatchewan, you want R-60 insulation in your ceiling. Your ceiling is leaking a lot of heat.

The way to prevent ice dams is:
1. Seal all air leaks between the heated interior and your attic, and
2. Improve your insulation layer.

When contractors can't perform these two tasks properly, they often recommend other measures to get rid of the leaking heat (for example, ventilation) or to prevent the ice dam from damaging your ceiling (for example, installing a rubber membrane under your roofing). But these two measures are signs of failure. They are only necessary for badly designed roofs.

My guess is the suggested work is more than you can handle, so you should probably hire a contractor to improve air tightness and insulation in your ceiling. Often, the best approach is to use spray polyurethane foam to address these problems.

Aug 19, 2010 11:16 PM ET

Wall vapor barriers
by Mike Blake

I don't see a clear consensus on this issue. Except Builder beware! No win situation it looks like to me. Why isn't there a clear standard. Even code built will be wrong it seems. I had problems with a unfinished basement where we had kraft faced insulation installed on a walk out exterior wall. The inspector called it a fire hazard and had to cover it or remove it the owner wanted to finish it later (never did) so we removed the Kraft facing. over the winter the OSB frosted & melted till warping and mold were an issue. would driwall prevented this or simply covered up a problem?

Aug 19, 2010 11:48 PM ET

by Francois Theriault

I'm considering an exterior wall composed of stucco, double wall paper vented inferiorly, 2" of polyiso, plywood, 2x6 filled with fiberglass bat, and no poly to enhance inward drying. This appears to meet the recommendations of Dr. Joe and others. Two concerns - 1) the need for a second rainscreen at the outer face of the plywood, and 2) the effect of high humidity in the bathroom. Does a vented rainscreen not increase the risk of condensation on the plywood during the heating season? Will high-humidity areas in the house not cause longterm harm? (Zone 6 - Okanagan Valley in B.C., 3500 heating degree days, 5 foot overhang).

Aug 20, 2010 4:36 AM ET

Response to Mike Blake
by Martin Holladay

Mike Blake,
Unfortunately, there is no substitute for the need for builders to study a little bit of building science so they can keep up with these issues. You're right that building to minimum code standards isn't enough to stay out of trouble.

If you install fiberglass batts in an exterior wall of a walk-out basement, you're asking for trouble. Basements are often damp, which means that the interior air has a high relative humidity. Fiberglass does very little to slow the flow of air, so your warm, humid indoor air has easy access to the OSB sheathing. The predictable result is lots of condensation on the back of the OSB. (By the way, OSB is the most vulnerable available sheathing -- much more likely to turn to oatmeal when wet than other types of sheathing like plywood.)

Fiberglass batts are the worst available insulation. In some applications, like yours, they can lead to real trouble. Your problems could have been easily solved with an adequately thick layer of exterior rigid foam. Another way to stay out of trouble would have been to insulate your basement walls with closed-cell spray polyurethane foam.

By the way, your building inspector was absolutely correct when he informed you that it is illegal to leave kraft facing exposed. It's a fire hazard.

Aug 20, 2010 4:41 AM ET

Response to Francois Theriault
by Martin Holladay

Francois Theriault,
Your proposed wall assembly should work fine. You don't want a rainscreen air gap between your plywood and your exterior polyisocyanurate -- the air gap would undermine the value of the insulation and make your plywood cold. If the plywood ever gets wet, your wall is designed to allow the plywood to dry to the interior.

There are two ways to address high indoor relative humidity in the bathroom:
1. Paint your walls with vapor-retarding paint.
2. Install a bathroom exhaust fan and use it.

If you take those steps, everything should be fine.

Aug 20, 2010 9:57 AM ET

perm ratings and condensation
by Andy Mueller

As a straw bale home designer and builder I have found that straw bale wall systems with perm ratings around 10 perform quite well in the harsh New England climate. These wall assemblies rarely experience the condensation issues so commonly found with todays conventional building practices.

Aug 20, 2010 3:57 PM ET

To follow up-The question is
by Andy Mueller

To follow up-The question is about “Green” insulating products in R&D which claim to posses the same bulk moisture storage and water vapor dissipation properties that of a plastered straw bale wall system? I understand that straw bale construction is still a leap of faith for many green builders and I would like to know if there is a transitional insulating product that isn’t high in embodied energy, recyclable, user-friendly, minimized failure and above all is affordable.

Aug 20, 2010 5:17 PM ET

Nonsense without accounting for climate
by Steve P

I live in the NorthEast USA. Almost every winter, we hit 30 below zero temperatures. This can go on for a week and then be followed by a week of above-freezing weather and substantial rain.

