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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. For more information on methods of fastening rigid foam or furring strips to a concrete wall, see Fasteners for Concrete and Brick.)

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. (For more information, see "Using a Dimple Mat to Keep a Basement Wall Dry.")

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

Apr 21, 2016 4:59 PM ET

Go rigid, not spray
by Dana Dorsett

At 3" closed cell foam would have to be applied in two lifts, with a curing period between them.

I'm not sure what the inch of foam sprayed onto EPS foam is buying you? (in option 1, part 3)

An inch of EPS with an R13 studwall meets code-min performance.

If you furred out the pony wall stud edges to make them flush with the foundation and installed cheap fluff in those furred-out stud bays (it compressed low density R30 or whatever, as long as it's full), then installed an inch of EPS floor to ceiling, seams taped, edges sealed, you can then install a 2x4 studwall on the interior of that.

Put an inch of EPS under the bottom plate of the stud as a capillary break & dew point control, no need for pressure treated. The pony wall portion would already be well over code min, but you'd still need to stuff the studwall with fiber where the concrete starts (but full height is also fine). Unfaced R15s would with margin, and would still be able to dry out after a flood. If the basement is always dry and flood risk low,kraft faced R13s would be fine.

If you left the pony walls empty you'd need at least R13 + 5 to meet code on your interior studwall which means 1.5" EPS instead of 1".

Apr 21, 2016 5:14 PM ET

Edited Apr 21, 2016 5:38 PM ET.

Reply to Dana
by Greg Volland

My thinking about using the closed cell spray foam over EPS is cost and time savings. The reason I was leaning towards some level of SPF is that it would help seal the space and irregular cavities/gaps and would be faster than EPS + caulking.

You kind of lost me with your suggestions... As my photo shows, I have faced R-19 in the pony walls already. Are you saying that if I furred out those walls to make them flush with the concrete foundation wall (about an inch), I'd have to put "fluff" in that small cavity between the EPS and the R-19? Not sure how I'd even do that..

I'm open to that idea of doing EPS floor to ceiling as you suggest and then the 2x4 walls inset. If I did that, would I need to insulate the interior 2x4 wall?

I'm also getting a bit confused as to why some commentators at GBA are saying SPF is great and now you're advocating for no SPF and rigid only. Please enlighten me.


Apr 21, 2016 5:26 PM ET

Response to Greg Volland
by Martin Holladay

Q. "I'm also getting a bit confused as to why some commentators at GBA are saying SPF is great and now you're advocating for no SPF and rigid only. Please enlighten me."

A. You'll hear many voices (and many opinions) on GBA. Here's my opinion: spray foam insulation is good for certain applications, but since most brands of closed-cell spray foam are expensive and use a blowing agent with a high global warming potential, it's best to use other insulation materials if these other materials work well. Save the spray foam for applications where it's the only option (like insulating a rubble-stone foundation wall, or certain air sealing tasks).

Apr 21, 2016 5:41 PM ET

Reply to Martin Holladay
by Greg Volland

I understand. What about the other part of my reply to Dana:

"As my photo shows, I have faced R-19 in the pony walls already. Are you saying that if I furred out those walls to make them flush with the concrete foundation wall (about an inch), I'd have to put "fluff" in that small cavity between the EPS and the R-19? Not sure how I'd even do that.

I'm open to that idea of doing EPS floor to ceiling as you suggest and then the 2x4 walls inset. If I did that, would I need to insulate the interior 2x4 wall?"



Apr 21, 2016 6:04 PM ET

Edited Apr 21, 2016 6:05 PM ET.

Response to Greg Volland
by Martin Holladay

Most of these decisions are up to you, not me or Dana.

If you want to make the interior finish material (probably drywall) of the lower concrete wall and the upper framed wall co-planar, you can. Or, if you prefer to have a shelf dividing the two walls, you can have that instead.

If you want to install EPS on the interior side of the insulated stud wall, you can. Or you can omit it -- as long as you meet minimum code requirements. The final R-value of these wall assemblies is up to you.

Apr 21, 2016 6:30 PM ET

Reply to Martin Holladay
by Greg Volland


I am planning to finish the inset 2x4 stud wall with drywall and want a continuous wall depth (no shelf).

What I am trying to understand is how best to insulate the exposed concrete foundation wall to avoid moisture/condensation issues. I have little concern for flooding, etc.

My specific question is this: if I furred our the existing foundation pony walls to be flush with the concrete foundation walls, and then ran EPS from floor to ceiling as Dana suggested, is it ok to have a 1 inch "void" between the EPS rigid foam and the existing R-19 that is in the current 2x6 pony walls atop the foundation wall?


