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How to Insulate a Basement Wall

If you want to avoid moisture problems and mold, choose your insulation materials carefully

Posted on Jun 29 2012 by Martin Holladay

Here at, we regularly receive questions from readers about the best way to insulate a basement wall. Since these questions pop up frequently, it’s time to pull together as much information as possible on this topic.

In this article, I’ll try to explain everything you always wanted to know about insulating basement walls.

Is it worth insulating a basement wall?

If you live in Climate Zone 3 or anywhere colder, it’s cost-effective and wise to install basement wall insulation. This advice applies to those who live in most of New Mexico and most of Alabama, as well as all of Oklahoma, Arkansas, and South Carolina, and anywhere colder than these states. (Click here to see a climate zone map.)

Canadian researchers who studied basement insulation methods and costs in five Canadian locations (Toronto, Ottawa, Halifax, Edmonton and Victoria) concluded that “for all types and sizes of basements assessed in this study, the lowest total life-cycle cost was associated with basements insulated internally, full-height to a nominal level of R-20.”

How much money will basement insulation save you annually? According to a report published by the U.S. Department of Energy, the annual savings attributable to R-20 basement insulation in a 1,500-square-foot home ranged from $280 per year in Washington, DC to $390 per year in Buffalo, New York, assuming that natural gas costs $0.72/thermUnit of heat equal to 100,000 British thermal units (Btus); commonly used for natural gas.. (However, energy consultant Michael Blasnik cites two Minnesota studies that show lower levels of savings. See his 6/29/12 comments posted below.)

What do building codes require?

The 2012 International Residential Code requires basement insulation in Climate Zones 3 and higher. Here are the minimum code requirements for basement wall insulation — assuming that you are insulating with foam, not fiberglass batts:

  • Climate Zone 3: R-5
  • Climate Zone 4 (except Marine Zone 4): R-10
  • Marine Zone 4 and Climate Zones 5, 6, 7, and 8: R-15.

Should I insulate the wall on the inside or the outside?

I used to believe that the best location for basement wall insulation was on the exterior. In recent years, however, I’ve decided that interior basement insulation makes a lot of sense.

However, there are valid reasons for both approaches, and either way can work fine. So if you prefer one approach, don’t hesitate to use it.

Here are the advantages of exterior basement insulation:

  • Exterior insulation keeps the concrete within the home’s thermal envelope; this increases the amount of interior thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. and reduces the likelihood of temperature swings if heating and cooling equipment stops working. (However, it’s worth pointing out that the advantages of interior thermal mass are often exaggerated.)
  • Exterior insulation protects the dampproofing or waterproofing layer from damage during backfilling.
  • Insulating on the exterior allows a builder to install an uninterrupted layer of rigid foam from the foundation footing to the rafters. While this approach isn’t required — you don’t have to insulate above-grade walls with exterior foam if you don’t want to — many builders like it.
  • Exterior insulation provides more interior space in your basement than interior insulation.
  • It’s easier to insulate and air-seal the rim joist area with exterior insulation than with interior insulation.
  • If you insulate on the exterior, you avoid the expense of interior studs and drywall.
  • Exterior insulation leaves the interior of the concrete wall exposed (assuming the basement is unfinished) so that the concrete can be inspected at any time for cracks.

Here are the advantages of interior basement insulation:

  • The insulation work integrates more smoothly with the construction schedule, since it happens after the building is dried in rather than when the excavation contractor is eager to backfill the foundation.
  • It’s easier to provide an uninterrupted connection between the below-slab insulation and the wall insulation when the insulation is on the interior. If you install the wall insulation on the exterior, the footing will usually interrupt insulation continuity. (For more information on this issue, see Foam Under Footings.)
  • If you insulate on the interior, you avoid the hassle of figuring out how to protect the above-grade portion of the exterior basement insulation.
  • If you plan to install brick veneer on your above-grade walls, interior basement insulation makes more sense than exterior insulation. (For more information on the incompatibility of brick veneer with exterior basement insulation, see Image #4, below.)

Briefly, how are basement walls insulated on the exterior?

After the basement wall has been protected with a dampproofing or a waterproofing system, insulation is installed from the top of the footing to somewhere near the top of the rim joist. Acceptable insulation materials include 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.), expanded polystyrene (EPS), closed-cell spray polyurethane foam, or mineral wool. Polyisocyanurate insulation should not be used because it can absorb water.

Below-grade insulation does not need to be attached to the concrete; it is held in place by the backfill. The best backfill material is a fast-draining granular material like gravel or crushed stone with a thin cap of soil or clay.

Above-grade insulation may or may not need to be attached to the concrete — fastening methods include foam-compatible adhesive, TapCons with washers, and specialty fasteners like Hilti IDP fasteners or Rodenhouse Plasti-Grip PMF fasteners — depending on the height of the exposed foam and the method used to protect it.

Some builders cantilever their 2x6 perimeter walls so that the basement insulation isn't proud of the siding. If the basement insulation ends up proud of the siding, you'll have to protect the top of the basement insulation with metal flashing. The top of the flashing needs to include a vertical leg that extends upward and is lapped by the housewrap; the flashing should be sloped, and the bottom of the flashing needs to terminate in a drip leg that extends beyond the insulation and the insulation protection materials.

If I insulate on the outside, how should I protect the above-grade foam?

The above-grade portions of all types of exterior insulation must be protected from physical abuse and sunlight. Among the products than can be used for this purpose are the following:

For more information on this topic, see How to Finish Exterior Foundation Insulation.

Briefly, how are basement walls insulated on the interior?

The best way to insulate a basement wall on the interior is with foam insulation that is adhered or attached directly to the concrete. Any of the following insulation materials are acceptable for this purpose: closed-cell spray polyurethane foam, XPS, EPS, or polyisocyanurate.

Rigid foam can be adhered to concrete with foam-compatible adhesive or can be attached with special fasteners like Hilti IDP fasteners or Rodenhouse Plasti-Grip PMF fasteners. (For more information on using Hilti IDP fasteners to attach rigid foam to a basement wall, see Marc Rosenbaum’s article, Basement Insulation — Part 2. For more information on Rodenhouse Plasti-Grip PMF fasteners, see New Green Building Products — June 2013.) To prevent interior air from reaching the cold concrete, make sure to seal the perimeter of each piece of rigid foam with adhesive, caulk, a high-quality European tape, or canned spray foam.

Building codes require most types of foam insulation to be protected by a layer of gypsum drywall. Many builders put up a 2x4 wall on the interior of the foam insulation; the studs provide a convenient wiring chase and make drywall installation simple. (If you frame up a 2x4 wall, don't forget to install fire blocking at the top of the wall. For more information on fire blocking, see Fire-Blocking Basics.)

