We just had an energy audit done on our 25 year old house.  It's a 2100sf cape in southern New Hampshire.  The inspector found that our house was reasonably tight and decently insulated (though our walls are only 2x4) ... except for our full, unfinished basement, which has simple poured concrete foundation walls (about 8' high) and no insulation in its ceiling.

This has obviously been a mild winter, so the basement this year hasn't been all that cold--- typically about 50 degrees. On very cold days, it will get colder down there, since the top 18" or so of the foundation is above grade.  I did have foam sprayed into the boxed area above the sill and alongside the rim joists, so that helps somewhat-- at least it eliminates air infiltration there.

Part of this energy audit deal is that the electric company pays 50% of any recommended remediation.  The auditor suggested that we have the basement ceiling (about 1400sf) insulated with fibreglass to prevent the basement from acting as a "cold sink" that would suck heat out of the house.  He agreed with me that insulating the entire basement walls with simple, fireproofed Polyiso foamboard would be the better solution, but it's a lot more money.

We would be out of pocket about $900 to do the basement ceiling and about $3,000 to do the walls.  I know the walls would be a more effective solution, but I really don't want to pay that much money for a project that would take a long time to pay for itself.

One option might be to apply Polyiso board to only the upper 4 feet of the basement wall-- would that be better than installing fiberglass in the ceiling? Obviously, the board would have to be surrounded by a bead of caulk to create a good seal.

So... is it worth it to insulate the basement ceiling or not?  I welcome any advice.

Thanks!

If you installed the Polyiso board, would it be on the outside or inside the basement?

If you installed the Polyiso board, would it be on the outside or inside the basement?

Oh, on the inside for sure. I understand that you can get Polyiso with a reflective fireproof covering so that you don't have to sheetrock over it.

Do you do anything in the basement for which you'd like it to be more comfortable? Comfort is worth something, even when the "payout" is too long. What kind of car do you drive? If it's an "upscale" SUV vs a low-end "basic transportation" vehicle, where's the payout for the extra bucks spent for "what you really wanted?"

Posted By DickRussell on 02 Feb 2012 07:48 PM
Do you do anything in the basement for which you'd like it to be more comfortable? Comfort is worth something, even when the "payout" is too long.

No, basement comfort is not a concern, since we really only use the basement for storage. The only reason I would insulate the basement ceiling or walls would be to cut down our heating oil consumption. Would the cost savings of insulating the bottom side of our ground level floors be worth it?

If you insulate the floor only, then as you note the basement will be significantly colder in winter, without heat from above it. Just how cold is hard to say, but the uninsulated foundation walls and slab will let some ground heat in and likely keep the space above freezing. If the foundation walls are readily accessible, this could be a DYI project you could complete over time at substantially lower cost than to have a contractor do it. This fix to your problem also would help resale of the home when that time comes. Imagine a prospective buyer walking into the basement in winter.

Yes, you most definitely should insulate your basement. Un-insulated basements typically account for 30 -40 % of heat loss in a home. The ground is always colder than the basement and heat is drawn into the cold. Warm air rises, heat travels from warm to cold. It is constantly trying to balance the temperature between the house and the ground. I recommend that the basement walls be fully insulated from the top of the ring joist down to the concrete floor. The myth about insulating only the top half of a basement wall was squashed 25 years ago. It is now code in Ontario to fully insulate the basement walls (hey we get some pretty cold winters up here..... Normally)

Do you have a furnace and ductwork in the basement. If you do, by insulating the ceiling, you are making the basement a cold zone and all your heating is then trying to warm the cold zone before it delivers the heat to the upper levels. Your energy consumption might actually go up. Insulate the walls with two or three inches of closed cell spray foam for the best results. If the basement is warm, the floors above will be warm, your feet will be warm, you may actually find you can lower the thermostat setting a few degrees. Do not use fiberglass batts against the concrete walls, they won't make the slightest bit of difference. Batts in the basement ceiling work best for sound control between floors. I would use Roxull batts for sound control. In fact I don't recommend fiberglass batts for any application.