Apr 21, 2016 7:30 PM ET

Edited Apr 21, 2016 7:57 PM ET.

Apology to Greg Volland
by Dean McCracken

My cautions regarding spray foam were based on a misunderstanding of what you propose. Please disregard them. Martin and Dana have thus far refrained from stating the obvious that "some people who post don't know what they are talking about and should be ignored." Their restraint is commendable. Listen to them, not to me!

Apr 22, 2016 6:01 AM ET

Response to Greg Volland
by Martin Holladay

Q. "Is it OK to have a 1 inch void between the EPS rigid foam and the existing R-19 that is in the current 2x6 pony walls atop the foundation wall?"

A. It would be best to fill this void with something -- either a continuous layer of 1-inch EPS, or compressed fiberglass (perhaps a half thickness of R-13 batt, compressed).

Apr 22, 2016 4:14 PM ET

Response to Martin and Dana
by Greg Volland

Thank you again for you insights. After closer inspection of the foundation wall and existing 2x6 exterior wall, I am having second thoughts about try to fur out the difference and fill the void because there are several obstacles in the way. What if I use attached plan? It calls for me to adhere 1 or 1 1/2 EPS to the concrete only (face and top, sealing the joints). Then build my new 2x4 wall inset. Would it be ok to then use R-13 Faced fiberglass on that 2x4 wall (ease of install and accessible)?

I think Dana was saying this would be an ok plan. Can you confirm?

Thanks! Hope to get going this weekend!



Apr 22, 2016 5:29 PM ET

Avoid the void!
by Dana Dorsett

As drawn you have no back-side air barrier to the 2x4 R13s from the top of the EPS on up, and nothing but air between the R13 and the facers of the R19s. This is a classic thermal bypass channel (and too often, a rodent-run).

How you deal with it depends on the depth of the ledge. If it's a fairly consistent inch, double layering the EPS and going floor to ceiling with the interior-side sheet would be good, since both layers can be taped air tight. If it's 1.5" or 2", same thing. If more than 2" cutting the ledge-top EPS at the edge and stacking low density batts, then going floor to ceiling with the 1" EPS, compressing the batts in place with the studwall framing would stuff the void with a mid-density more air retardent fiber layer, and allow you to detail the foam as an air barrier. If the ledge is too inconsistent in depth to fill with EPS and under 2" you can use split 19s, which are very compressible down to about an inch or less, but would still be a reasonable density at 2". But the key is to have no voids, which means lower infiltration, no unimpeded thermal bypass, and no unimpeded convective routes around insulating layers.

As drawn the plywood subfloor and floor EPS stops at the bottom plate of the studwall, and the bottom plate is in contact with the slab. It's better if the plywood & floor EPS extend all the way to the wall EPS, which puts a thermal and capillary break between the slab and all of the wood (including the bottom plate) and exposes more bottom plate for attaching the drywall & kick board trim.

Apr 22, 2016 6:30 PM ET

Reply to Dana
by Greg Volland


Two thoughts based on your reply:

1) There is a 3 inch drain line on the wall that runs the length of the space (see photo). I can move it out from the wall enough to put the EPS on the existing studs + foundation wall in a continuous floor to ceiling application, but I would still need to bring out the new 2x4 wall far enough from that to account for the drain line. Is that ok in your view? Any suggestions? Should I just skip insulation in the new 2x4 walls?

2) Regarding the floor/stud walls, a few of the stud walls are already in place and the bottom plates are already down (PT + sill sealer roll foam) so, unfortunately the EPS is not going under those areas... I will see the way you suggest is done for the other areas.

3) there are a few posts on concrete footings in the space and the plan is to build 2x4 walls around them to "box them in". My question: should the slab areas within those "boxes" also be covered by the EPS? I assume so...



Apr 29, 2016 2:46 AM ET

by Greg Volland

Where did everyone go?!

Apr 29, 2016 5:55 AM ET

Response to Greg Volland
by Martin Holladay

1. You'll get more responses to your questions if you post them on GBA's Q&A page. Here is the link:

Lots of readers visit the Q&A page daily. Far fewer readers are likely to notice Comment #161 on a 4-year-old article. (In general, the comment section under our articles is used to comment on the article itself -- not so much to post a question about a renovation problem. But of course we try to help people out, wherever the questions are posted.)

2. You've posted 9 questions in a row here, and you've gotten detailed technical answers to all but the last posting. Sometimes donor fatigue sets in. But we'll do our best to help you.

