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Helpful? 4

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, GBA Advisor

Here at GBAGreenBuildingAdvisor.com, 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.

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.

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

1.
Fri, 06/29/2012 - 10:26

Heads up Martin
by 5C8rvfuWev

Helpful? 0

Unless my aging eyes deceive me, Martin, I think your text is referencing the CZ map GBA uses at the top of the Q&A section while the map you link to is kind of cart before horse.

Or am I the one confused this time?

Good summary by the way. Thanks. Makes me wish I had a basement.


2.
Fri, 06/29/2012 - 10:27

Case study at Marc Rosenbaum's house
by John Rockwell

Helpful? 0

An excellent example presented over three blogs posts can be found at Marc Rosenbaum's blog "Thriving on Low Carbon":

http://blog.energysmiths.com/2011/07/basement-insulation-progress.html


3.
Fri, 06/29/2012 - 10:39

Edited Fri, 06/29/2012 - 10:51.

is poly always a problem? Or simply near useless at best.
by Dustin Harris

Helpful? 0

The idea of encapsulating wood and fiberglass behind poly certainly is scary, but I can imagine that concrete/foam/poly/stud/sheetrock might be acceptable....and (perhaps) even a very slight improvement.

Keep up the excellent articles. Good stuff!

(and I see you took care of the climate map link)


4.
Fri, 06/29/2012 - 10:46

Response to Joe W and Dustin Harris
by Martin Holladay, GBA Advisor

Helpful? 0

Joe and Dustin,
Thanks for catching my error with the bad climate zone map link. The error has been corrected in the text.


5.
Fri, 06/29/2012 - 10:58

Response to Dustin Harris
by Martin Holladay, GBA Advisor

Helpful? 0

Dustin,
Q. "Is poly always a problem?"

A. As far as I'm concerned, yes. You don't want it anywhere near the interior of a basement wall. If water beads up on one side of the polyethylene, it's going to stay there. Just say no to polyethylene.


6.
Fri, 06/29/2012 - 12:34

savings overstated?
by Michael Blasnik

Helpful? 0

Martin- Lots of great information about how to insulate basements, but you may want to reconsider the savings claims.

I've seen a few studies of basement wall insulation and the measured savings are much smaller than the numbers you cite. You might want to check out this study in Minneapolis http://www.ornl.gov/info/reports/1991/3445606042537.pdf which was sponsored by ORNL and found average savings of 92 therms of gas for interior insulation which would be just $66/year using the same energy prices that were used in the source you cite. That source (also ORNL, strangely enough) claimed savings of $400 for R-10 in Minneapolis.

You might also want to check out this other Minneapolis study http://www.mncee.org/getattachment/c09d3ced-ab1f-49ae-be32-649adf7190f7/ where they found 197 therms average savings from interior insulation which would be $142 using that same energy price.

In both of these studies, the payback periods were fairly long even when excluding the costs for finishing the wall. One study did conclude that for intentionally conditioned basement the payback may be reasonable (in Minneapolis). But in the more common unintentionally conditioned basement, much of the reduction in conduction losses shows up as a warmer basement.

A basement wall insulation retrofit can provide lots of benefits -- including more comfortable and finished space, but the payback based on energy savings alone may not be that fast.


7.
Fri, 06/29/2012 - 13:17

Edited Fri, 06/29/2012 - 13:21.

Response to Michael Blasnik
by Martin Holladay, GBA Advisor

Helpful? 0

Michael,
Thanks for your comments and links. I have edited the text of my article to reflect your comments.

I welcome other GBA readers to share any links to relevant studies or monitoring data from their own homes or projects.


8.
Fri, 06/29/2012 - 14:09

Comfort and Maintenance
by Armando Cobo

Helpful? 0

I've designed and built homes with and without basement insulation in my builder days, and there are no comparisons on the comfort side, even though the ROI may not be there. Also, how much sheetrock and carpet mold will cost to fix when rigid foam is not installed?


9.
Fri, 06/29/2012 - 14:16

Response to Armando Cobo
by Martin Holladay, GBA Advisor

Helpful? 0

Armando,
Joe Lstiburek agrees with you -- he says, “Insulating the slab and basement walls controls mustiness and mold in the summertime. Now moisture doesn’t condense on cold surfaces in the basement. We insulate less for energy efficiency than for comfort and odor control.”


10.
Fri, 06/29/2012 - 15:47

Edited Fri, 06/29/2012 - 15:50.