Few (if any) homes have central AC, and what AC is used amounts to one or two window units used a few days of the year. But the relative humidity for June to September will still be quite high - it's simply the climate we enjoy.

The Prairies actually have it better. It may be nominally cooler, but it is also much drier most of the year. In Maine, I'm not sure there even is a "dry" season.

There are many valid points in the above story and comments, but they all seem to be searching for some "magic pill" - the "one way" of building that will work everywhere (maybe even on the moon :-)

In Canada, they have been building "super-insulated" (by US standards) R2000 homes for 25+ years. You will note most of Canada's population lives within 100 miles of the US border, so it stands that most of these homes would be close to the northern USA.

Of course there were problems. First, builders don't like conforming to standards. Homeowners didn't like running HRVs all the time. But the principles of R2000 are still valid and applicable to similar climates (with adaptations for new technology).

That means never trapping moisture within an assembly, controlling human-produced moisture at or near source (with HRVs) and making sure liquid water has no ingress from the outside (above and below ground).

Personally, I am adverse to spray foam. As mentioned, new wood contains a lot of moisture. Spray foam creates such a hermetic seal that - unless it is encasing inorganic, moisture-free material - increases the chances of locking in that moisture (and any added from structural movement or design failure).

Time will tell.

Aug 20, 2010 6:29 PM ET

Now what really makes sense . . .
by Capn John

from this point of view, is balloon construction. Air from a vented basement space rises up through the walls carrying moisture to the vented attic--we hope--with it.

Aug 20, 2010 9:44 PM ET

exterior foam board insulation
by glen

is it a good idea to add foam board to the exterior sheeting to increase the total wall r value when residing and new windows

Aug 21, 2010 4:23 AM ET

Response to Capn John
by Martin Holladay

Capn John,
I hope your comment was tongue-in-cheek. It is certainly true that most attic moisture problems can be traced to a damp basement or crawl space -- and an inadvertent air leakage connection. Weatherization contractors see this problem all the time.

Needless to say, deliberately designing a house with such an air leakage connection would be a big mistake.

Aug 21, 2010 4:25 AM ET

Response to Glen
by Martin Holladay

You asked, "Is it a good idea to add foam board to the exterior sheeting to increase the total wall R-value when residing and installing new windows?"

The answer is yes, as long as:
1. The rigid foam is thick enough to prevent condensation in the walls, and
2. The installer does a good job flashing windows and penetrations.

In most cases, such exterior foam retrofit jobs require the inclusion of a rainscreen gap between the foam and the siding.

Aug 21, 2010 8:27 PM ET

What to do with plastic?
by Dan Freiheit

When I was in school my instructor said to put the plastic on the inside, but back then we used black board on the outside. From all the commets it seem to me that the zone is key to the problem. If it's 7-8ish then think plastic. As you go to warmer areas the progam has to change.

Aug 22, 2010 8:06 AM ET

by Marc

My house is in the Mid-Atlantic region and is constructed of concrete block walls with the brick front. As I have been remodeling, I have been lining the block walls with 1" rigid foam in the basement with the DOW fiberglas free batts inside metal stud walls. Is that a mistake? Additionally, I have a wood framed addition which I planned to remove the exterior siding and replace it with 1" rigid foam and Certainteed foam backed siding, and finish off with the 1" interior and dow battts. Any advice?

Aug 22, 2010 12:23 PM ET

Response to Marc
by Martin Holladay

I haven't heard of "Dow fiberglass-free batts." What are they made of?

Aug 22, 2010 3:55 PM ET

Increasing air flow in your personal steam shower
by John Whipple

"The drying potential of an assembly decreases with the level of insulation and increases with the rate of air flow." * from the above article

I have been experimenting with air flow in my steam shower renovations. I have built a few mock ups and have nearly finalized my design. The design is simple - move air through the steam room to speed up the drying times. These designs are so sleek and contemporary today and a busy steam shower can be in a constant state of saturation if used by the entire family at multi times in the day.

By moving air through the steamer the rate at which it dries out is increased dramatically. With a quick squuegee a typical 4'x5' steamer dries out before you are ready to go to bed or out to work.

Aug 22, 2010 9:19 PM ET

DOW Fiberglass Free Batts
by Marc

I am not sure. I trashed the packaging. They are sold at Lowe's and require no gloves, they don't itch and can be cut with scissors. There is a clear backing membrane if you will, that we left on against the inside of the house against the sheetrock. It states that it is not a source of food for mold and that mold cannot grow under any circumstances. It looks like blue jeans.