3. In response to your latest questions: Yes, it's OK to move the drain pipe and the 2x4 wall. And yes, you should install EPS against the slab before you box in anything that you intend to box in.

Apr 29, 2016 1:35 PM ET

Reply to Martin Holladay
by Greg Volland

Understood. Thanks Martin!

Jan 13, 2017 10:32 AM ET

wet basement walls
by kim dolce

We are about to address the basement in the renovation of an 1860's carriage house in mid-coast Maine (zone 6). The building sits almost directly on a ledge at the front, jutting out about 15' into the basement and hugs one side of the building then opens up to about 6' of headroom towards the back. The foundation walls are stone and mortar closer to the ledge and change to brick as the basement opens up. The floor is dirt and after a decent rain or good thaw we have water running freely from the ledge and through at least one area of the brick.

We intend to address the water as best we can with French drains (exterior) and gutters, but the ledge guarantees we won't be entirely successful. We discovered a drain set in the basement floor, and though no sump pump exists water does seem to flow toward the drain and I don't anticipate the problem of standing water, just moisture passing through. We only intend to use the basement for storage of things like crates and tools, but want it as dry as possible.

We've had a parade of people through to quote the job and almost to a man everyone said we should spread gravel then poly sheeting (anything from 6-20mil depending on quote) and pour a rat slab which would provide a place for water to be channeled under the slab to the existing drain. They then all recommended 2"-3" of cc spray foam on the walls.

It all seemed to make sense, but I thought I'd come back to GBA one more time to make sure we weren't missing anything. Your article seems to point to one flaw in this plan, it doesn't seem to address our wet walls. Is the application of a dimpled membrane the answer? I'm not turning up much in a search, but is something like Superseal what I should be looking for? Also, did I also noticed the mention of an air space between the membrane and the insulation? If so, what will the spray foam adhere to?

Thank you and by all means, let me know if you see anything else we're missing.

Jan 13, 2017 10:42 AM ET

Resonse to Kim Dolce
by Martin Holladay

1. If I were you, I would want to know where the floor drain leads. Does it run to daylight? If not, where does it go? How will you prevent it from clogging in the future if you don't know where it terminates?

2. The standard solution in a case like yours is to install dimple mat on the wall before you spray the closed-cell spray foam. The dimple mat should extend down to the crushed stone layer under the rat slab so that any water that trickles down the wall will have free access to the crushed stone layer.

Jan 13, 2017 11:13 AM ET

Resonse to Kim Dolce
by Charlie Sullivan


If you post your question in the Q&A section rather than in the comments of an article like this, I think you'll get more people seeing it and responding to it. Martin's comments are usually the most thorough and helpful--the rest of us just function like a Greek chorus to help emphasize what he says. But sometimes what we say is useful.

My suggestion is to specify Lapolla 4G spray foam, because the standard closed-cell spray foams us a blowing agent that has a huge global warming impact (~1000 times worse than CO2).

Jan 13, 2017 11:55 AM ET

Martin, We do plan to put a
by kim dolce

We do plan to put a hose in that drain to see if we can figure out where it goes. There's a retaining wall at the back of our property and my money has been on that. Funny, until this moment it hadn't dawned on me to examine the retaining wall for a gusher during a heavy rain. Thanks!

Jan 13, 2017 11:57 AM ET

Charlie, Thanks for the tip.
by kim dolce

Thanks for the tip. I usually spend my time on the articles and blogs so don't tend to think of the Q&A.

Dec 6, 2017 1:24 PM ET

Edited Dec 6, 2017 1:25 PM ET.

XPS VS EPS - Making my purchase today to begin project
by Brad Ogden

Before I finalize my decision and place order today I am hoping to get some reassurance / advice. I am finishing my 2500 sqft basement which has poured concrete walls. I originally was going to do 2" XPS on perimeter of wall (Pink Stuff) and 1" XPS foamular 150 on floor with T&G OSB or plywood sub floor on top. However when at HD, I saw a product called DuroSpan, also goes under the label DuroFoam in Canada, that is 2" EPS with foil like film on both sides which cost almost $10 less than XPS. I was considering using this product for the walls, however decided against it because I am concerned the foil like lining would permit any moisture from concrete wall from being absorbed and dry inward. Although XPS has a low permeability, my thought is at least if some minor leakage did occur from a hair line crack at least it could be absorbed and dry inward, otherwise, if / when there is a minor water leak where would the water actually be able to dry to? However now I have found 2" closed cell EPS non-faced R8 for only $10 / sheet which would be over $15 savings per 4x8 sheet, being I need over 70 sheets this is a significant savings. My thought is the un-faced EPS would allow moisture / water to be absorbed in the event of a minor leak and dry inward, plus save me over $1,000 on material. For the concrete floor I would still use 1" XPS (Pink stuff), and have found the Foamular 250 as opposed to the formula 150 for even greater compression strength for only a $1 to $2 per sheet more. Am I missing anything? Will the EPS perform as good or better in this installation? XPS would be R10, EPS is R8, however I will be framing and filling stud cavity with batt insulation anyways. I will attempt to link to the EPS I am considering here: - Thank you for any insight back. I appreciate it. (Also I live in Climate Zone 5 in South East Michigan, however I don't think that is relevant to my question.)