I knew it!!!
by Armando Cobo

Helpful? -1

I don't want to brag here, but Dr. Joe is taking credit for my thinking.... ;-))


11.
Fri, 06/29/2012 - 18:49

Martin, Nice compilation ...
by Milan Jurich

Helpful? 0

Martin,
Nice compilation ... thank you! Question ... which method, interior or exterior, would make most sense if the upper walls were framed with 2x6 construction, OSB, 2" rigid foam, housewrap, 1x3 furring and fiber cement cladding below which would be a 42" water table of synthetic stone? Is it best to align all of the rigid foam in 1 plane from footing to the roof truss before covering with housewrap; then attach lathe and cladding to the furring exterior to the foam? Trying to visualize what would work best ... interior or exterior insulation in this case. Would appreciate a sketch from someone as to how that might look from a wall section perspective. Thanks!


12.
Fri, 06/29/2012 - 22:15

worry
by John Klingel

Helpful? 0

I have no experience, but I would worry about vapor that does get to a concrete wall with foam over it (interior insulation). Won't that wall now be even colder than when naked, with no good way to dry? Unless your air sealing is perfect, some water will condense. What happens to it? Curious.


13.
Fri, 06/29/2012 - 23:14

Edited Fri, 06/29/2012 - 23:16.

I'm with Klingel, interior
by aj builder, Upstate NY Zone 6a

Helpful? 0

I'm with Klingel, interior rigid foam will get interior air behind which equals crap.
Better, spray foam or exterior...

Best ICF or Thermomass.


14.
Sat, 06/30/2012 - 04:55

Response to Milan Jurich
by Martin Holladay, GBA Advisor

Helpful? 0

Milan,
I'm not a fan of synthetic stone, which is hard to make look right. There are many pitfalls -- for example, the synthetic stone has to extend below grade to look right, and that sets certain limits on permissible details.

In short, you could install either interior or exterior insulation on your basement wall, but exterior insulation will probably be easier to detail in your situation. Good luck: synthetic stone over OSB is risky if you don't know what you're doing.


15.
Sat, 06/30/2012 - 05:05

Edited Sat, 06/30/2012 - 05:08.

Response to John Klingel
by Martin Holladay, GBA Advisor

Helpful? 1

John,
Concrete is a wonderful substance, and it holds up well when exposed to water. Remember, the exterior side of most basement walls and basement slabs are constantly damp -- and builders haven't been worrying about what that means for the last 100 years. You drive by concrete bridge abutments every day, and I doubt that you stay up at night worrying about the damp concrete. Nobody is running out with a hairdryer to keep the concrete bridge abutments dry every time it rains.

When a basement wall is covered with rigid foam or closed-cell spray foam, the interior air is no longer contacting the basement wall. It's a great system. There is no magical hidden condensation going on under the foam, because the interior air isn't there. The interior air is on the other side of the foam.

So now let's look at your comments.

"I would worry" -- please don't -- "about vapor that does get to a concrete wall with foam over it (interior insulation)." -- there really is very little vapor that makes it through the foam; that's physics. We can do the calculations so we know how small the number is -- "Won't that wall now be even colder than when naked" -- yes, that's the whole idea of insulation -- "with no good way to dry?" -- well, most concrete walls can dry a little bit at the top, especially if the above-grade portion of the exterior is exposed to some air. And remember, concrete likes to be damp, because that makes for stronger concrete. "Unless your air sealing is perfect, some water will condense." -- So now you are worried about moisture piggybacking on air rather than vapor that gets through the foam. But there is no air moving through the wall, because concrete is a good air barrier. So there isn't a stack effect or a pressure differential or wind operating to move any air. "What happens to it?" Well, if the concrete is damp, it's damp. But it's on the outside of your thermal envelope. The soil is damp too. Just stop worrying. You'll never make the environment outside your thermal envelope completely dry unless you move to Arizona.


16.
Sat, 06/30/2012 - 05:11

Note to GBA readers
by Martin Holladay, GBA Advisor

Helpful? 0

I won't be able to respond to questions posted on this page for a while, because I'll be on vacation for the next two weeks. I imagine that this dialog will continue, however, even if I can participate. I'll be back on July 16.


17.
Sat, 06/30/2012 - 09:16

As John Brooks would say, 3D
by aj builder, Upstate NY Zone 6a

Helpful? 0

As John Brooks would say, 3D networks, air will be getting past rigid foam sheathing. Spray foam, ICF, Thermomass much better.