Aug 23, 2010 7:11 AM ET

30 yr experience w/ poly
by Anonymous

When we built our house in '78-'79 (eastern VA), we put poly on the interior walls even though some people were starting to recommend against it at the time. We have recently done some remodeling for a new fireplace, sliding glass door replacement, and repairs to rot from improperly flashed decking. In all cases we had to tear into the old wall structure. There was no sign of moisture damage or condensation due to the interior poly in any of the locations.

Aug 23, 2010 7:37 AM ET

Response to Anonymous
by Martin Holladay

Your situation is typical. Like you, I built my house with interior poly, and subsequent renovation work has not revealed any problems. Most houses with interior poly are not experiencing problems.

The worst-case situation for interior poly involves air-conditioned houses with stucco, manufactured stone, or brick veneer over a vapor-permeable sheathing (without exterior foam). Those houses usually have serious problems.

Of course, new-home builders can't be reassured by the fact that 90% of homes with interior poly are performing well. If they were to choose a construction method that resulted in 10% failure, they would soon be out of business.

Aug 23, 2010 7:49 AM ET

Response to Marc
by Martin Holladay

It sounds as if you have been installing Dow SafeTouch batts, which are manufactured from polyethylene terephthalate fibers. According to my research, the plastic facing is a vapor barrier.

You wrote, "I have been lining the block walls with 1" rigid foam in the basement with the DOW fiberglass-free batts inside metal stud walls. Is that a mistake?"

My guess -- yes, it's probably a mistake. You don't want a vapor barrier in that location. Moreover, I'm not sure whether polyethylene terephthalate fibers behave better or worse than fiberglass when used in a damp location like a basement. I wouldn't recommend fiberglass for basement walls.

According to the Dow Web site (see above link), "SAFETOUCH™ Insulation should not be used in the exterior walls of Types I, II, III or IV construction, per the International Building Code. It is important to understand the building code that is being enforced in the location of building design to ensure compliance. If foam plastics are used in the wall constructions, then the exterior walls are required to meet the requirements of Section 2603.5 of the IBC. Specifically, SAFETOUCH™ Insulation has not been tested per Section 2603.5.5 (NFPA 285 Test) of the IBC in an exterior wall system that includes foam plastics and is therefore not approved in this application."

Although the Dow Web site is terribly written -- wake up, Dow! Give us the facts! -- it seems that they are admitting that their batts are not vapor-permeable enough to comply with the requirements of the IBC for Types I, II, III or IV construction. If anyone from Dow is reading this, feel free to tell us all what your Web site really means.

I would suggest that you use only rigid foam or closed-cell spray polyurethane foam on a basement wall.

Aug 23, 2010 9:48 AM ET

rigid foam insulation
by vi

does anyone have a opinion on using 1"ridgid blue foam sheets to cover the underside of a floor with or with out fiberglass batts. there is a 4' crawl space that has 2"extruded foam on exterior walls. the house is in Montana, very little humidity and winter temps that get to zero and below quite ofter during dec. and jan.

Aug 23, 2010 10:03 AM ET

Response to Vi
by Martin Holladay

Sealed conditioned crawl spaces make more sense than unconditioned crawl spaces. If you have insulation on your crawl space walls, and if your crawl space has no vents, then you should remove all insulation from the joist bays above the crawl space.

Be sure that you have a ground cover (heavy polyethylene) on your crawl space floor. You probably want to add more rigid foam to your crawl space walls.

Aug 23, 2010 10:45 AM ET

by vi

additional info on the above ?. the floor joist are 11" boise casscade manufactured joists, the sub floor is OSB. the foam would be attached the the bottom of the floor joists, encapsulating the entire bottom with the seams taped. if i use fiberglass batts they would be staped to the bottom of the sub floor with the vapor barrier up toward the sub floor. the purpose would to be to increase the r value of the floor and derease the use of fossil fuels during the cold months. any insight would be great. thanks

Aug 23, 2010 10:47 AM ET

by vi

we have exterior vents that we close and seal during the winter months, also we do have poly on the raw dirt floor.

Aug 23, 2010 10:48 AM ET

Response to Vi
by Martin Holladay

Your vents should be permanently sealed. Insulate your crawl space walls, not your joist bays. You want a conditioned crawl space.

Aug 23, 2010 10:56 AM ET

thank you.
by vi

thank you.

Aug 23, 2010 11:13 AM ET

Vapor barriers
by gotchaa

i did not see any mention of open and closed-cell foam in walls, etc. This does same things but I believe to a greater degree. Why wasn't it mentioned?

Aug 23, 2010 11:19 AM ET

Response to Gotchaa
by Martin Holladay

What your point? I'm not sure what you are driving at.