One brand of rigid foam, Dow Thermax polyisocyanurate, meets code requirements for a thermal barrier and can therefore be left exposed in a crawl space (and in some jurisdictions, in a basement) without the need for a layer of gypsum drywall. If your basement doesn’t need wiring, studs, and drywall, then Thermax is probably the brand of insulation to use. (However, be sure to check with your local building official before going this route.)

If you plan to insulate your basement walls with spray foam, the best approach is to frame your 2x4 walls before the foam is sprayed, leaving a gap of 1 to 2 inches between the back of the studs and the concrete wall. The gap will later be filled with spray foam.

For information on insulating rim joists, see Insulating Rim Joists.

If you live in an area where termites are a problem, your local building code may require that you leave a 3-inch-high "inspection strip" of bare concrete near the top of your basement wall. To find out what details are required in your area, talk to your local building official.

What if the basement has an interior French drain?

Some basements have an interior French drain at the base of the basement walls. This type of French drain may be installed on just one or two walls, or it may be installed around the entire perimeter of the basement. Usually, the French drain (a shallow trench) includes perforated drain pipe that leads to a sump; the drain pipe is usually surrounded by crushed stone.

Before insulating a basement wall with an interior French drain, you'll probably want to cover the wall with a dimple mat. The dimple mat allows any water that seeps through the wall to find its way to the French drain at the base of the wall.

Can I insulate on the interior with fiberglass batts, mineral wool batts, or cellulose?

No. Fiberglass batts, mineral wool batts, and cellulose are air-permeable. When this type of insulation is installed in contact with concrete, the moisture in the interior air condenses against the cold concrete surface, leading to mold and rot. That’s why I advise builders that fiberglass batts, mineral wool insulation, and cellulose should never be installed against a basement wall.

The risk of moisture problems is reduced if the concrete is first covered with a continuous layer of rigid foam or closed-cell spray foam. If that is done, some builders then install a 2x4 wall on the interior side of the foam insulation and fill the stud bays with fiberglass batts. This approach is less risky than installing fiberglass directly against the concrete. However, I don’t think that fiberglass batts belong in a basement. My advice: if you want a higher R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. , just install thicker rigid foam, and leave the stud bays empty.

If you decide to combine rigid foam installed on the interior side of the basement wall with fluffy insulation between 2x4 studs installed on the interior side of the rigid foam, you may be wondering, "How thick should the rigid foam be to keep the wall free of moisture problems?" A conservative approach for below-grade walls is to follow the guidelines for above-grade walls detailed in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

Does interior basement insulation need to be vapor-permeable?

No. The idea that a damp concrete wall should be able to dry towards the interior — in other words, that any insulation on the interior of a basement wall should be vapor-permeable — is mistaken. In fact, you don’t want to encourage any moisture to enter your home. Your concrete wall can stay damp for a century; that dampness won’t hurt the concrete.

For more information on this topic, see Joe Lstiburek Discusses Basement Insulation and Vapor Retarders.

Should I include a polyethylene vapor barrier?

No. Basement wall systems should never include any polyethylene. You don’t want poly between the concrete and the insulation; nor do you want poly between gypsum drywall and the insulation. You don’t want poly anywhere.

Paul Ellringer, an energy and mold consultant in Saint Paul, Minnesota, has a collection of slides showing moldy basement insulation. In most cases, these basement walls were insulated with fiberglass batts, and included two layers of polyethylene — one on each side of the studs. Ellringer calls this a “diaper wall,” and reports that most of them are a mess. “Fibrous insulation and poly are inherently problematic, and should not be used in below-grade walls,” says Ellringer. “Sometimes when you open it up, the fiberglass is soaking wet. If the house is two to four years old, the studs are often beginning to rot.”

What about ICFs or the ThermoMass system?

If you build a new basement with insulated concrete forms (ICFs) or the ThermoMass system, your wall already includes insulation, so you don’t need to add any more.

Both approaches work. The main disadvantage of these systems is their high cost compared to conventional poured concrete walls.

ICFs have a core of concrete sandwiched between two layers of rigid foam. ThermoMass walls have a core of rigid foam sandwiched between two layers of concrete. It seems to me that the ThermoMass sandwich makes much more sense than the ICF sandwich: since foam is more fragile than concrete, it makes more sense to protect the fragile layer with concrete than to put the fragile material on the outside of the sandwich.

If you decide to use either ICFs or the ThermoMass system, pay close attention to the wall’s R-value. Many ICF and ThermoMass walls have relatively low R-values. If you’re going to buy such an expensive wall system, be sure to specify thick foam.

What do I need to know if I am installing insulation on an existing house?

If you want to insulate an existing basement, you’ll probably be working from the interior. Before installing a layer of foam insulation on an existing wall, the first step is to verify that the basement wall doesn’t have a water-entry problem.

Diagnosing and fixing water-entry problems in existing basements is a big topic in its own right, and is beyond the scope of this article. Suffice it to say that if your basement walls get wet every spring or every time you get a heavy rain, the walls should not be insulated until the water-entry problem is solved.

Among the possible solutions to this problem:

  • Adjusting the grade around your house so that the soil slopes away from the building on all four sides;
  • Installing roof gutters connected to conductor pipes that convey the roof water away from the foundation;
  • Excavating the exterior of your foundation and installing new footing drains leading to daylight;
  • Installing an interior French drain around the perimeter of your basement and connecting the drain to a sump equipped with a sump pump; and
  • Installing a layer of dimple mat against the basement walls before insulating.

For more information on these issues, see Fixing a Wet Basement.

If your basement has stone-and-mortar walls, you can’t insulate the walls with rigid foam. The only type of insulation that makes sense for stone-and-mortar walls is closed-cell spray polyurethane foam.

If your basement has poured concrete or concrete-block walls, you can proceed with the same methods used for new construction — as long as you’re sure that the walls don’t have a water-entry problem.

What about crawl space walls?

Crawl space walls should be insulated with the same methods used for basement walls. For more information on insulating a sealed crawl space, see Building an Unvented Crawl Space.

Basement wall insulation is a cost-effective measure

Remember: if you live in Climate Zone 3 or anywhere colder, installing basement wall insulation is almost always cost-effective. Performing this work will lower your energy bills, and will also provide an important side benefit: insulated walls are less susceptible to condensation and mold.

That means that insulated basements stay dryer and smell better than uninsulated basements.

Last week’s blog: “Understanding Energy Units.”

Click here to follow Martin Holladay on Twitter.

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Image Credits:

  1. Fine Homebuilding
  2. Thermomass
  3. Building Science Corporation
  4. Journal of Light Construction

Aug 9, 2012 12:24 PM ET

polyiso correction -Thanks
by darek jansen

Thanks for clarifying (and so quickly).
As a web developer, I fully understand how hard it can be to reconcile information in different areas when entered at different times.

As a homeowner-embarking-on-a-large-addition, I'm just grateful I don't have to come up with a new plan.