Enjoy the vaca.... Shade is better than sunblock.... :)


18.
Sat, 06/30/2012 - 20:59

Edited Sat, 06/30/2012 - 21:01.

Yes, true
by John Klingel

Helpful? 0

Ok, so my suspicions have been confirmed. Yes, concrete likes being damp, and, no, I don't worry about wet concrete all over the country in bridges, etc; that stuff is not in my house. Too, I'm not worried about the concrete; it's the possible kooties. If concrete is damp, chit is going to grow on it. THAT is what I am concerned about. The fact that there won't be much air movement is a good point; not much air, not much water. Not much water, not much growing. OK; I quit worrying.... mostly. That said, were I to insulate a concrete wall on the inside, I'd make sure the humidity was low inside, just in case.


19.
Sun, 07/01/2012 - 19:22

Sorry, Martin, but I STILL don't understand..
by Dustin Harris

Helpful? 0

Since some moisture and air can get between foam insulation boards and create condensation, wouldn't spray foam be better? And wouldn't poly over foam boards = spray foam?

I certainly understand that one would not want anything encapsulated that has bad reactions with moisture. Also understand that spray is the only suitable solution on uneven surfaces like stone and mortar as air would circulate in gap behind foam boards.


20.
Sun, 07/01/2012 - 21:25

Taming the basement
by Ron Keagle

Helpful? 1

For any future basements, I would split every hair possible to achieve maximum perfection. I would strive to prevent any contact between the foundation concrete and the ground; and also between the foundation concrete and the interior air.

Concrete in contact with the ground will always have a moisture content from absorption of ground moisture. And if the interior surfaces of the concrete is exposed to interior air, the moisture in the concrete will always diffuse to the house interior if the interior vapor pressure is low enough, which is likely to be most of the time.

Preventing moisture from entering the exterior of the concrete would be adequate to stop this inward diffusing vapor, however, this is not the only source of moisture in the concrete. During high summer humidity, interior vapor is likely to condense on cool interior surfaces of basement concrete. This condensation will be absorbed into the concrete with a sponge action. It will tend to charge the interior zone of the concrete with perhaps a higher moisture content than what the outside soil is wicking into the exterior of the concrete. Then when the ambient humidity level drops a few days later, this inward-accumulating wetness in the concrete reverses itself and beings diffusing back into the living space.

So, in the summertime, you have moisture migrating through the basement walls from exterior ground moisture, and you have moisture periodically charging into the interior of the basement walls from the absorption of condensing ambient vapor from the interior. And then much of that periodic interior moisture charging comes back into the interior when ambient humidity drops.

Sealing interior surface of the concrete with an impermeable layer might solve the entire problem. It would prevent moisture in the concrete from entering the air of the living space. However, it still might be possible for highly humid interior air to condense on the surface of the interior sealing membrane. This may or may not be a problem, depending on the extent and whether this membrane was enclosed in a finished wall structure with limited air circulation.

Insulation on either the exterior or the interior might keep the wall interior temperature from falling below the summertime ambient temperature, and thus prevent condensation of highly humid interior air on the interior side of the basement walls. However, the insulation would need to be impermeable as is the case with closed cell foam.

I really do not know what microbial problems this basement dampness can cause, but I am convinced that it causes some. Concrete is a sponge, and we all know that a damp sponge is an ideal incubator for bacterial growth. I suspect that it is the interior air that transports the seeds of this microbial incubation to the moist sponge of the basement walls. In short, I don’t mind a basement, but I don’t want to live with one. So the key is to encapsulate the basement in such a way that it cannot absorb moisture from both inside and outside.


21.
Mon, 07/02/2012 - 07:16

taming the basement
by David Metzger

Helpful? 0

Well put Ron.
I put an apartment in my walk out basement in zone 6. I incapsulated and insulated the concrete inside and out. I did every detail right--except insulate under the footing--and I'm not losing sleep over that detail. The basement on average is 15% more humid than the upstairs and never goes above 55% unless you open the windows. But since I have an ERV, there's no need to open the windows. I did, however, use open cell icynene on the interior and sheet rocked over that. In retrospect, it might have been more prudent to use moisture and mold-resistant sheetrock on the walls--though I was smart enough to use metal studs. Now I guess I'll just hope there's nothing growing on that paper backing of the sheetrock. Time will tell.