Dec 6, 2017 1:52 PM ET

Edited Dec 6, 2017 1:53 PM ET.

Response to User-6948667
by Martin Holladay

You have double-posted your question, which is confusing. It's best to post your question just once.

Here is a link to the other place where you posted your question; you'll find my answer there: Interior Basement Insulation - XPS vs EPS selection.

Jan 2, 2018 1:45 PM ET

Broken link (Click here to see a climate zone map.) Read more:
by William Sherman

The climate zone map link is broken.

Jan 2, 2018 1:52 PM ET

Response to William Sherman
by Martin Holladay

Thanks for notifying us of the broken link. We appreciate it. The link has been fixed.

Apr 9, 2018 3:36 PM ET

Exterior XPS w/ dimple mat?
by Bryanw511

I would prefer to insulate the exterior of my new home's basement wall. I also plan to use a spray on elastomeric waterproofing membrane and a dimple mat. After the membrane, would you recommend the XPS and then the dimple mat, or the other way around?

Apr 10, 2018 10:27 AM ET

by Jay

My basement does not get very much air flow. Which would you say is better for indoor air quality:
1) DOW XPS with BLUEDGE (non HBCD flame retardant) but diffusing blowing agent
2) EPS with HBCD flame retardant
Thanks in advance!

Apr 10, 2018 10:48 AM ET

@ Bryanw511, for insulating
by Michael Maines

@ Bryanw511, for insulating foundation exteriors, I recommend type 2, 15 psi EPS treated with borates for insect resistance. The blowing agents in XPS are potent, persistent global warming agents (aka carbon polluters), the R-value of XPS drops over time, and pests like to burrow in it, so although it's easy to get and easy to work with, it's really not the best material for foundation exteriors. The dimple mat can go on either the interior or exterior of the insulation, since water won't affect the foam, the air gap is small enough that it won't affect the insulating value significantly, and the reason for the dimple mat is to keep water away from the concrete.

Apr 10, 2018 10:54 AM ET

@User 7xx, the diffusing
by Michael Maines

@User 7xx, the diffusing blowing agents in XPS are not a health concern, as far as I know, they just destroy the atmosphere. I don't know enough about the health concerns of flame retardants to advise you, but would ask why you don't consider increasing your ventilation to healthy, comfortable rates?

May 30, 2018 4:55 PM ET

Edited May 30, 2018 5:06 PM ET.

85 year old walkout with joists set in concrete
by user-7062640

An energy audit identified foundation insulation as my most important step. While I would prefer to dig and insulate the exterior, a slab-on-grade addition on one side of my house makes insulation from the exterior nearly impossible for that side. Other sides have complications too, so insulation from the interior seems to be my option.

I'm in Southeast Alaska, which is wet and has an average Dec-Feb low of 26 degrees F.

My 85 year old walkout basement has some efflorescence and dampness, but no bulk water problems that I'm aware of. The slab floor has been completely dry. During a 100 year flood event last fall a failed storm drain broke like a fire hydrant next to my foundation, and the only impact was some dampness and efflorescence on the cement foundation wall there. With french drains, sloping, and some bituthene, I'm addressing exterior drainage to hopefully limit what efflorescence and dampness I do see.

Roughly 5 years ago the prior owner painted a cement sealant onto the interior foundation wall. That suggests there had been at least dampness in the past. It also means drying to the inside is already limited.

I see a few cracks in the concrete foundation wall, but no water coming through them. The cracks are in areas that were sealed by the prior owner, and areas that were not sealed by the prior owner. Given that it's 85 year old concrete, I find a few, dry cracks to be not so bad.

Complicating matters, my joists are set in the concrete foundation wall. It appears they were dipped in tar first, to limit susceptibility to water from the concrete. While I see efflorescence around them, they are hard and appear not compromised despite being in concrete for 85 years. Still, they're my primary concern.