Sep 20, 2012 12:14 PM ET

Flogging a dead polyethylene horse
by Jonathan Teller-Elsberg

Martin, I'm sorry to say I'm still unclear on just why "Basement wall systems should never include any polyethylene." Here's my potential situation: I have an old house in Vermont with a fieldstone foundation/basement walls. They are leaky, and we've had the interior perimeter of the basement piped to the sump pump, with dimple mat extending about 4 inches above the floor. It has been recommended to me that we use spray foam insulation up around the sill and upper portion of the basement walls, which are dry areas to my knowledge. (The recommendation was that extending the foam all the way down to the floor would be overkill.) It has also been recommended that I put up a layer of radiant barrier bubble insulation (e.g., against the wall, and apply the foam onto that, so the foam is not attempting to adhere directly to the potentially damp stone and old, crumbly-in-places mortar. This barrier would be tucked in behind the dimple mat at the bottom. It would be applied so that order of materials would be: earth, stone wall, aluminized side of bubble sheet, bubble interior of sheet, white polyethylene side of sheet, basement air or foam and then basement air, depending on how close to sill. This bubble layer would offer a small bit of insulation value, but the recommendation was made based more on its usefulness in enabling better application of the foam.

Any water collecting on either side of the bubble sheet should drop by gravity into the perimeter drain system, right?

What am I missing w/r/t moisture problems? Or is this use of polyethylene sufficient uncommon that you didn't have it in mind when making your recommendation?

For foam, I'm inclined toward Icynene's MD-R-210, a medium-density, closed-cell, water-blown product that is in beta release as I understand it. I am trying to avoid the more common spray foams given the concerns outlined by Alex Wilson at I'm wide open to comments on the advisability of this choice.

Many thanks,

Sep 20, 2012 12:32 PM ET

Response to Jonathan Teller-Elsberg
by Martin Holladay

You're on the right track.

As you can imagine, there are many ways to skin a cat. I'm not much of a fan of bubble wrap / foil products, however.

Many experts would recommend the installation of a plastic dimple-mat product on your wall, followed by spray foam. Others (like Joe Lstiburek) might install a layer of Grace Ice & Water Shield.

The idea is to be sure there is an air gap behind the foam insulation -- enough of an air gap that the water can find its way down to the French drain at the bottom of the wall.

It don't have any experience with the spray foam you are asking about. In most cases, high-density closed-cell spray foam is the type of foam to use in a damp environment. It has a proven track record.

Sep 20, 2012 5:09 PM ET

Follow up on bubble wrap as barrier
by Jonathan Teller-Elsberg

Martin, thank you for your prompt reply. If you have more you think worth adding on why you aren't a fan of bubble/foil, I'd appreciate your comments -- certainly if they are relevant to this particular use. Given that you don't mention any deal-killing flaws with the bubble option, I'm inclined to stick with it for the following reasons:

1) The size of the bubble roll will make for easier installation (6 ft vs. 4 ft for Ice & Water Shield -- it's a low ceiling, so a 6 ft roll will allow me to do the job in one continuous sheet and no need for taping up seams). I'm confident in guessing that it will be much easier to install than dimple mat.

2) The bubble roll looks to be (very) slightly less expensive than Ice & Water Shield and, I'm confident in guessing, much less expensive than the dimple mat.

3) The bubble roll will provide a smidge of insulation value.


Oct 4, 2012 5:57 PM ET

This is great information but
by Brandi Borkgren

This is great information but I am still worried about your concerns with polyethylene. It is actually recommended for the application you are describing on the following website.

Oct 4, 2012 9:26 PM ET

Response to Brandi Borkgren
by Martin Holladay

Since this is your third comment posted on GBA including a link to a site with R-value exaggerations, I'm beginning to realize that your are planting spam.

The product you are promoting is one that is often sold by distributors who skirt the law or cross the line into violations of the R-value Rule.

As I responded to the last time you posted a comment, I've written many articles on the topic; one of them was called Martin’s Useless Products List.

Here's what I wrote in that article: "Distributors of foil-faced bubble wrap 'insulation' have a rich history of exaggeration and fraud. A September 2003 exposé in Energy Design Update documented several wild exaggerations by manufacturers. Although foil-faced bubble wrap has an R-value of about 1 or perhaps 2, several manufacturers have falsely claimed R-values ranging from 5 to 10. In hopes of avoiding FTC enforcement action, the manufacturers, caught red-handed, sent EDU a comical cavalcade of apology letters. The bottom line: foil-faced bubble wrap costs just as much as — and in some cases much more than — 1-inch-thick rigid foam. As building scientist John Straube pointed out, 'I might recommend it if it were half the price of R-5 rigid foam, but if it costs more than R-5 foam then you have to be crazy or stupid to use it.' ”

Oct 23, 2012 10:19 AM ET

Martin, As a first time DIY
by Brian Doe


As a first time DIY basement, I have been working on finishing my basement for almost a year. I have steel studs and tracks up for all soffits and walls. I have already run electric for lights and outlets thought the studs. I was getting to the point of insulation. I originally thought about using spray foam insulation, but with a price tag of $1800, I wasnt sure i wanted to put all my money into this system. The biggest concern with this system is the fact that some people suggest they can smell the foam in their house and it has ruined it for them. Also i dont understand how the slab of the floor isnt going to let in moisture. I don't plan on having XPS on the floor and then a subfloor.

Because I am limited with what I can do with the walls and electric already in place, about the most I can do outside of spray foam would be to use fiberglass insulation with kraft or foil backing or slide in 1/2" sheets of XPS behind the wall. I was leaning towards the 1/2" XPS behind the wall, but it is my understanding that in zone 5 of IL, that 1" at 5 R value is required?

Is it worth putting in the 1/2" or do i really need 1"? Do i even bother with this approach or just go with batting? I cannot tear down all the walls, it would be way to much to start over again. what about a product like Roxul

Read more:

Oct 23, 2012 10:37 AM ET

Response to Brian Eck
by Martin Holladay

I'm sorry to hear about your cart-before-the-horse problem. As you now realize, you should have installed a thick layer of rigid foam on your walls before you began installing your steel framing.

You won't get much thermal benefit from 1/2 inch of rigid foam. I don't recommend the installation of air-permeable insulation products like fiberglass batts or Roxul, because these types of insulation can allow warm, humid interior air to contact cold concrete (or the relatively cold surface of a thin layer of 1/2-inch rigid foam).

Here are your choices:
1. You could bite the bullet and pay for spray foam. (Even if you install spray foam, however, your steel studs are much too close to the concrete. You really want to have enough room for a thick layer of insulation between the cold concrete and the steel studs, which are conductors.)

2. You could disassemble your stud walls and insulate with a layer of rigid foam. Then you can put your stud walls back up.

Oct 23, 2012 10:49 AM ET

So you believe that 1/2
by Brian Doe

So you believe that 1/2 ridged foam is not enough? How much xps foam is enough? The studs are generally greater then 1 - 1 1/2" away from the walls already. It is just very hard to put a 1" XPS foam board behind it with the studs already in place. I would feel comfortable with the spray foam application behind the studs. My biggest concern is the odor and or safety of it. Finding a qualified contractor in the chicagoland arean. To dismantle the studs would be a gigantic setback. Of over a years worth of effort.