22.
Mon, 07/02/2012 - 08:40

what about the floor
by D Mikulec

Helpful? 0

Martin,

Thanks for your article about insulating basement walls. You hint at the benefits of insulating the floor. Can you give your thoughts for doing this in a completed home? Old DIY articles suggested a poly sheet on the concrete followed by 2x4 sleepers with the voids filled with foam and the whole thing covered with 3/4" plywood. I think this solution would cause a problem at the stairs. If carpet and pad are installed directly over the concrete, what advice do you have for the pad?

Thanks,

Don


23.
Mon, 07/02/2012 - 14:20

Seal and insulate Basements
by Bruce Haynes

Helpful? 0

I am finishing up a total (daylight) basement remodel that will be living space. I excavated the perimeter, pressure washed the foundation, removed and reinstalled the footing drain, (filled with sediment from 1970), damproofed the wall, installed 2" closed cell foam board, covered the foam with painted galvanized metal roofing screwed with tapcons to the concrete (use impact driver, not hammer drill!) , the front third i am installing a nice rock veneer over mesh screwed through the insulation to the concrete on 6" centers. Inside on the west half i used existing furring strips but beefed them up to be 2.25 and infilled with rigid foam, then sheetrock. On the floor I installed warmwire, then schluter ditra and tile over. I was pretty pleased with the results except realized a thermo break on the floor would have been a benefit and i did not air seal above the walls, just used fiberglass insulaton.

Now three years later based on what i see (disturbing) in my new fluke infra red thermal camera, i am doing the east half inside. I removed all furring strips, installed 4.5" polysci insulation in the overhead joist to rim joist area, caulked all joints before the blocks then after the blocks were installed too. Applied 1" t and g closed cell foam board to the walls, then screwed (tapcons) thrrough the 2x4's flat against the 1" foam on 2' centers with 1.5 foil covered styrofoam (white stuff) in between the strips (makes a flush surface). On the floor i layed a strip of 3/4 " thick foam under the horizonal base 2.4 strip to keep it off the floor. Sheetrock will cover all. On the flloor I am using warm wire again but this time i am putting down 1/4" cork then the warm wire then tile. Oh and around the window framing inside I am putting 3/4" foam, then the trim.

It will be interesting to see the difference in the west and east sides after the floor is in. This project is at the 4000' level on Mt Hood in Oregon so we have snow on the ground from November to May.


24.
Mon, 07/02/2012 - 21:13

Additions are better places
by aj builder, Upstate NY Zone 6a

Helpful? 0

Additions are better places for added space verses basements which always are a compromise.


25.
Wed, 07/04/2012 - 18:09

basement that has an exisiting vapor barrier
by kathy olmstead

Helpful? 0

I have a basement that was previously insulated with rigid foam against the poured concrete wall but it also has a layer of poly under the drywall (my home is in Minnesota and where poly vapor barrier used to be a code requirement.) There is no evidence of mold or damp; should I be concerned? Should I consider removing the drywall and the vapor barrier and redoing it, even though there does not appear to be any problem?


26.
Wed, 07/04/2012 - 23:08

Avoiding foam?
by Jason Holstine

Helpful? 0

Martin, highly useful to have a clear resource for this tricky subject. But, a simple question: what if one wishes to avoid foams for cradle-grave or health concerns (for insulating/reinforcing existing basements)? You brush by this briefly, but what's the concern with a 2x4 stud wall installed 1" off the concrete filled tightly with ComfortBatt? What of using Roxul's new semi-rigid rockwool boards (ComfortBoard or CavityRock)?


27.
Wed, 07/04/2012 - 23:18

Kathy, the poly is redundant
by aj builder, Upstate NY Zone 6a

Helpful? 0

Kathy, the poly is redundant over the foam and is fine. Leave it.


28.
Wed, 07/04/2012 - 23:30

Edited Wed, 07/11/2012 - 10:50.

Jason, the dryer the cellar
by aj builder, Upstate NY Zone 6a

Helpful? 0

Jason, the dryer the cellar the more I say your idea is fine. Wet cellars should have no contents stored within and have no living space constructed. Better to build an addition or upgrade homes. I drylocked 2 coats an entire cellar including the floors and it made a huge difference. Dehumidifiers and sump pumps, etc. Add heat as needed. Control the humidity and temperature and much of the battle is won.