Walls that include open-cell spray foam or closed-cell foam still need to be analyzed from a hygrothermal perspective to be sure that the wall assembly is able to dry. Just because you use spray foam doesn't mean you can ignore a moisture analysis.

Open-cell foam is quite vapor permeable, and several builders have had wet-wall and wet-ceiling problems in homes insulated with open-cell spray foam. The usual cause of these problems: high indoor relative humidity, and no vapor retarder on the interior side of the foam. In a cold climate, this combination spells trouble.

Closed-cell spray foam is an effective vapor retarder.

Aug 23, 2010 1:49 PM ET

Humid South
by Anonymous

Remember that this problem is reversed in the humid south. In Baton Rouge today there will be high humidity and probably a 95-98 temp. Water vapor from the outside can condense in wall because of the sheetrock side is cool from the air conditioning. This is a particular problem where the ceiling vent for the air handler is blowing cooled air on an exterior wall, which may cool down dramatically.

Aug 23, 2010 8:48 PM ET

Shower/Steam Room on Outside Corner of House
by Sam

Timely discussion. I think I am convinced no poly on interior of walls (house is in DC area).
So for remodel will use "Modified" GBA Wall = latex paint/drywall/2*8 studs & unfaced fiberglass (can't afford 2lb foam)/0.5" Ply Sheathing/1.5-2" Closed Cell Foam (taped & sealed as best I can)/0.75" RainScreen/Cementitious Siding -- So question is: for shower/steam room on outside wall do I omit outside foam and use air/bulk water barrier (kills wall R value)? Or what? House is very small so using false wall(s) is not optimum. Moving the room to be all interior is possible but $$. Any way to vent stud cavities that has some chance of working?

Aug 23, 2010 11:17 PM ET

Reflective Heat Barrier
by Anonymous

I've seen pitched, reflective foil barrier for exterior walls and attics. Any experience and recommendations for or against this stuff?

Aug 24, 2010 5:04 AM ET

Renovation of older home
by Pat

I am adding insulation to an existing brick/block home in Cincinnati - in most cases I have added 1 1/2" rigid foam directly on the block. In a few cases I have installed stud walls with kraft faced fiberglass batting. I'm wondering if there is the potential for water problems as a result, particularly with the fiberglass batt construction....any comments?

Aug 24, 2010 5:27 AM ET

Response to Sam
by Martin Holladay

You wrote that you plan to install "1.5 - 2 in. Closed Cell Foam (taped & sealed as best I can)" on the exterior side of the plywood sheathing. Do you mean rigid foam board (like XPS or polyiso)?

In the vicinity of the steam room, I would suggest one of two approaches:

1. You could switch to 1.5 in. or 2 in. of EPS foam -- slightly more permeable XPS or polyiso -- and include MemBrain on the interior.

2. You could use any type of rigid foam on the exterior, but instead of using fiberglass in the steam room, you could use spray polyurethane foam between the studs (for example, one or two portable Handi-Packs).

Aug 24, 2010 5:34 AM ET

Response to Anonymous
by Martin Holladay

You asked a question about radiant barriers. Here's what the GBA encyclopedia has to say on the issue:

"Radiant barriers are shiny sheets of material—aluminum foil, for example—with a low-e (low emissivity) surface. When an air space has a low-e material on one or both sides, the R-value of the air space or building assembly increases.

"The effect of a radiant barrier on a building assembly's R-value may be significant or insignificant, depending on whether the assembly is well insulated or poorly insulated. Radiant barriers do not significantly benefit well-insulated assemblies. A poorly insulated assembly, however, will benefit from a radiant barrier. That's why radiant barriers make the most sense when installed in an uninsulated steel warehouse; they make the least sense when incorporated into residential walls or roofs, as these assemblies are already required by code to be insulated.

"Radiant barrier products can be foil-faced kraft paper, foil-faced polyethylene film, or foil facings on rigid insulation or wood-fiber sheathing. No paints — not even aluminized paints — qualify as radiant barriers. If the radiant surface is touching another material, it won't work; an air space is required on at least one side of a radiant barrier in order for it to function as designed. Otherwise, it functions as a conductor.

"If radiant barriers are installed horizontally with an air space above—for example, on an attic floor—their usefulness rapidly deteriorates due to dust accumulation.

"Radiant barriers have no R-value. However, if installed adjacent to an air space, they can help raise the R-value of the air space."

Read more here: Insulation Choices.

Aug 24, 2010 5:38 AM ET

Response to Pat
by Martin Holladay

Of the two methods you've been using, the installation of rigid foam is far preferable to the steel studs with fiberglass batts.