Oct 23, 2012 11:00 AM ET

Edited Oct 23, 2012 11:01 AM ET.

Another response to Brian Eck
by Martin Holladay

You are in climate zone 5. The minimum code requirement for basement wall insulation in your climate zone is R-10. That means at least 2 inches of XPS.

Your steel studs are much worse thermally than wood studs; they act like radiating fins, conducting heat through any insulation you install between them.

Somehow, you need to get at least 2 inches of foam insulation between the back of your steel studs and the concrete wall, or it's hardly worth insulating. That is the minimum code requirement.

Oct 23, 2012 11:27 AM ET

I'm not sure where you came
by Brian Doe

I'm not sure where you came up with R-10, can you elaborate? Im looking on this site and i see R-5, which is 1 inch foam.
Zone 5 R-5 for 2x4 walls; R-7.5 for 2x6 walls

Nov 4, 2012 6:41 PM ET

Brian Doe's wall foam r-value question
by Eitan Yanich


You are confusing the minimum thickness of foam on the outside of above-ground framing with the code required r-value for basement wall insulation using foam. The chart you cite is for the above-ground framing; the code cited by Martin is for basement walls.

Nov 4, 2012 7:52 PM ET

How to insulate a daylight basement wall
by Eitan Yanich

I am in Washington state, Maritime zone 4. I have a daylight basement which has a concrete stemwall ranging from 1 to four feet high, with 2x6 wood framing above that. Twenty years ago, when I built it, I insulated the exterior of the stemwall with 2 inches of foam but never finished the interior. Last year we hired a builder who framed an addition. He put 4 inches of foam on the outside of the new stemwalls, and an inch of foam on the outside of the plywood above grade, plus a rainscreen. On this new section, I was planning on leaving the interior concrete exposed, and after reading your article, that seems like a really good idea. But for the older sections that have two inches of exterior foam, we put an inch of foam on the inside and framed a 2X4 wall inside that which is still just exposed framing. My builder planned to blow fiberglass into the entire wall cavity, though I was contemplating using cellulose instead of fiberglass due to a few nasty stories I read here about blown-in fiberglass. But then I ran out of money.
Now we are trying to finish this project, but after reading your article I am a bit confused. It seems like where the stemwall is only a foot or two high, we might be able to get away with blowing in something since there are 6-7 feet of wood-framed wall above that will breathe. And the 2 inches of foam on the outside of the wall, combined with a winter ground temperature of 45-50 degrees one foot down, should result in the wall staying warm enough that it won't ever be dripping wet. Maybe. We do have excellent drainage around the foundation, and the exposed wall never looks damp. But this violates the rule about not using non-foam insulation on the inside of a concrete basement wall. And where the stemwall is four feet, it seems to me that there is a greater potential for condensation on the wall.
Other options: Should I cut off the 2x4 wall just above concrete height and isolate the concrete part from the wood framing above it, and then use only foam on the concrete part, leave the 2x4 wall uninsulated, and use whatever I want on the above-grade wood-framed wall? Or should I install a whole bunch of blocking in the 2x4 wall to isolate the concrete section below from the wood-framed section above, and then blow in insulation in the above-grade section while leaving the below-grade section of the 2x4 wall empty?
Oh, and next time? I would stick to insulating the concrete on the outside and leaving the inside surface exposed. When you use foam and a rainscreen on the wood framing above that, the transition to the concrete isn't so bad, the house is tight, and i wouldn't have to be scratching my head wondering how best to make this work now.

Nov 5, 2012 5:49 PM ET

Edited Nov 5, 2012 5:50 PM ET.

Response to Eitan Yanich
by Martin Holladay

I think it's a good idea to avoid the use of fiberglass batts and cellulose on below-grade walls. However, not every expert agrees. Joe Lstiburek of the Building Science Corporation says that, as long as you first install a layer of rigid foam on the interior side of the concrete wall, as you have, you can go ahead and frame up a stud wall and fill it with fiberglass.

In most cases, that approach will probably work. Since basements sometimes flood, however, I think that fiberglass batts are still a bad idea.

It's your call how to proceed, but I think that your comments show that you have a good understanding of the relevant issues.

Dec 30, 2012 4:07 AM ET

Extreme Climate Presents Challanges
by Anthony Strang

Hi Martin,

First of all I'd like to thank you for providing what I believe to be an excellent source of information on insulating basements.

I have been struggling with air quality issues in my home for the last 5 years as a result of trapped moisture and mold growth inside the existing finished basement walls. I have asthma and a general heightened concern regarding the quality of air I'm breathing.

Your advise on the use of XPS and other types of foam makes sense and I have been looking for confirmation on similar ideas for a long time all while getting very mixed and confused feedback from local contractors.
I live in a very extreme Canadian Climate. In my climate zone the temperatures will fall as low as -37F for weeks at a time in the winter months, (late Oct- late April) and as high as 95F in the Summer months.
Inspecting the unfinished areas of my basement (only Fiberglas and poly over wood studs) I find a thick frost covering the concrete. In this same area there is a major draft at the bottom of said wall cavity as the air has begun to actually circulate as warm air is pulled down by somewhat of a reverse convection over the freezing cold concrete. It's a dramatic effect at the bottom of the wall where the poly is not quite sealed allowing the draft to travel over the floor with reasonable force!

The rest of the basement is finished with regular drywall over poly/wood studs and fiberglass against the concrete. I know there is something similar happening behind the drywall as I can smell a musty odor in the spring which would be when the ice condensate would start to melt.
It's not so bad the carpet gets wet but you can smell the dampness in the room.

To make matters worse my home is a raised bungalow. The concrete foundation walls extend roughly 5 feet above grade which makes for a large area of climate exposure to naked concrete. I can not insulate from the outside above grade as the design of the home wouldn't allow for it.

I have been considering tearing down the inside walls, and addressing any mold issues that may be present. The home is 20 years old and doesn't have any obvious foundation cracks or seepage issues. For the re-construction I like the idea of interior rigid XPS straight over the concrete sealed to the nines allowing very little chance of air meeting the concrete. I'd like to have spray foam professionally done on the top portion in between the floor joist header area after the XPS panels are installed for the rest of the wall seems like it would help seal everything up well.

What do you think of this combination of foam? Also, I would still like to use roxul inside the wall cavities that will make up the home theater area. This is strictly for acoustic treatment. I'm guessing no poly, just foam on the concrete then the roxul, then the mold resistant drywall ontop of wood 2x4's. What are your views on this?

Here my province the building code still specifies 6 mil poly over fiberglass / wood studs in basements below grade. Most of the new homes I have had a chance to see will have about 2" of ice and frost on the concrete underneath the poly, fiberglass. Obviously this is NOT the right thing to be doing.