We have a boiler in the basement, and I've insulated every hot water pipe down there. I am also not concerned about the basement temp being comfortable, since we only use it for storing stuff. As I mentioned, I also don't want to invest in putting up fireproof polyiso board, let alone spray foaming the 180 feet circumference of our 8' high walls.

All I really want to know is if it's worth $900 in terms of potential heat savings to put fiberglass batting on our basement ceiling? That's the only option I'm considering.

Thanks!

Posted By Amazer98 on 04 Feb 2012 09:54 AM
We have a boiler in the basement, and I've insulated every hot water pipe down there. I am also not concerned about the basement temp being comfortable, since we only use it for storing stuff. As I mentioned, I also don't want to invest in putting up fireproof polyiso board, let alone spray foaming the 180 feet circumference of our 8' high walls.

All I really want to know is if it's worth $900 in terms of potential heat savings to put fiberglass batting on our basement ceiling? That's the only option I'm considering.

Thanks!

Batts in the ceiling aren't going to buy you much (other than mold on the cool edges of the joists and is practically impossible to air seal with the all of the plumbing electrical and heating penetrations.  But with the boiler in the basement insulating the walls will.  The heat loss of even a 50F basement to a 25F average mid-winter exterior temp is huge, and it's a loss you're paying for with standby and distribution losses on the heating system.  Making it colder down there will only increase that standby & distribution loss and the net effect on the heating bill would be quite modest, maybe not even measurable.

In a basement retrofit on an older home the fire-rated iso could be a mistake, since the foundation probably doesn't have a capillary break between the footing and the foundation wall, or between the top of the foundation and the foundation sill.  Foil faced iso is a vapor barrier, and would increase the moisture content of the foundation, forcing it to dry toward the above-grade exterior, and raising the average moisture content of the foundation sill. It's better to use 2-3" of unfaced EPS or fiber-faced iso which are semi-permeable, letting the concrete dry toward the interior of the basement.  To meet code with the non fire-rated fam you can either build a an interior studwall, or use furring through-screwed to the foundation on which to mount half-inch gypsum as the ignition barrier. 

If you use reclaimed goods from commercial re-roofing the cost of the foam is quite modest (25-35% of virgin-stock, and WAY cheaper than fire-rated iso). If you can't find a closer source on craiglist or something, there's always insulationdepot.com.  For the same whole-wall R the reclaimed foam option often cheaper per square foot than insulating with batts & studwalls You're looking at a total of about 1500 square feet if you do the walls.  At R10-R12 you'd probably be able to get the foam costs down to 25-35cents per square foot for reclaimed goods- sometimes less for maybe $500 for the foam end, throw in another $50-$100 for some 1-part foam or duct mastic to seal all of the seams.  Then figure out what the sheet rock would set you back- it's not a bank-breaker, and you can do a lot for a grand.  And what you end up with is a less moldy basement, a warmer floor, and a double-digit percentage reduction in the heating bills, whereas with batts between the joists it would barely register on the bill, (but might make the floors warmer than they are now.)

You DO have to air seal and insulate the band joist & foundation sill, which can be a lot of cut'n'cobble but it's not too bad.  Whatever you do don't just stuff batts up against the band joist, since convection transported moisture hitting the cold band joist all winter would load it up with moisture.  EPS or XPS cut'n'cobbled and air-sealed at the edges with can-foam is the way to go.

FWIW: I did my basement in central MA with reclaimed 3" fiber-faced iso @ $20 for a 4x8 sheet (63 cents/ foot for R19, or a bit more than 3 cents/R-foot), and I've seen it cheaper since.  It reduced my heating bill by more than 15%.  YMMV.

Dana, thanks for the knowledgeable advice. I knew from other posts (including yours) that foamboard on the walls was the best way to go, but thought that for a relatively modest cost that putting fiberglass in our basement ceiling might have some value. You've convinced me that it's not worth it, so for the time being we won't do anything.

If I get it together to take on the project of installing foamboard to our concrete walls, what's the best way to secure the pieces to the walls? Do I use a masonry drill bit to make holes in the wall and then screw in the foamboard?