I plan to insulate from the inside with 2" EPS against the foundation, and 2" polyiso to the interior of that. That achieves R20, avoids polyiso's R loss in the cold, and avoids the need to frame a wall thanks to polyiso's smoke and flame rating. Both materials have fairly limited greenhouse gas implications.

I'll appreciate any advice, and have a few specific questions:

1) How concerned should I be about making any changes since my joists are in the cement? I may be making the cement wetter if it can't dry to the inside, but my hunch is that it's fairly wet already, can't dry much to the interior already, and my exterior drainage work is making it less wet, regardless of the interior insulation. Is there any benefit to not insulating the cement rim joist area? Is insulating my foundation at all out of the question?

2) Is there any benefit to insulating from the exterior where possible, and having overlap sections where both interior and exterior are insulated to avoid heat moving through the concrete there? Or should just I stick with the continuity of entirely interior insulation?

3) It's been suggested I install drainage mat against my interior foundation wall before rigid foam. But since I do not have an interior french drain or bulk water problems right now, I think that would just allow warm air to get behind the rigid foam and cause moisture problems while reducing the insulation value due to cold air flowing down the drainage mat.

4) Since I do see some cracks in the concrete wall, should I be concerned that insulating the foundation could cause it to occasionally freeze, and crack more significantly, even though it's slightly below grade at the cracks? Should I be insulating less than R20, as suggested for a brick building here:

Thanks much for any advice folks have. I enjoy and appreciate GBA tremendously! Despite my inability to figure out how to add my name, I'm a GBA subscriber and it's a worthwhile investment.

May 30, 2018 5:08 PM ET

Edited May 30, 2018 5:20 PM ET.

There is no point to using EPS on the exterior
by Dana Dorsett

There is no point to using EPS if the average winter low is +26F. Inch for inch even the worst polyiso out there will beat it's performance with the cold side at 26F, and below grade the temperatures are much warmer.

In the attached file take a look at Figure 2 and compare the the derating curve for R5.2/inch roofing polyiso, and compare that to the Mineral Wool Batt curve (which is comparable to EPS). When it's 25F on the cold side, 65F on the warm side, the mean temp through the foam will be about 45F.

At 45F a polyiso- only solutoin would be delivering R5/inch. If the outer half is mineral wool (or Type II EPS), the mean temp through the outer layer would be 35F, and at best breaking-even with the performance of polyiso.

Below grade it's warmer than that, and the polyiso wins every time, even on the outer layers.

Most foil faced polyiso would outperform the straw-man polyiso example in that picture, thus beating EPS by even more. (R6-R6.5/inch @ 75F mean temp rather than R5.2.)

polyisoderating.pdf 115.42 KB

May 31, 2018 7:24 AM ET

Edited May 31, 2018 7:26 AM ET.

Response to User-7062640 (Comment #178)
by Martin Holladay

First of all, can you tell us your name?

Q. "How concerned should I be about making any changes since my joists are in the concrete?"

A. This is a hard risk to evaluate. There are lots of variables. Briefly, it's important to insulate your foundation and rim joist area; and this work will make the concrete colder, increasing the risk that your joist ends will rot.

The classic solution is to cut off the embedded ends of the joists with a Sawzall and support the joists with a new framed wall on a secure footing.

Q. "Is there any benefit to insulating from the exterior where possible, and having overlap sections where both interior and exterior are insulated to avoid heat moving through the concrete there?"

A. Again, this is hard to evaluate. Ideally, either all of the insulation will be on either the interior or the exterior of the wall. But with embedded joists, an argument can be made in favor of installing as much exterior insulation as possible, even if the exterior insulation doesn't extend all the way down to the footing.

Q. "Should I be concerned that insulating the foundation could cause it to occasionally freeze, and crack more significantly, even though it's slightly below grade at the cracks?"

A. No. The problem you describe almost never occurs.

May 31, 2018 8:42 PM ET

Edited May 31, 2018 8:50 PM ET.

Thanks Dana and Martin
by user-7062640

Thanks for your insight. Dana, that graph is super helpful. Martin, I really appreciate your answers as always.

If you have any thoughts about the drain mat, or rain slicker, between the insulation and foundation, please do let me know. I plan not to use drain mat, and to secure the foam board with anchors (not adhesive) so I can aways cut the spray foam around the edge and take a piece of polyiso off 5 years down the road to see how things look.

-Shawn Salias

Jun 1, 2018 6:04 AM ET

Response to Shawn Salias
by Martin Holladay

The interior dimple mat is optional. If you see no signs of water entry, and if there is no interior French drain, the typical approach would be to skip the dimple mat.