Dec 30, 2012 7:09 AM ET

Response to Anthony Strang
by Martin Holladay

Your experience is a good confirmation of the reasoning that has led me, for years, to advise builders to avoid the use of fiberglass batts and polyethylene on below-grade walls. Your plan to correct the errors in your house makes sense.

Q. "What do you think of this combination of foam?"

A. Your plan to use XPS on the walls and spray foam on the rim joist area should work fine. Be sure that you meet your local code requirements for covering the foam with an ignition barrier or thermal barrier. If the foam is covered with gypsum drywall, you should be fine.

Q. "Also, I would still like to use Roxul inside the wall cavities that will make up the home theater area. This is strictly for acoustic treatment. I'm guessing no poly, just foam on the concrete then the Roxul, then the mold resistant drywall ontop of wood 2x4's. What are your views on this?"

A. That should work.

Q. "Here my province the building code still specifies 6 mil poly over fiberglass / wood studs in basements below grade. Most of the new homes I have had a chance to see will have about 2 inches of ice and frost on the concrete underneath the poly, fiberglass. Obviously this is NOT the right thing to be doing."

A. I agree. Considering your climate, this example of code stupidity is particularly glaring.

Jan 21, 2013 10:43 AM ET

Edited Jan 21, 2013 10:57 AM ET.

interior insulation on extrerior masonry above grade
by Adam Brindowski

First off I like your style. Secondly, I have gutted the old 60's paneling in my upstairs 1940ish cape cod. Being a retrofit I obviously want too maximize my results for energy efficiency. Here's my dilemma; After tearing down the old drywall on the exterior brick wall the framing is only 2" in depth. I would love to use 2" rigid foam board over the whole wall and reframe the son of a gun on the inside of the foam board, but I don't want to lose and more sq footage or have to build the window sill out further. The main reason I ask is the framing is not true 16 on center.. Would metal studs make sense to save room? Should I cut the foam board and glue it to the brick in between the existing stud framing and seal up the edges and seams with "great stuff" spray foam or caulk ? Should I use Kraft faced fiberglass? Whichever one I use do I need a plastic over the studs and behind the drywall. There is no sheathing over the brick. I can easily touch and chip away excess mortar. The insulation that was under there is fiberglass batts with no plastic and the older wood is not rotted or any sign of mold.., Not sure how many wythes of brick. Not sure what kind of brick. Not sure if the brick is hollow or honeycombed. Its a larger block-like, ruffled edge, kiln dried clay brick, like 4" high 10" long and maybe 7“ or 8" deep. It is most peculiar. I live in Milwaukee WI. So its damn cold in winter and hot and muggy in summer, especially since I'm 6 blocks from Lake Michigan. Please share your thoughts and advise a lost novice.

2013-01-21_08-57-46_752.jpg 2013-01-21_08-58-52_222.jpg

Jan 21, 2013 11:18 AM ET

Response to Adam Brindowski
by Martin Holladay

I don't think those are bricks. I think those are terra cotta block. More information here: Structural Terra Cotta.

If you insulate with rigid foam on the interior, it would be best if the insulation is installed in a continuous layer, not in small strips between the studs. Are the studs even necessary? If the only purpose of the studs is to provide a way to nail up the paneling, they can be removed.

Jan 24, 2013 10:55 PM ET

Thermax interior insulation
by Milan Jurich

When insulating interior basement walls for new construction in zone 5 (cold climate), is it acceptable to apply Dow Thermax along the inner poured basement wall from the footing to the the top of the wall. Original plan was to use 2" of XPS or EPS Type IX under the basement slab and 1" around the slab perimeter. Can the Dow Thermax replace the short 1" thick portion around the slab perimeter ... run it continuous from the footing to the top of the poured wall? If not, is it better than to apply the XPS/EPS at that area and then transition to the Thermax above the poured concrete slab?

Jan 25, 2013 8:48 AM ET

Response to Milan Jurich
by Martin Holladay

If I were doing the work, I would use XPS or EPS for the vertical insulation around the perimeter of the slab.

You might be fine using polyiso in this location -- after all, the horizontal foam under the slab should keep your slab dry -- but why take a risk? The location is potentially wet -- so I'd stick with XPS or XPS.

When you come back later to install the polyiso on the interior of the concrete wall, be sure to seal the seam at the bottom of the polyiso with canned spray foam or a high-quality European tape.

Jan 25, 2013 9:16 PM ET

Response to Martin Holladay
by Milan Jurich

Thanks Martin ... appreciated!

Feb 10, 2013 9:56 PM ET

how to decide when it is safe to insulate basement walls?
by Erich Riesenberg

Martin, thanks for your blog and comments. I have bought a house which is now vacant and am improving it before moving in.

The first task was to install rigid foam in the rim joists - expanded polystyrene sealed with Fill & Seal and caulk.

A next task would be to insulate basement walls. However, there is a mechanical waterproofing system around the floor. It is a thin strip of metal conduit which works by collecting water and pumping it out. However, I don't see any signs of a water problem such as mold or discoloration.

I plan to add ground around the house and slope it away from the house for a "passive" solution.

My question is, how is it determined whether a basement is waterproof? Should I wait a couple years to make sure there is no water? I would lke to get this done before moving in but will be patient if that is best. Just curious what a knowledgeable person would do with a new used basement. Thank you.

Feb 11, 2013 7:07 AM ET

Response to Erich Riesenberg
by Martin Holladay

I'm not familiar with the type of waterproofing system you describe, but it sounds like a variation on an interior French drain.

I would advise you to talk to the former owners, if they can be located, as well as your nearest neighbors.

Here in Vermont, the time for basements to get wet is April. If I want to know if a basement is wet, I wait until May.

I'm not sure where you live, but if you live in a cold climate, wait until the snow melts, and you have a big rainstorm. That will tell you something.

Feb 11, 2013 8:30 PM ET

Thank you Martin. I will move
by Erich Riesenberg

Thank you Martin. I will move on from the rim joist to the attic.

I may keep the "thermal mass" and do something else. Thanks for your blog and comments.

Jun 26, 2013 5:04 PM ET

polyiso facings
by joell solan

I am trying to decide what type of rigid insulation to use on my basement walls and floor. I have been leaning towards XPS but am wondering about polyiso; my concern is what type of facing would be best against concrete., i have access to polyiso from a roofing project backed with black paper/felting (?) on both sides or foil on one side and the black material on the other. would it be best to use foil against the concrete? and xps on the floor for better compression strength?

Jun 27, 2013 5:35 AM ET

Edited Jun 27, 2013 5:36 AM ET.

Response to Joell Solan
by Martin Holladay

As long as your basement wall has no signs of moisture entry or moisture problems -- a prerequisite for any plan to install interior wall insulation -- you can use polyisocyanurate insulation on your walls. The type of facing doesn't matter.