29.
Thu, 07/05/2012 - 07:11

Edited Fri, 07/06/2012 - 09:47.

interior and exterior insulation?
by Mark Fredericks

Helpful? 0

I purchased a 60 year old home last summer and quickly upgraded the insulation on the basement interior with XPS foam and a 2x4 wall. I'm now planning to excavate the exterior of the home to address some drainage issues and run my downspouts into a common footing drain. I know this exterior work should have been done before interior insulation but we were short on time for energy rebates and it needed to be done during the winter.

Outside, I hope to dig out around the foundation, installing a new footing drain and damp-proof the exterior concrete walls. I'm wondering while I have these trenches dug, is it still valuable to also wrap the exterior of the foundation in foam board too? Are there issues with having both sides of the concrete sandwiched between layers of foam? and will this exterior insulation still be cost effective?

Thanks for the help!


30.
Thu, 07/05/2012 - 08:33

Edited Sat, 07/07/2012 - 09:07.

Poly against a basement wall
by Kevin Ring

Helpful? 0

I an a HERS Rater in MA. We have many builders who insulate finished basements with fiberglass batts. Why do you categorically proscribe the use of poly anywhere in such an assembly? It seems to me that a single layer of poly directly against the concrete would keep exterior moisture away from the studs and fiberglass. Painted wallboard inside would serve as an air barrier that would minimize the amount of interior moisture coming in contact with the potentially cold poly, and it would permit it to dry to the inside. Is this not better than batts in contact with the concrete?

What should I recommend to my clients who refuse to use anything but FG Batts in their above grade and below grade walls?


31.
Thu, 07/05/2012 - 10:51

Reply to Kathy Olmstead
by Ron Keagle

Helpful? 0

Kathy,

Leave the poly vapor barrier. I cannot see how it could cause harm, and I do see a role for it. You did not mention whether the rigid foam you used was closed cell or open cell. It is true that closed cell foam would act as a vapor barrier, so in terms of the foam sheet alone, a poly vapor barrier would be redundant.

However, the foam sheets have joints between the sheets, and between the sheets and fir strips. If these joints are not meticulously sealed (I am guessing that they are not), vapor can pass through them, contact the cold concrete, condense there, and cause wetness that will fill the joints by capillary action, or simply be trapped between the back of the foam and the concrete. This wetness could damage the drywall and/or cause mold.

The poly vapor barrier will cover the joints on the warm side and prevent this problem. If I were insulating the interior of a basement with expanded polystyrene foam, which is a vapor barrier, I would still apply a poly sheet between the foam and the drywall as you have.

There is another point to consider, however. Moisture can wick through the wall from the exterior ground contact, depending on how the exterior is moisture proofed. If your interior insulation system blocks vapor, it can trap the incoming dampness inside of the interior wall system. So, in addition to preventing outward condensation of vapor, the proper system overall will also stop this incoming moisture absorption of the concrete by some type of exterior treatment.

I can certainly see a role for polyethylene sheeting both inside and outside of a basement wall.


32.
Fri, 07/06/2012 - 09:10

Exterior insulation positives
by bo jespersen

Helpful? 0

Here in Maine we have done both interior and exterior insulation many times and have had good results. I will read the Minn. studies to see if they address the air leakage reduction when sealing up the rim joist in basements. We see very large heat savings with reduced air leakage in addition to the insulation when this is done right.

Two items I would like to add to the pros column of exterior insulation. It can; A.) protect your waterproofing/damproofing from the grinding of backfill. We use a rubberized, waterproofing membrane for concrete that requires a nice warm protector like XPS. And B.) it allows the interior to stay exposed (if left unfinished) to see any cracks or seams that have opened up.


33.
Fri, 07/06/2012 - 17:59

Edited Sat, 07/07/2012 - 15:31.

Foundation Insulation & Insects
by William Sanner

Helpful? 0

In Western Pennsylvania, concrete block foundations are the most common kind. I have seen many cases where carpenter ants or termites have used the wall cavities created on interior side of the insulated basement walls as clandestine access to the interior. Many of the cross sections proposed here should at least be augmented with, for example; a thin, stainless-steel shield separating masonry wall top from the sill. This shield should be curved down over the top of the interior wall cavity and frame. A suspended ceiling could be installed and although it would conceal the shield, panels of the ceiling could be periodically raised to look for mud tubes or other evidence of an invasion.