Jun 19, 2018 2:30 PM ET

Edited Jun 19, 2018 2:32 PM ET.

half basement with concrete sill
by Jared Weber

What is the best way to insulate my basement wall? (picture attached)


Jun 19, 2018 2:49 PM ET

Edited Jun 19, 2018 3:00 PM ET.

Response to Jared Weber
by Martin Holladay

Well, if he's still there, the easiest way would be to tell the spray foam guy in the photo to aim his wand at the walls.


Spray foam guy.jpg

Jun 19, 2018 2:52 PM ET

Second response to Jared Weber
by Martin Holladay

I'm not sure there is a "best" way.

The above-grade portion (the portion with stud walls) is insulated the same way as any above-grade wall.

The concrete portion of the wall is insulated as described in the article on this page.

If you want the interior wall to be co-planar, you can install floor-to-ceiling studs on the interior side of the insulation once the insulation work is complete, and then install drywall or plaster.

Jun 20, 2018 10:06 AM ET

half basement insulation
by Jared Weber

That handsome guy in the super-suit was painting the ceiling joists (which will be left exposed)... same guy will be doing the insulation...

Thanks for your input. I guess I have a couple additional questions.
First, is the standard pink fluffy stuff I see in Home Depot the way to go for the above grade portion? I also see there are alternatives at the store such as Rock Wool and various types of foam panels
Second, for the space between the ceiling joists, which is hard to see in the picture... I saw a video where XPS foam and "great stuff" was used to insulate the rim joist area -- my rim joist area isn't exactly the same as the video because it doesn't' but up right against the foundation wall -- what would you suggest for this area?
Lastly, and maybe this is overkill... but if I used traditional insulation between the studs would it be a good or bad idea to put 1/2" XPS over the studs before I install my drywall to get a little bit more R-value down there?

Jun 20, 2018 12:28 PM ET

Edited Jun 20, 2018 12:30 PM ET.

Third response to Jared Weber
by Martin Holladay

Now that I think about it, I should have realized that the guy in the suit is a painter, not a spray foam contractor, since his wand has only one hose, not two hoses.

Q. "First, is the standard pink fluffy stuff I see in Home Depot the way to go for the above grade portion?"

A. It sounds like this type of work is new to you. If possible, try to find an experienced contractor to guide you. Either fiberglass batts, mineral wool, cellulose, or spray foam can work, but whatever insulation material you use, it has to be installed properly. In case you end up using fiberglass batts, I'll provide a link to a relevant article: "Installing Fiberglass Right."

Q. "I saw a video where XPS foam and Great Stuff was used to insulate the rim joist area."

A. Here is a link to an article to advise you on that work: "Insulating rim joists."

Q. "If I used traditional insulation between the studs would it be a good or bad idea to put 1/2 inch XPS over the studs before I install my drywall to get a little bit more R-value down there?"

A. Installing interior rigid foam is fine, but most green builders avoid the use of XPS, which is manufactured with a blowing agent that has a high global warming potential. Either EPS or polyiso would be a better choice. Here are links to two articles you should read:

"Walls With Interior Rigid Foam"

"Choosing Rigid Foam"

Jul 12, 2018 10:02 AM ET

Insulating over dimple mat
by Jay Lista

If I already have dimple mat installed with protruting ramset fasteners halfway up the interior wall (to grade level) and the top half of the interior wall is exposed concrete, does it make sense to do something like 2" of polyiso on the top half, 1 1/2" of polyiso on the bottom half, and then 1/2" polyiso (to get to R13 or greater on the full assembly) as a second layer over the entire wall to level everything out/cover the seams?

The rigid foam would be flush against all concrete surfaces, but I doubt I could fully airseal the bottom half of the wall between the platon mat and the rigid foam (potential condensation issues), and I'd also be concerned about the protruding ramset nails puncturing the back of the foil backing on the polyiso as I attempt to level everything out on the wall (potential moisture transfer through the nails direct into the polyiso foam, but it would still have the dimple mat between it and the concrete)

Would spray foam insulation be a better choice or am I OK with the rigid foam approach? For cost reasons I'd obviously prefer the rigid, but not if it will cost me more down the road. Thanks in advance.

Jul 12, 2018 11:26 AM ET

Edited Jul 13, 2018 4:27 AM ET.

Response to Jay Lista
by Martin Holladay

It looks like you have posted your question twice, on two different pages. I have already answered your question on the page where you first posted it.

Here is the link: "Foam over Platon dimple mat on foundation."