However, if you are installing rigid foam on top of an existing concrete floor, you should choose either EPS or XPS, not polyiso. For more information on retrofitting rigid foam above an existing slab, see these links:

Fine Homebuilding: The Stay-Dry, No-Mold Finished Basement

Fine Homebuilding Q&A: Finishing a basement floor

GBA Q&A: Basement floor insulation retrofit

GBA Q&A: Floating plywood floor

Green Basement Renovation.

The High Cost of Deep-Energy Retrofits.

Jul 2, 2013 1:52 PM ET

post polyiso install dampness
by Chris van Wilgen

Im at a crossroads here. I have recently installed 2" of what appears to be a fiberglass paper faced polyiso which I purchased ( firestone 95+ specd roofing 2nds) I spoke with the firestone tech and he saw no problem using this on below grade interior foundation walls. You also mentioned not to be concerned with the facing, which I was. So I proceeded. I left a 2" gap above the floor and prepared to sheetrock. I am noticing a heavy dampness on the botton 2-3 inches of the panel against to concrete. It wets my fingers. Concerned, I removed it to take a look. The rest of the panel and most of the wall were dry, but I am also noticing condensation in the walls to about 2-3 feet above the floor.
How should I proceed? I never had wall dampness before. Should I just replace with an xps?

Jul 2, 2013 1:57 PM ET

My other thought was to place
by Chris van Wilgen

My other thought was to place a 1/2" of tuff r or some foil faced, taped against the concrete and then reinstall the 2" polyiso?

Jul 2, 2013 2:09 PM ET

Edited Jul 2, 2013 2:11 PM ET.

Response to Chris van Wilgen
by Martin Holladay

Assuming that the polyiso was installed in an airtight manner (with the perimeter of each sheet sealed with caulk, construction adhesive, or canned spray foam), it really isn't possible for the moisture to be condensation. The source of the water may be your soil; the concrete may be getting wet from the exterior side. However, there is no easy way to diagnose the source of the moisture over the internet.

After you have removed one of the pieces of polyiso, I suggest that you perform a test to see if your concrete wall is damp. Tape a 12 inch square of clear polyethylene sheeting to the concrete wall in the area that worries you. Leave it there for a few days. If you see beads of moisture forming between the concrete and the poly, then the concrete is damp (and the moisture is coming from the soil).

Sep 5, 2013 7:11 AM ET

How wet is too wet?
by George Heinrich

So I was wondering how dry a concrete basement wall must be in order to use the closed-cell spray foam? My basement weeps a little moisture (walls feel slightly wet to touch) maybe once every 2 years when we get a hurricane or numerous days of rain in a row. It only weeps like this in a few places (2-3).
Otherwise the walls are dry. Would even this limited amount preclude using the spray foam?
I'm just wondering because the cost/effort balance is pretty high for a few small leaks that only appear every 2 years or so...
A related question...can the spry foam contain the moisture in the wall or will the moisture push the foam off the wall? Is this a problem if no warm air is getting to the moisture?
Thanks, George

Sep 5, 2013 8:38 AM ET

Response to George Heinrich
by Martin Holladay

My answer to your question is, "I'm not sure." I would advise you to ask your spray foam contractor, explaining the situation just as you have here.

I would also check the grade and slope on all sides of your foundation. Increasing the slope of your grade to encourage water to flow away from your foundation can solve many problems like yours.

Oct 18, 2013 6:18 PM ET

Moisture Control
by Brian Koenig

Nice post -- basements can be very difficult to insulate. And moisture control is one of the biggest factors. Leaks and cracks must be addressed first. Source control will keep the moisture from getting through the walls' cracks.

Oct 28, 2013 1:01 AM ET

Installing a layer of dimple mat against the basement walls
by Mike Wall

Martin, You briefly mention a water control strategy is "•Installing a layer of dimple mat against the basement walls before insulating." This is something I am considering in a house with a poured concrete basement with 8' walls and grade at 6'. The dimple mat should terminate at grade level. What would be acceptable methods of insulating this wall? Should I be worried about moisture condensing on the membrane between the insulation. We have the classic cold Canadian climate. The most cost-effective insulation would be fibreglass batts against the membrane and maybe some XPS against the top 2' of concrete. What would you recommend in this situation.

Oct 28, 2013 5:35 AM ET

Response to Mike Wall
by Martin Holladay

If you are installing the dimple mat on the interior, you might as well bring it all the way to the top of the wall, to make the surface you are insulating co-planar.

The only two insulation types to consider (as I wrote in my article) are spray polyurethane foam and rigid foam. Don't use fiberglass insulation on the interior of a basement wall.

If your basement has a water entry problem, you need to install an interior French drain, connected to a sump, at the base of the wall before installing the dimple mat.

Dec 13, 2013 5:43 PM ET

Why use equal insulation thickness on unequal heat loss?
by Nick Welch

From the reading I have done, the heat loss in a basement wall is predominantly near the top, and it tapers off greatly as the wall descends into the ground. Since multiple layers of foam with staggered seams are a good idea anyway, it seems logical to install less insulation on the lower parts of the wall and more on the upper parts (for example, 2" on the bottom 1/3, 3" on the middle 1/3, and 4" on the top 1/3). It should yield less heat loss for the same foam budget.

Conveniently, this would also leave more room on the lower parts of the walls for electrical boxes, allowing you to use thin wall framing such as 2x4s on their sides, without having to dig out holes in the foam.

Is this as good of an idea as it seems to me?

Dec 13, 2013 5:50 PM ET

Response to Nick Welch
by Martin Holladay

You can take this approach if you want. The trouble with the approach is that heat doesn't just move through the concrete horizontally; it also moves through the concrete vertically. Concrete is an excellent conductor. It acts like a thermal bridge.

The most effective and easiest way to address all of these issues is to follow the code (or do better than the code) and install a uniform layer of insulation on your basement walls. You won't regret it.

Dec 16, 2013 12:14 AM ET

Cognitive dissonance re condensation
by Keith H


I'm struggling with a bit of cognitive dissonance regarding the use of vapor barriers in basement and condensation at the interior boundary of the concrete foundation.

If it is achievable to seal a piece of polyiso well enough to prevent condensation at the concrete-PI boundary, why isn't it achievable to seal a sheet vb such as polyE against the concrete? Or to use a paintable VB? If so, why wouldn't this allow the use of a batt product such as Roxul ComfortBatt? I'm guessing you are going to say because Roxul is air permeable and needs an air/vapor barrier on the interior to prevent interior-generated moisture from entering the stud cavity. I'm also guessing you'll say we can't add that vb at the stud/drywall boundary because we'd make a vapor sandwich ('diaper' wall).

To which I ask a followup question: if we use PI at the interior concrete boundary, then won't any interior vapor barrier at the drywall/finished surface create a diaper wall?

I'm going to post a question with more details in Q&A but thought that my question re VB placement/avoiding double VB would be useful to some people reading this thread.