I don't think your walls will have any moisture problems (as long as there were no moisture problems before you started insulating). But when you install fiberglass batts between steel studs, you have almost no insulating value whatsoever. The heat is so easily transmitted through the steel studs (which act like thermal bridges) that the fiberglass batts might as well not be there at all.

For more information, see Steel Studs.

Aug 24, 2010 10:47 AM ET

Best Insulation for new home constuction in Galveston, Tx
by Bill T

We are building a new home in Galveston. The exterior is vinyl siding over 5/8 inch plywood with 2x4 studs and with interior sheetrock. We are trying to achieve the best R factor as possible and not have the problems of mold, mildew, inner wall rot and other related problems. The a/c -heater heat pump will be seer rated 16.

Aug 24, 2010 10:54 AM ET

Response to Bill T
by Martin Holladay

Bill T,
There really is no upper limit on the possible R-value of your wall. Most builders or homeowners don't strive for the "best possible" R-value; they usually stop at a point that seems economically reasonable.

Obviously, the minimum R-value of a wall is determined by your local building code. Many energy experts advise builders to install twice the code minimum R-value, if that amount of insulation is affordable.

For some reason, you've decided to install vinyl siding over plywood over 2x4 studs. This construction is incompatible with your desire for a high-R-value wall.

Ideally, your wall would be sheathed with exterior rigid foam -- as thick as you can afford. Exterior rigid foam greatly reduces thermal bridging through wall framing.

If you are absolutely set on vinyl siding over plywood sheathing over 2x4 studs, the highest R-value wall would include closed-cell spray polyurethane foam between your studs, and then as much interior rigid foam over your studs as you can afford.

Aug 25, 2010 9:30 AM ET

Exterior Walls
by Adam Bert

In my experience, in humid Florida, it seems that the best mindset to have when trying to determine the materials to use for an exterior wall assembly that keeps dry is to have no vapor retarder at all but rather a vapor barrier or two. For example, typical Florida homes are constructed with CMU on the first floor and wood sheathing on the second floor and above. On the first floor we typically will install stucco or cemplank over a permeable house wrap which is attached to the block. On the inside of the wall we'll install rigid iso board, furring strips and drywall or furring strips then permeable foil wrap followed by drywall. On the second floor again we'll install stucco or cemplank over top of a house wrap which is fastened to the plywood with insulation installed between the studs then the drywall over top of the vapor barrier. The idea in both of these instances is succumbing to the fact that there will be moisture in the walls, no matter what, and dealing with it. Don't stop it. Let it flow to the outside when it's cold or to the inside when it's hot...and dry out. I can recall back in 2004 when Hurricane Charlie came to town that so many homes suffered from moisture problems from the driving rain, not because of water getting through the vapor barriers but rather hitting the vapor barrier, traveling down the walls and having no place to weep out (improper flashing is a whole other topic).

Aug 25, 2010 6:26 PM ET

hot AND cold
by Midwesterner

What about here in the Midwest where the temps are -20 to 40 during the cold months and we also have high humidity with temps around 100 for weeks during the summer? Most of this has seemed to tilt towards "this works in warm climates" and "this works in cold climates" ....?

Sounds like you don't want a vapor barrier on either side in that case? Permeable house wrap on the outside, and kraft paper on interior batts? Assuming 2x4 walls, OSB/plywood sheathing and hardiboard/engiineered wood/vinyl on the exterior -- that's what you typically see for framing in a new-construction neighborhood.

Does this mean that for really decent insulation you should have 2x6 exterior walls in this setup?
Perhaps with open-cell insulation instead of fibreglas?

Aug 25, 2010 10:46 PM ET

Two walls
by Mike Hoyt

I wish to consider the insulation of two walls. Location Upstate New York. Winters to 0 F, summers to 95 F. Baseboard heat.
1. Old house built as follows from inside out - plaster / lathe, 2x5 studs, brick mortared between studs (Insulation? thermal mass?), clapboard siding. (No sheathing) I wish to discard the siding and install plywood sheathing, foil-faced polysio foam and vinyl siding. I have been told to put the polyiso under the plywood since the reflection will cause problems with the siding. I intend to leave the brick in place. What is the correct order of insulation, what other layers should I consider and what other suggestions would you have.

2. New house wall consideration. Same climate. Radiant floor heat. Desire to have envelope of externally-insulated masonry thermal mass. Interior wall structure to be 8" thick Azar Dry-Stack Block containing electrical and plumbing runs subsequently filled with grout. Next would be 3-4'' foam sprayed in place. Exterior to be covered with stucco over metal lathe attached to Azar wall with ties through foam. Comments as to practicality, unmentioned layers, potential problems would be welcomed.

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