Jul 12, 2018 11:36 PM ET

Edited Jul 12, 2018 11:40 PM ET.

Hi Martin
by Jay Lista

I think my post was confusing - I'm insulating the basement on the inside, not the outside. I reposted here since this thread made more sense than the last one that i now see was discussing insulating on the outside with dimple mat. Sorry for the mixup.

Jul 13, 2018 4:29 AM ET

Second response to Jay Lista
by Martin Holladay

Using rigid foam would work, in spite of the imperfections you mention. Closed-cell spray foam would be easier and would perform slightly better. Is spray foam worth the higher cost? You decide.

Jul 20, 2018 11:21 AM ET

Interior +/- Exterior XPS
by user-7122242

Thank you for your very helpful article - I am new to this and I have been learning a lot from you. We are building a house in Nova Scotia, Canada, Zone 6A. It is near the ocean and the weather is often wet. The lower level of the house is 20 x 50, one 20 foot wall is all below grade, one 50 foot wall is a step-down foundation and the 2 other walls are above grade (the hill slopes in 2 directions). The lower level will be finished and I very much want to minimize any moisture. The builder plans to install a double run of footing drains and I believe a surface drain is also planned. We will have a metal roof with 2 foot overhangs. The current plans for the basement part is to have a 10" concrete wall, 2 inches of XPS (one 2 inch layer, interlocking sheets, seam sealed etc.), and a 2x4 framed wall with rock wool batts in cavity. Our code requires a vapour barrier before the dry wall - I am hoping they will accept a smart barrier like Membrain as a substitute. My questions for you are:

1. Should we also add 2 inches of XPS on the exterior? Are there any issues with sandwiching the concrete between 2 non-breathable layers? Do you think any benefits would be worth the extra expense? If we go this route the builder wants to slightly cantilever the house over the foundation.

2. Should we consider increasing the interior XPS? Interior space is not an issue for the 20 foot wall so we could easily increase the thickness there. Space is more of an issue on the 50 foot with the step-down foundation but it wouldn't be impossible to add another inch.

3. Are there benefits to using 2 1-inch layers of XPS, rather than a single 2-inch layer (e.g., to offset the sheets, etc.).

4. Any advice on avoiding the vapour barrier? The builder understands my concerns but thinks the inspector will require one.

I'd appreciate any advice you have. Thanks very much - Janet Ingles

Jul 20, 2018 12:26 PM ET

Response to Janet Ingles (Comment #192)
by Martin Holladay

Q. "Should we also add 2 inches of XPS on the exterior?"

A. In the U.S., codes require basement walls in Zone 6 to be insulated to R-15. Your current plan (2 inches of interior XPS, which equals R-10, plus 3.5 inches of mineral wool, which equals at least R-13) gives you an R-23 wall, which exceeds U.S. code requirements.

So the answer to your question depends partly on your R-value goal. If you add another R-10 to the exterior, you'll be up to R-33. That's not crazy, if you can afford it.

However, green builders avoid the use of XPS, which is manufactured with a blowing agent that has a high global warming potential. What you want is EPS, not XPS. More information here: "Choosing Rigid Foam."

Q. "Are there any issues with sandwiching the concrete between 2 non-breathable layers?"

A. No.

Q. "Do you think any benefits would be worth the extra expense?"

A. You'll have to do the math. The answer depends in part on the cost to install the foam and your local energy cost.

Q. "If we go this route the builder wants to slightly cantilever the house over the foundation."

A. That's a normal approach.

Q. "Should we consider increasing the interior XPS?"

A. I may sound like a broken record, but the answer depends on your goals and your budget. Are you building a Passivhaus? Can you afford the upcharge? Or are you looking for a quick 5-year payback on any incremental work? I can't answer these questions for you -- your values and budget affect the answer.

Q. "Are there benefits to using 2 1-inch layers of XPS, rather than a single 2-inch layer (e.g., to offset the sheets, etc.)."

A. Yes. The thermal performance of two layers with offset seams will be slightly better than the thermal performance of one layer. Is it worth the upcharge in labor? You decide.

Q. "Any advice on avoiding the vapor barrier?"

A. Pray for an intelligent, well-educated building inspector. Failing that, try gentle education. If that doesn't work, install MemBrain.

