Insulating floors above basements with fuel-fired boilers\furnaces.

There has been some back and forth in my circles about whether or not to insulate floors above basements with fossil fuel boilers and furnaces. 

My sense is that there is no doubt that it is prudent to insulate floors to R-19 or better for houses located North of the Carolinas (or thereabouts), IF the heating appliances are high-efficiency, BUT it can be very situation dependent when the heating appliances are providing a good deal of inadvertent heat (i.e. atmospherically-vented, open combustion units heating the basements to temperate levels).

I always foam the rim\band joists, but I am starting to question the cost-effectiveness of insulating basement ceilings in retrofit applications. Thoughts from the pros?

Thanks in advance!


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Hi Patrick,

Basements are funny places, they seem to stay reasonably warm without any extra effort, so why change anything?  But that is obviously far from the truth.  The idea of being inadvertently heated means it isn't a thermostat controlling the temperature down there, but the balance between the miscellaneous heat gain and the abundant heat loss.  The resulting temperature and heat loss, is therefore under our control.  Insulate the floors and the temperature goes down.  Insulate the basement and the temperature goes up.  Both reduce the net heat loss from the home, but insulating the floor will result in a colder basement and a somewhat colder floor above.  From my northerly perspective it seem that insulating and air sealing the rim joist and exposed foundation walls (I always suggest an extra foot below grade) is a total no brainer.  Besides it is often very easy.  Insulating the entire wall then becomes subject to moisture and water considerations and the intended use, but at least the worst part is done.  The result from this improvement should be a noticeable increase in basement temperature, plus some warmer floors above. Regardless of the approach, moisture issues need to be addressed. 

From a total heat loss perspective, both is better, but one should run the numbers to see if those extra dollars could be better spent elsewhere.


Thanks Bud. Here are a couple of follow-up questions I have.

One, is addressing moisture as simple as controlling for bulk moisture and then running a dehumidifier with the right capacity (in most cases)?

Second, what are your thoughts on hos to insulate the interior side of basement\foundation walls? I have employed a few strategies, but still feel that there are probably a few I have yet to attempt.

Thanks in advance Bud!

Wood and fiberglass just seem to be the highest source of mold problems.  That's not to say all such installs will go bad, but I have seen many.  Another strange one is the rodent infestations.  One home had installed 2x4's down 4', filled it with unfaced fg and covered it with clear plastic.  It looked like an ant colony, all the tunnels, nests, and potty spots. The wife freaked out. 

Building science is strong on rigid insulation, but it should be covered with a fire barrier.  The Dow Thermax has a thicker foil covering and has been approved in some locations for a direct install.

However, any approach selected will still be at the mercy of water and moisture.  If the soils outside are moist, which 99% are, a VB on the inside will allow that moisture content to equalize all the way to behind the VB.  Thus, the pink or blue insulation, w/o a VB will allow some moisture diffusion all the way through to the inside.  This drying to the inside reduces the moisture build up and is slow enough that conditioning of some sort will handle any resulting moisture.

It's always all of the other conditions that determine what approach is best.  Things like dry walls are only really dry in the desert.


I like where this is going. I love polyiso rigid board, but I have been unable (so far) to get my hands on the Thermax product. I have found that only big distributors carry it, and they will not deal with the "smaller" home performance pros. Have you had any luck acquiring it?

I have used a good deal of Dow Tuff-R in the past. I seal the space(s), slap rigid polyiso board up, seal all seams and joints with a good foil tape, and finish up with 1/2" gypsum board. Nevertheless, I have been kept up on a few occasions at night wondering whether there is moisture building up behind that gypsum board or behind the polyiso board.

Care to weigh in on this?

Thanks again Bud!

When any of the foil faced rigid insulation is used, it is a VB and thus there would be a potential for moisture accumulation.  However, depending upon the adhesive used, there might not be any food for the mold to grow on.  If you ask, some code officials might accept the Tuff-R. 

As for locating a source, perhaps Scott could give you some direction.  He posts her, but here is his contact info.

Scott Cummings Sr. Technical Service Engineer Dow Building Solutions Charlotte, NC

You could probably give him a friend request and he would reply.


Thanks Bud. I just shot Scott an e-mail.

As for the issue of code officials and their (relative) reluctance to allow the use of the foil-faced rigid foam in a variety of applications, do you have any advice for approaching such conversations with them?


Maine has approved the use of the Thermax and discussed the issue with all code officials, they all got the same notice. The two I talk to want to see drywall or the Thermax as that is what the state fire marshal wants to see.  Now, there is some wiggle room, as Maine has just started to implement state wide building codes and some smaller town don't even have codes in place as yet.  But since I'm just an auditor only, I can only advise.  If I were to be the installation company, I don't see where I could not follow the state guidelines no matter where the home was located.