To put foamboard on the wall using furring, first use foam-board construction adhesive (available at box-stores) in acorn-sized blobs ~16" o.c. to glue it to the wall. You have about 10-15 minutes to adjust it before the adhesive starts to set. Seal the seams & edges, then install 1x furring 16-24" o.c., with Tapcons sufficiently long to get at least 3/4" into the concrete, and hammerdrill/roto-hammer right through the furring+ foam into the concrete. If your foam is thicker than 2.5" you may have to buy the fasteners online or at specialty hardware shops, but most box stores carrry them at least 4" long. You'd only need fasteners 24" o.c. and you can run the furring either vertically or laterally, but keep it off the concrete floor where it might wick moisture.

Or, you can glue up the foamboard and trap it there with a non-structural studwall snugged rigth up against the foam. DO put a capillary break between the bottom plate and the slab (I prefer to use 1/2-1" XPS, but 6-mil poly + foamy sill gasket also works.) Anchor the bottom plate of the stud framing to the slab with Tapcons, and you can nail the top plate to the joists. Single plates are fine- it's not supporting the house, it's trapping the foam and holding up the drywall. With a studwall it's worth putting in UNFACED R11 or R13 batts, and it's pretty easy to hit ~R20 whole-wall with 2" of foam (any type, as long as it has no plastic or foil facers) + batt-filled studwall.

The material cost of doing foam on the foundation wall isn't much more than doing R30 batts in the joists for most homes, if you use reclaimed foam, and in many homes there's even less foundation wall area than basement ceiling area.

As a general rule it's almost always better to keep the mechanical systems fully inside both the pressure and thermal boundary of the house, and make thermal boundary adjascent to the pressure boundary. That way the ducts, pipes, boilers, air-handlers etc don't lose or gain heat or air to the exterior, giving the system a chance to run at the equipment's optimal efficiency. (Ducts running above the insulation in attics of slab-on-grade homes are a common but significant hit in efficiency, even with the ducts insulated sealed, but an energy DISASTER when they're not, often nearly doubling the peak heating & cooling loads.)

The I=B=R output ratings of boilers is a crude model the anticipated loss of system efficiency when not located within the conditioned space. A typical mid-sized boiler with a D.O.E. rating of ~100KBTU/hr usually has an I=B=R of only ~87K. Do you really want to "throw away" 13% of the boiler's output by installing insulation between the boiler and the conditioned space, instead of putting the insulation between the boiler and the great outdoors? Right now the lack of foundation insulation is a big hole in your thermal envelope, but the fix is to insulate the foundation, not the floor.

Thanks, Dana, for the good advice about how to install the foamboard. I'll save this on my computer and use it when I move ahead with the project.

Dana,

Is it necessary or cost affective to insulate the basement floor also?

For new construction it's cost-effective to put R5-R8 insulation under basement slabs in NH climate, R12-R15 if the slab is used for radiant heating.

As retrofit the cost of demolishing and replacing a slab makes it not cost effective, but when finishing a basement putting R5 above the slab and under a sub-floor is worth it from both a comfort and heating-costs point of view, so long as you have the headroom to work with. That usually means reworking the basement stairs to deal with the change in floor level at the bottom step.

Here is my advice: if you plan to use XPS or EPS boards on the inside of the foundation, be REALLY careful about applying them with the foundation tightly, taping the seems and seal the bottom and top of the boards carefully. The boards should be really airtight. Do not allow warm air to go between the boards and the foundation by convection, or you could have condensation...and then significant mold...especially on the upper section of your foundations walls. The best advice that I can give you is to be careful about that. The safest way to insulate your foundation walls from the inside is to use sprayed polyurethane foam. It sticks to the foundation, so no air gaps are possible between the insulation product and the foundation.

I agree, sealing the top & bottom edges of the rigid with 1-part expanding foam is required here, but the amount of condensation you'd get between the rigid & concrete isn't likely to get ahead of the drying capacity the foundation has toward the exterior, and indeed the dew/rain splash back is likely to exceed any winter condensation volumes from a wintertime convective loop behind poorly sealed rigid foam, at least in THIS climate (southern New Hampshire, not Whitehorse Yukon.)