Hairline cracks and other minor foundation defects commonly develop with time in many of these block foundations and the pests get through them. I think this issue, along with potential for complex moisture management oversights and those attendant problems, when combined with a relatively small documented monetary return by energy saving force me to remind you that basements are not living spaces. They should be well-lighted, well-equipped, ventilation-capable work, machinery and storage areas. In the last 20 years I've seen one atypical 'insulated' basement space in a high-end house that solved these issues by locating key service / mechanical / utility functions in an an approximately 4-foot wide 'hall' that separated the exterior sub- and above-grade masonry walls (on their interior side) from the insulated framed-wall side of the hall.


34.
Sat, 07/07/2012 - 03:25

Don's question about floors
by David Martin

Helpful? 0

Can anyone answer his question? He asks about carpet directly over the slab -- I'd say no way to that -- but what about the poly over the slab, then foam board between sleepers covered by 3/4" plywood? That's best practice, no? I thought this article would address the floor and the walls at the same time.

To the website admins: I forgot my password and couldn't reset it using the reset password button. Something seems wrong there. I tried emailing you through the "contact us" links, but didn't get an answer. I made a lucky guess after numerous attempts and got it right.


35.
Sat, 07/07/2012 - 17:24

Edited Sat, 07/07/2012 - 17:26.

High Performance Basement Philosophy
by Ron Keagle

Helpful? 0

Basement space is relatively expensive, but the cost is offset because you fundamentally need a foundation. There are cheaper foundations than basements, but they don’t offer any useable space. So once you commit to the cost of some type of foundation, it makes economic sense to add a little more cost and reap the benefit of basement space.

However, for average construction where cost restraint is king, the basement is often left as being merely adequate space for utilities, storage, workshops, etc. Bringing it up to the same quality standard as aboveground space would further increase its cost relative to the aboveground space. So basement space quality has always sort of hung in limbo. If more living space is needed, it often makes more sense to acquire it by expanding the aboveground space.

In reading a book by Mark White called Superinsulated Truss-Frame House Construction, Mr. White makes a convincing case that new houses should be built without basements, and use a concrete pier foundation instead. He cites a host of bad effects arising from basements.

I can see his point, especially when considering a high performance, superinsulated house where one is spending extra money to achieve a higher performance in several ways. With that objective, a substandard basement space is a greater contrast to the aboveground living space. Not only that, but the negative effects of the average basement space can spill into the highly perfected aboveground space and drag its quality down. A basement can be like an infection that makes the whole house sick.

I see no reason why the basement space could not be of the same quality as the aboveground space, and for high performance houses, I would work toward that objective for the basement. It would further increase the cost of the basement, but superinsulated houses are generally compact and space efficient, so the basement is relatively small, thus limiting the cost of its perfection.

However, the exact route to achieving this level of basement space perfection is something that deserves a great deal of study, particularly in the soil and drainage engineering of the surrounding ground. The whole zone extending from the roof overhang to the footings needs to be designed like a high quality machine.


36.
Wed, 07/11/2012 - 10:54

Mark Fredericks, don't
by aj builder, Upstate NY Zone 6a

Helpful? 0

Mark Fredericks, don't combine gutters into footing drains. Big no no. They go their own way, away from the home. And there is no problem with foam on two sides of concrete. The only issue with foam is it getting too wet and also that ants, termites etc love it. ICF foundations are foam on two sides, they have information on their sites as to how to protect the foam. roof membrane, stucco etc.


37.
Mon, 07/16/2012 - 06:34

Response to Dustin Harris
by Martin Holladay, GBA Advisor

Helpful? 0

Dustin,
Q. "Since some moisture and air can get between foam insulation boards and create condensation, wouldn't spray foam be better?"

A. Perhaps -- although I don't think that properly taped or caulked rigid foam boards will allow air to get in contact with the concrete. Moreover, many builders are leery of spray foam because of the possibility of lingering odors.

Q. "Wouldn't poly over foam boards = spray foam?"

A. I'm not sure what you mean. Rigid foam boards are an effective air barrier and vapor retarder, so there is no need to use polyethylene if you have rigid foam.


38.
Mon, 07/16/2012 - 10:53

Response to Don Mikulec
by Martin Holladay, GBA Advisor

Helpful? 0

Don,
Q. "You hint at the benefits of insulating the floor. Can you give your thoughts for doing this in a completed home?"