Jul 20, 2018 2:43 PM ET

Interior +/- Exterior XPS Continued
by user-7122242

Martin - Thanks so much for your helpful and prompt comments. Cost aside, how much impact would adding exterior foam and/or increasing interior foam have on basement moisture? One of my primary goals is to avoid the damp/musty smells that I associate with basements - Janet

Jul 20, 2018 2:53 PM ET

A few comments for Janet Ingles
by Dana Dorsett

The R10 XPS eventually drops to R8.4 (the same as 2" EPS of the same density) as it loses it's climate damaging HFC blowing agents. The manufacturers will warranty it to 90% of the labeled R value as long as it's not in a location where it can become saturated wet. That would make it R9. So from a design point of view consider it to be R8.4-R9 above grade (where it can drain), and "who knows, way better than nothing", below grade. The better drainage you have on the backfill, the more likely it is to still perform between R8.4-R9 in 25 years. But using Type-II EPS would be R8.4 now, and R8.4 fifty years from now, which is fine.

An R13 studwall is not the same as R13 continuous- it's closer to R9- R10 "whole assembly" for that layer after accounting the thermal bridging of the studs. From a thermal performance point of view you're looking at about R20 "whole-wall", with credit given for the R-value of the gyproc & concrete, and the interior & exterior air films, the 2" foam, and any finishing materials over the exerior of the foam, etc.. That would meet code for US climate zone 6 (which is comparable to the climate of NS). The thermal mass of the concrete also gives some benefit, but not enough to really matter in your climate & stackup.

With the 2" of foam on the exterior you could put the vapor barrier between the studwall and concrete (rather than on the interior side) and still meet the requirements of the NBC. I'm not sure if NS changed any of that for local codes. See Table on p.30 (PDF pagination):

All locations in NS have fewer than 5000 (base 19C) annual heating degree days (Halifax runs about 4200 HDD/year) so as long as the foam-R/cavity-R ratio is more than 0.2, the code enforcers should be happy with the vapor barrier on the concrete. With the foam derated to a fully depleted R8.4 and R15 rock wool in the stud wall there would be a ratio of 0.56 at center cavity, which is good enough to meet code even in a 9,999 HDD(C) climate. With the labeled R10/R15 it would be a ratio of 0.66. In the US the IRC code prescribes a minimum of R7.5 on the exterior of a 2x4 wall for zone 6, to be able to use standard interior latex paint as the interior side vapor retarder:

Your stackup would meet/beat that too.

Putting the vapor barrier between the studwall & concrete provides a capillary break between the concrete and the fiber insulation & wood, both of which could otherwise wick ground moisture if the footing is seasonally wet.

See also:

Jul 20, 2018 3:13 PM ET

extra foam insulation
by Jon R

The payback on extra foam will be much better above and near the surface. So consider adding exterior foam that doesn't extend all the way down to the footings.

Plan on doing some dehumidification to prevent musty smells.

Jul 20, 2018 3:40 PM ET

Response to Janet Ingles (Comment #194)
by Martin Holladay

Q. "One of my primary goals is to avoid the damp/musty smells that I associate with basements."

A. The origin of any musty smells won't be your walls (if the walls are detailed as described). The origin will be your slab floor (if it isn't insulated). The solution -- the key to a basement that doesn't have the musty smell -- is to install a continuous layer of horizontal rigid foam under your slab.

Jul 20, 2018 3:49 PM ET

Edited Jul 20, 2018 3:50 PM ET.

Musty smell mitigation...
by Dana Dorsett

The musty basement smell can also be mitigated by insulating under the slab (all of the slab). The subsoil temperatures in NS are lower than the summertime outdoor dew points, so moisture tends to collect on anything resting on the cool slab, creating mold and "musty basement smell" conditions. An inch of EPS does a lot for the musty basement smell, but 2" can still be financially rational on a lifecycle basis if you energy costs are high or your average construction costs low.

It's fine to use Type-II (1.5lbs per cubic foot nominal density) or some somewhat less expensive 1.25lb density Type-VIII EPS under a residential slab, but not under the footings. Building materials reclaimers usually have it in stock, stripped from large commercial buildings during demolition or re-roofing. (They also tend to have roofing polyisocyanurate, which is higher R/inch, but not suitable for use under a slab due to it's moisture wicking characteristics.) Used roofing EPS is typically less than 1/3 the cost of new foam.

edited to add:

(Looks like Martin is posting while I'm typing, again... :-) )

Jul 20, 2018 7:15 PM ET

Edited Jul 20, 2018 8:02 PM ET.

Interior +/- Exterior XPS Continued++
by user-7122242

Thank you both very much for your comments...We were planning 4" rigid foam underneath 4" concrete (under the entire slab with vapour barrier between). We want to avoid musty smells but also prevent too cold winter feet...
ps. should we be insulating under the footings too?

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