Concrete is very porous, and quite tolerant of that level of moisture drive. Seal it, sure, but there is no need not be hyper-obsessive in that endeavor.

but the amount of condensation you'd get between the rigid & concrete isn't likely to get ahead of the drying capacity the foundation has toward the exterior

I can assure you it can. I've seen a foundation whose top was 3 feet over grade (more than enough drying capacity toward the exterior) and the upper half of it was infected with mold...you couldn't believe it, the mold was so dark that it was ugly.

The problem is that the insulation board acts like an imperfect barrier between the inside and outside temperature. So, the temperature on the foundation gets so cold that you don't need excessive inside humidity to get the dew point for a prolonged period of time. It's hard to figure out, but think like a energetic window that has lost it's low emission gas...you have condensation between the two layers of glass. The convection looping is then providing constant warm and humid air there, so you have substainable condensation (water) and mold has it's food to live and grow.

Regarding the sealant, I'm not so sure about the residential urethane product. As far as I know, it's a good product, but it's made to last 20 years or so. I would try for something more durable, like silicone (if the gap is narrow) or another made to last 50 years or more caulking product.

I may sound excessive, but I've seen what the problem looks like and how much it can cost to solve it. Go see the New York protocol and you'll see that it's a big job to do just for the decontamination part (I'm not talking about demolishing your actual walls, reinsulate better, rebuild the walls, etc.), so the time and cost spended to be sure that it will not happen are very well spent.

Cheers!

The problem is that the insulation board acts like an imperfect barrier between the inside and outside temperature. So, the temperature on the foundation gets so cold that you don't need excessive inside humidity to get the dew point for a prolonged period of time. It's hard to figure out, but think like a energetic window that has lost it's low emission gas...you have condensation between the two layers of glass. The convection looping is then providing constant warm and humid air there, so you have substainable condensation (water) and mold has it's food to live and grow.

Posted By johanson on 21 Feb 2012 04:38 AM
The problem is that the insulation board acts like an imperfect barrier between the inside and outside temperature. So, the temperature on the foundation gets so cold that you don't need excessive inside humidity to get the dew point for a prolonged period of time. It's hard to figure out, but think like a energetic window that has lost it's low emission gas...you have condensation between the two layers of glass. The convection looping is then providing constant warm and humid air there, so you have substainable condensation (water) and mold has it's food to live and grow.

The key is just how substantial that loop is, and the humidity of the interior air. With two flat surfaces with 1mm or less of separation the air sealing details don't need to be nearly as careful as when there is a more substantial gap, the total volume of air that move in a micro-gap during the winter driven solely by convection don't carry much water in a southern NH climate.  But the colder the climate, the more important it becomes.

Regarding the New York protocol mold issues, there may be some confusion as to the source of the mold-inducing moisture.  Not all moisture in a foundation wall is coming from minor convection behind the foam, in fact most of it would be coming from elsewhere.   The amount of moisture wicking up from the footing will exceed that deposited via convection from room air with 40F dew point if the rigid foam is even nominally sealed.  By installing the rigid foam it decreases the rate at which the wall can dry toward the interior, leading to higher moisture content higher up in the wall assembly.  In a New York City climate even the rain/dew & snow-melt getting into the above grade portion from the exterior will exceed any wintertime convective process behind the foam.  It's common in some areas to leave the bottom ~12" of the foundation wall un-insulated to allow the ground moisture wicking from the footings to dry toward the interior rather than wicking further up the wall. 

Every foundation insulation project needs to be assessed relative to the in-situ conditions, but bulk water & capillary draw is an order of magnitude or so bigger issue than micro-convection behind imperfectly sealed rigid foam, and convection is an order of magnitude or so bigger than vapor-diffusion through 2" of XPS.  Where there isn't a capillary break between the footing and foundation wall, using more-permeable EPS or fiber-faced iso is a better option than XPS, since the higher vapor permeance allows the ground moisture to dry toward the interior more quickly.

Guys,

I live in nyc and I notice there are mold build up on the foundation walls. I'm pretty sure the wall is coming from the outside. What are my options in insulating and not have to worry about moisture coming from outside?