Dec 16, 2013 7:27 AM ET

Response to Keith H
by Martin Holladay

The glib way to answer your question about using a polyethylene vapor barrier to keep concrete walls dry would be, "The reason we know that it doesn't work is because thousands of builders have tried it, and thousands of remodelers and home performance contractors have had to clean up the moldy mess caused by these failures. We know it doesn't work because of field experience."

However, a more thoughtful response would explain that fiberglass batts and Roxul batts contain air. The air between the fibers is basement air, and it contains moisture. It isn't some kind of miracle, dessicated air. So condensation is possible. Polyisocyanurate sheets have no air; the only air trapped is a very small amount of air near the seams and between the foam and the wall.

The air between the fibers of the fiberglass or Roxul insulation is not divorced from the basement air. As temperature conditions change, there is a pumping action that creates air exchange between the air in the insulation and the air in the room. When the air is warm, it expands. When it is cool, it contracts. As this happens, pinholes in the poly allow for air exchange. This process introduces new moisture into the insulation cavity.

Dec 19, 2013 4:00 PM ET

Question about finishing a basement in a new home
by Aaron De Boer

I am currently looking at finsihing up my basement in my 3 year old home. After much research along with this article I am convinced that these are the best practices to follow. However that being said, my basement is very dry and no issues (that are noticeable). I live in a very harsh climate where it can get to -30c in the winters.
The basement was left with R12 fibreglass from floor to rim joists stuffed in between framing along all the walls , pressed directly on the concrete wall. A loose sheet of poly is stapled along all the framing.
So my question is what do you recommend going forward to effectively finish these walls?
1) Remove all poly, fibre and framing and start from scratch following the practices as indicated in the article
2) Remove the fibre and poly and fill in the 2 foot sections in between the framing with rigid insulation and foam all the edging within the framing. However this would mean the framing is still up against the concrete directly too.
In either option can I reuse all the R12 and put on top of the rigid insulation again (with no poly) and cover with drywall without issues or just toss the fibreglass?
Any feedback is greatly appreciated

Dec 19, 2013 4:35 PM ET

Response to Aaron De Boer
by Martin Holladay

The safest approach is to remove the polyethylene and fiberglass, and remove (or loosen) the stud wall components so that the stud can be moved inward, creating a gap between the studs and the basement wall. That would make it possible to install closed-cell spray foam.

However, all that work is daunting (and potentially wasteful). If you don't find any dampness or mold behind the fiberglass, you could adopt another approach: watchful waiting. If your basement is so dry that you don't have problems, you could leave well enough alone. It's your choice.

Dec 26, 2013 4:35 PM ET

Edited Dec 26, 2013 4:45 PM ET.

An issue with one wall
by Pete R

Hey, this is a great article with some great questions and answers. Thanks for taking the time to do this.

I have an issue that I would greatly appreciate some input on. I am in the process of finishing my basement. I've adhered 2" foam board around nearly the entire foundation perimeter. The 2x4 framing is about 2 inches from the foamboard and I figured I'd add roxul insulation between the studs to help additionally with climate comfort for the northeast winters.

My problem is the one wall (about 7 feet wide) is right next to the stairs coming down to the basement. I only have 1 inch between the stair stringers and the concrete wall. I can't get even 1.5 inch foamboard in there so I was going to put up some furring strips, cover with heavy mil plastic and then apply a mold safe sheet rock to the strips. Is that acceptable or do I have other options?


Dec 26, 2013 5:15 PM ET

Edited Dec 26, 2013 7:37 PM ET.

Response to Pete R.
by Martin Holladay

If I were you, I would slide 1 inch of rigid foam between the concrete wall and the stair stringer. Your other option is to remove the treads and slide the stringer out a little bit further from the wall. Then cut back the treads and reinstall them.

Dec 31, 2013 12:48 AM ET

Roxul section comments
by Randy Cook

Martin - Can you comment on any pitfalls of the recommended basement wall section outlined in the Roxul Comfortboard IS Basement Application Sell Sheet? The section shows a concrete foundation wall, then an "Air/Moisture Barrier", then the Roxul Comfortboard, then stud wall with Roxul batts, then a vapor barrier, then the drywall...

I've been researching this topic for several months and had landed on this basement wall section, until I read this article.. But then I think twice that the Roxul team would recommend a section that has been a proven to failure... Do you feel this section is different enough from older diaper wall/poly sections that it could survive? I'm in St. Louis, MO, Zone 4A / Mixed-humid. Currently I have rolls of CertainTeed Membrain to use both against the concrete and behind the drywall, and lots of Roxul on order. I'm more concerned first about the mold issue, second about R value.

Additionally, the back wall of the basement is a studded wall (Walkout basement). Based on your article, would you also not recommend poly (CertainTeed Membrain in my case) on this wall section? The walkout wall is vinyl siding, Tyvek wrap (lapped and Tyvek-taped properly by me one night after the builder got it wrong), 1/2" insulated sheathing, 2x6 studs.. See attachment.

Attached are the wall sections I have planned currently, but after reading, I'm wondering if I should scrap a lot of that and go with the rigid foam?

I certainly appreciate any commentary. I really try to avoid this type of pointed question, but in months of searching I've found so many conflicting articles and so many just seem to simply be outdated... I was surprised to read the Lstiburek post you link to where he admits some of his older advice is obsolete... Which is why I hope my question can clear up a lot of questions for not only myself, but for other readers as well.

Wall_Sections_Tabloid.pdf 84.62 KB

Dec 31, 2013 7:30 AM ET

Response to Randy Cook
by Martin Holladay

I hate to be blunt, but Roxul is in the business of promoting the use of their products. In that respect, they don't differ from any other insulation manufacturer. They want to sell more mineral wool.

If you have a dry basement with no water entry issues and perfect interior drainage, and if the interior relative humidity of your basement stays low all year round -- including during the summer -- it's perfectly possible to use fiberglass batts or mineral wool to insulate the interior of your basement walls. Plenty of people have done it, and if your basement has the conditions I described, fiberglass or mineral wool can work well.

The problem is that many basements aren't as dry as the one I described. Once we're dealing with a basement that is occasionally humid, we're stepping into a new territory, and the use of interior air-permeable insulation becomes risky.

The Roxul brochure you linked to isn't based on building science. The brochure specifies the use of an "air/moisture barrier" (material not specified) between the Roxul and the concrete wall; but there is no need for an air barrier at this location, because concrete is an air barrier. If polyethylene is used, it's very possible that moisture will condense on the interior side of the poly when the interior air is humid. This moisture will dribble down the poly and puddle near the bottom plate.

Determining which material to use depends on many factors: not only on cost, but on how much risk you want to assume. If you're like me, and don't like risk -- and you don't want to tear out your basement walls in 6 years to fix them -- I suggest that you use rigid foam or spray foam to insulate the interior side of your basement walls.