A. Insulating an existing basement floor will rarely be high on the list of cost-effective retrofit options. But if you are aiming for the Passivhaus standard or willing to spend the big bucks required for a deep-energy retrofit, then it may make sense to insulate your existing basement slab. Here's a link to an article that discusses insulating basement slabs as part of a deep-energy retrofit: The High Cost of Deep-Energy Retrofits.

Q. "Old DIY articles suggested a poly sheet on the concrete followed by 2x4 sleepers with the voids filled with foam and the whole thing covered with 3/4" plywood. I think this solution would cause a problem at the stairs. If carpet and pad are installed directly over the concrete, what advice do you have for the pad?"

A. You can't install carpeting directly over a basement slab, even if you include a carpet pad, because of the risk of summertime condensation on the cold concrete. You'll end up with mold under the carpet if you install it on the concrete.


39.
Mon, 07/16/2012 - 10:57

Response to Kathy Olmstead
by Martin Holladay, GBA Advisor

Helpful? 0

Kathy,
Q. "I have a basement that was previously insulated with rigid foam against the poured concrete wall but it also has a layer of poly under the drywall (my home is in Minnesota and where poly vapor barrier used to be a code requirement.) There is no evidence of mold or damp; should I be concerned?"

A. No.

Q. "Should I consider removing the drywall and the vapor barrier and redoing it, even though there does not appear to be any problem?"

A. No.


40.
Mon, 07/16/2012 - 11:01

Response to Jason Holstine
by Martin Holladay, GBA Advisor

Helpful? 0

Jason,
Q. "What if one wishes to avoid foams for cradle-grave or health concerns (for insulating/reinforcing existing basements)? You brush by this briefly, but what's the concern with a 2x4 stud wall installed 1" off the concrete filled tightly with ComfortBatt?"

A. Roxul ComfortBatts are made from air-permeable mineral wool. The concern is that the insulation is incapable of preventing moist interior air from contacting the cold concrete. The result will be condensation that runs down the concrete wall and pools near the bottom plate of your wall.

Q. " What of using Roxul's new semi-rigid rockwool boards (ComfortBoard or CavityRock)?"

A. The same problem applies to these products, which are both air-permeable.


41.
Mon, 07/16/2012 - 11:05

Edited Mon, 07/16/2012 - 11:08.

Response to Mark Fredericks
by Martin Holladay, GBA Advisor

Helpful? 0

Mark,
Q. "I'm now planning to excavate the exterior of the home to address some drainage issues and run my downspouts into a common footing drain."

A. AJ Builder is right: you don't want to combine pipes that convey water from your roof gutters with your footing drain. If you do, you will be introducing more water to soil near your footings, and that's definitely not what you want. Conductor pipes that convey water from your roof gutters should be solid PVC pipe, not perforated pipe, and should slope to daylight or to a drywall far from your foundation.

Q. "Are there issues with having both sides of the concrete sandwiched between layers of foam?"

A. No -- it's done all the time with insulated concrete forms (ICFs). So if you want more R-value in your basement walls, you should definitely go ahead and insulate the exterior of your foundation when it is exposed for the repair work to your footing drains.


42.
Mon, 07/16/2012 - 11:15

Response to Kevin Ring
by Martin Holladay, GBA Advisor

Helpful? 0

Kevin,
Q. "Why do you categorically proscribe the use of poly anywhere in such an assembly?"

A. Because it is unnecessary and leads to moisture problems and mold.

Q. "It seems to me that a single layer of poly directly against the concrete would keep exterior moisture away from the studs and fiberglass."

A. It seems that way, doesn't it? But you are wrong. There is no way you can install poly against a concrete wall in a way that will prevent moisture from entering your basement.

Q. "Painted wallboard inside would serve as an air barrier that would minimize the amount of interior moisture coming in contact with the potentially cold poly, and it would permit it to dry to the inside."

A. It's almost impossible to install wallboard in such a way as to ensure that it is a perfect air barrier, especially if the wall includes electrical outlets and switches. Moreover, why do you think that the air between the wall studs and the air between your fiberglass batts will be dry? That is is basement air, and it's moist. There isn't any way to be sure the air in the wall is dry unless you install a miniature dehumidifier between each pair of studs. (Don't try this.)

Q. "Is this not better than batts in contact with the concrete?"

A. I don't like to play the "what's worse?" game. The fact is, you don't want to use polyethylene or fiberglass batts to insulate your basement wall, because we have ample experience in the field about how these materials perform. Sometimes they work, especially in a very dry basement, but very often they fail. And when they fail, they really stink.