Jan 22, 2014 11:38 PM ET

What about terracotta, also frost failure from insulation?
by K T

Great article, for retrofitting insulation in existing home, what is the reasoning behind the statement "If your basement has stone-and-mortar walls, you can’t insulate the walls with rigid foam. The only type of insulation that makes sense for stone-and-mortar walls is closed-cell spray polyurethane foam."? Does this apply to structural terracotta foundations as well? Does this have something to do with moisture? Could spray cellulose work?
Also what about frost failure (freezing of wet soils, which push into foundation causing cracks) when foundation is insulated, is this a problem or myth?

Jan 23, 2014 8:24 AM ET

Response to K.T.
by Martin Holladay

The reason that you can't install rigid foam on the interior side of a stone-and-mortar wall is that the stone wall is too uneven and bumpy.

The advice given for stone-and-mortar walls does not apply to terra-cotta walls. If the terra-cotta wall is even (co-planar), there is no reason you can't insulate it on the interior with rigid foam.

Spray cellulose insulation should not be installed on the interior side of a foundation wall, for all of the reasons listed in my article. This is a damp environment, and cellulose doesn't handle water well. Cellulose needs to stay dry.

Decades of experience has shown that insulating basement walls will not cause walls to fail due to the freezing of the soil. Needless to say, it's always a good idea to manage the water on the exterior side of your foundation. Normal practices call for some mechanism to handle water that is shed by your roof and to handle rain (for example, by proper grading and the use of swales). It's never a good idea to have saturated soils near your foundation walls. For more information on these issues, see Fixing a Wet Basement.

One more comment on frozen soils and foundation walls: in New England, many barns have basements. Although the soil around these barns freezes, the barns have stood for 150 years.

Jan 30, 2014 5:40 PM ET

Basement with batt insulation and having moisture problems
by Greg Beitler

This article was extremely helpfull by the way. My wife and I live in the lehigh valley in pennsylvania. Our basement has the batt insulation installed on the entire interior basement walls.. This insulation was installed by the contractor shooting the nails into the concrete to affix the insulation to the concrete. Currently our winter temperatures have been in the twentys for about 3 weeks now. Last night I was in the basement and noticed that were the insulation does not cover the concrete there is moisture or almost ice buildup forming on the concrete. Also what I noticed is that the batt insulation does not go all the way to the basement floor there is about a 4 in. gap and in this gap there is condensation forming.

Note: Half of my basement is above grade, the condensation is only on the above grade areas.

So what is the recommendation to help with this problem.

I was hoping to start to finish my basement next year but don't want to start this project till the moisture issue is taken care of.

Do I need to remove the batt insulation in the above grade area and replace with foam board?

Mar 20, 2014 9:49 AM ET

Martin: Please add info on fire stops!
by Mark Hays

Thanks Martin for another helpful article and follow-up on basement insulation.

Please add info on firestops for basement walls, however -- which are required by the 2012 IRC and most inspectors. Foam board and spray foam that runs along a concrete wall, for example, must be interrupted every 10 linear feet by a firestop, e.g. a PT stud that is mounted and sealed tightly against the concrete wall, or a gap that is packed tightly with mineral wool. Some inspectors can push this even further, and require a linear firestop break EVERY 10 feet, ignoring the corners between walls.

The top of the interior stud wall also need to be firestopped, e.g. with a continuous strip of 3/4 inch OSB that bridges the gap between the top plate of the wall and the mudsill -- or pack the gap tightly with mineral wool or unfaced fiberglass. Some inspectors will not approve mineral wool or fiberglass, and prefer solid blocking. Ask your inspector before you start.

Holes in the studs or top plate, e.g. to run electrical wiring or pipes, must also be filled with fire resistant material, e.g. 3M Fire Barrier sealant.

Here are a couple of helpful resources:


We also ran into a problem with a commonly used "firestop" product: the orange "Fireblock" version of DOW Great Stuff foam sold at Home Depot and Lowes. I was surprised to discover that Fireblock Great Stuff ignites at 240 degrees, and DOW does NOT recommend it as a "firestop"!! Here is a quote directly from the DOW website:

"This product is defined by the International Building Codes as the use of approved building materials installed in concealed spaces to resist the migration of fire and hot gases. GREAT STUFF™ FireBlock is tinted orange to be more recognizable to building code officials. This product is not approved for use in firestop systems."

The third sentence contradicts the first two, and the label on the can. (Source: See the "reviews" option.)

I contacted DOW ans asked them for clarification, with no luck. With the clout of GBA and the Taunton Press, maybe you can convince DOW to reply.

In summary, this "basement insulation" guide needs to be updated -- and a new guide to firestops would be very helpful to your readers. There is a lot of confusion out there.

Thanks for all of your good work!


Mar 20, 2014 11:18 AM ET

Response to Mark Hays
by Martin Holladay

Thanks for your suggestion. I have edited my article to include some information on fire blocking.

Mar 25, 2014 9:41 AM ET

Interior basement insulation advice
by Matt M


Thank you for your research and faciliting this good discussion. I installed an interior wall insulation system 4 years ago to take advantage of govnt rebates, but after further consideration, I may have been too hasty. I am at the start of finishing the basement (currently just studs and insulation), and I am considering a change to the insulation system.

I live in southwestern Ontario with sub-zero temperatures from Dec-Mar, reaching -40C with windchill.

Here is the current arrangement:

1. Poured concrete wall
2. Typar house wrap. Installed with white side facing the wall: moisture movement from the concrete wall into the house is reduced, but there is stil moisture permeability from the interior towards the wall. Not sure whether this was necessary.
3. 0.5" rigid board XPS. Sealed all seams with Tuck tape.
4. Spray foam insulation in the headers, and about 24" below the
headers, on top of the rigid board XPS.
5. 2x4 studs with a 2" air gap between the studs and the XPS.
5. Roxul exterior wall, moisture-resistant batts for 6" studs on 16" O.C. (R-24)

I am using 1.5"x3.5" SPF wood studs, fastened with tapcom concrete
screws, 2" from the XPS, to give a 6" wall-to-front-of-studs gap.
The bottom sill plate is fastened on top of the sub-floor which is
made of 3/8" dimple wrap and 1/4" OSB plywood.

I am considering the following changes:

a. Remove the Typar house wrap
b. Add 1.5" of XPS for a total of 2" of XPS. Stagger the install and seal with tuck tape.
c. The XPS is not fastened against the wall. Since I'll add another layer of XPS, I'll use screw/washer fasteners to secure both XPS boards against the wall.
d. Install the studs against the XPS to remove the air gap
e. Reseal any gaps with two-component spray foam
f. Cut off batt insulation in the vertical plane to reduce compression on installation.
g. Building code in my area requires a vapour barrier on the warm side, so I am considering the MemBrain technology, even though I would prefer to install nothing.
h. Replace the OSB subfloor with plywood

What are your thoughts?

Thank you for your time,

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