Q. "What should I recommend to my clients who refuse to use anything but FG Batts in their above grade and below grade walls?"

A. You tell them, "I won't do any work that I can't stand behind, so I'm not going to install fiberglass batts to insulate your basement wall. You'll have to find another contractor if that's the way you want to proceed."


43.
Mon, 07/16/2012 - 11:59

Response to Bo Jespersen
by Martin Holladay, GBA Advisor

Helpful? 0

Bo,
Thanks for suggesting two other advantages to exterior foundation insulation. I have edited the article to reflect your useful additions.


44.
Mon, 07/16/2012 - 12:17

Response to David Martin
by Martin Holladay, GBA Advisor

Helpful? 0

David,
Q. "Concerning Don's question about floors: Can anyone answer his question?"

A. Sorry for the delay; I was on vacation. See my reply above (comment #38).

Q. "What about the poly over the slab, then foam board between sleepers covered by 3/4" plywood?"

A. That's one way. I prefer to install a continuous layer of rigid foam over the existing slab rather than interrupted strips of foam, however. For more information on insulating an existing basement floor, see:

Finishing a Basement Floor.

Green Basement Renovation.

The Stay-Dry, No-Mold Finished Basement.

The High Cost of Deep-Energy Retrofits.

Q. "I thought this article would address the floor and the walls at the same time."

A. No article can explain everything; the topic of this article is "How to Insulate a Basement Wall." If the links which I have provided don't answer all your questions, feel free to post further questions on our Q&A page.


45.
Tue, 07/24/2012 - 10:52

re: what about the floor
by D Mikulec

Helpful? 0

Martin,

Thanks for your response. Also, thanks for the links in your response to David Martin and for the comments from other readers. Most of these solutions require the layering of insulation and plywood over the concrete floor. Will these require the indoor stairs to be redone? My basement has a walkout sliding door. Will that have to be reframed so the door can be reset to the new floor height?

Don


46.
Tue, 07/24/2012 - 18:52

Response to Don Mikulec
by Martin Holladay, GBA Advisor

Helpful? 0

Don,
Q. "Will these require the indoor stairs to be redone?"

A. If you want your stairs to be safe and have consistent riser heights -- then yes.

Q. "My basement has a walkout sliding door. Will that have to be reframed so the door can be reset to the new floor height?"

A. Yes.


47.
Thu, 07/26/2012 - 15:00

your response and another question
by D Mikulec

Helpful? 0

Thanks for your reponses to my questions. I have one more: with insulation and plywood built up on top of the concrete, what is the likelihood of these drying should the basement flood?

Don


48.
Thu, 07/26/2012 - 15:32

Response to Don Mikulec
by Martin Holladay, GBA Advisor

Helpful? 0

Don,
If you have a plywood or OSB subfloor, standing water on the floor is always bad news, of course. If the flood was very brief, and you managed to clean it up immediately, the subfloor might be OK. But in most cases, you'll get buckling and delamination.


49.
Thu, 08/09/2012 - 07:38

Edited Thu, 08/09/2012 - 07:38.

Did i miss something about polyiso being used "below grade"?
by darek jansen

Helpful? 0

I was considering using two staggered layers of polyisocyanurate to insulate the interior of a new basement... but using two types of foam. Since I will not be able to get around to finishing my basement right away but will have to do that slowly over the next several years this will leave the foam exposed.

So, the layer on the interior will be the Thermax. The layer next to the cement wall would be Super Tuff-R (b/c it's cheaper per R and I'll be using it in a number of other places).

However, for Polyisocyanurate, the rigid foam board page on your site (http://www.greenbuildingadvisor.com/green-basics/rigid-foam-insulation) states: "... Because it can absorb water, polyiso is not recommended for below-grade applications. ..."

Isn't the interior of a basement considered "below grade"?
Do I need to use something else for the first layer (next to the cement)?


50.
Thu, 08/09/2012 - 08:00

Response to Darek Jansen
by Martin Holladay, GBA Advisor

Helpful? 1

Darek,
The information given in this blog is correct. As I noted above, "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."

Thanks for alerting us to the unclear wording that you cited in another GBA article. That encyclopedia article has been corrected. It now reads, "Because it can absorb water, polyiso is not recommended for use under slabs or on the exterior of foundation walls."

Thanks for keeping us on our toes.


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