Through a help forum, I talk with people all over the country and some have rules and some don't, so the advice always ends with check your local codes as specific codes are under local control.  Depending upon your work radius, those are people you should meet and talk with as your recommendations and work should match their requirements. 

I will be interested to hear what Scott has to say.  As for a source, I assume I have two, I'll have to ask.


I definitely hear on this Bud. It can be very disheartening at times, as code officials (usually of the fire safety bent) remain somewhat in the dark on the use of high-efficiency air sealing and insulating techniques. 

I'll whip out the blower door and show them all the air sealing I have performed, and some will become quite defensive about the idea that homes need to breathe. I explain the idea of breathing right, but it just doesn't get through to all of them. I will then throw in the fact that tighter homes also act to reduce the risk of fires spreading, and still they remain on the defensive.

I may have quite the battle ahead when it comes to convincing some code officials that rigid foam products, when artfully applied, are one of the best solutions to reducing heat loss\gain. It's just like the vapor retarder vs. vapor barrier debate (I want to say there is no debate, but that would not serve me well to do so).

I just cannot, in all good conscience, sit idly by as many contractors continue to push FG batts because they are easy to install and cheap, all the while as the Kraft facing is touted as an air and vapor barrier (and at times applied dangerously close to heat sources). I cannot tell you how many times I have had to remove FG batt insulation that is full of rodent droppings, wet to the touch, and dirty as can be, and yet I find many code officials who see it as a perfectly good way to insulate. I def have my work cut out for me. 

Thanks for all your help and advice so far on this topic Bud!


I'm planning on using a product called Insofast to insulate basements wall to the outdoors. It has built in studs for the sheet rock and just need construction adhesive to apply to wall.

Bud, what do you think. I'm in the same boat as Patrick rethinking my approach to the whole basement question. To F.G. or not.


Paul and Patrick,

You mentioned the key works, fast and cheap and unfortunately, I face an even bigger problem as I bottom feed, with people that have no extra money.  There are so many that can't or won't try to qualify for assistance, yet they are going broke trying to stay warm.  Bless mother nature so far this winter.

I mentioned that I recommend covering exposed concrete plus one foot below grade.  That is the area of greatest loss, thus greatest gain from improvement.  Some caulking, rim joist and at least one inch of rigid down to the one ft below mark and it makes a world of difference.  I know because I'm testing on my house.

As for the insofast, that looks great. But when you start talking drainage behind it you should be draining it somewhere and the with the whole wall covered, that's not a retrofit, that's a makeover and a good one.  And there's going to be a big price for it.  I bookmarked it just in case I want to test it in my basement, wink, wink.  Seriously, it does look good, and the price I saw did say $2 a sq ft.  Pink or blue is usually about $0.50 a board ft, so $1 a sq ft at 2". 

As for rethinking basements, I cringe every time a basement question comes in on the help forum as there are so many details needed to be able to give the right advice.  If any of us built a home tomorrow we would be doing a lot of work from the outside.  The homes we retrofit didn't get that attention and it isn't cost effective to back up and start over and that isn't the answer someone wants when they just purchased a new for them home with plans of finishing the basement.  They will just keep asking contractors until one of them says fg is quick and cheap.  Sniff sniff, what's that funny basement smell?

Hope some of that helps.  Duct board is another product I have looked at, but haven't tested it.


You're hitting on all the right points and issues Bud.

I have also considered using duct board (FSK faced rigid FG). It is used by some contractors in Mass, and code officials seem to be fine with it.

Paul, that Insofast system is interesting because I have experimented with such an approach before. My technique was as follows:

1. Wrap 1X4's with house wrap, and then use mechanical fasteners into the foundation (OR use a lot of construction adhesive). I wrapped the 1X4's because of concerns of wood against the foundation.

2. Apply 1" foil-faced polyiso board between 1X4's, and then add a second layer of 1" polyiso as a continuous insulation layer. Tape all the edges of the polyiso board so that none of the actual rigid foam is exposed. Also, tape all joints between the boards.

3. Slap up 1/2" sheetrock, and viola (you just added roughly R-12 to the foundation walls)!

Any issues with this Bud, or other pros? I have whipped out the IR camera and seen a significant difference, especially at the 1-2 feet at the top of the foundation walls. Can I get away with 1" polyiso between the studs, and just another 1/2" continuous? (Keep in mind that I'm in Massachusetts). My real concern at this point relates to Bud's point about drainage behind such a wall system.


Here is my thinking regarding the drainage plane built into the panels. Install these to one foot below grade, sheet rock and leave the bottom open to the built in drainage grooves which will let any moisture that does comes through a chance to dissipate. But on the other hand will leaving these grooves open negate the insulating quality of the panel? I would think not much.

So, my whole idea about these is that yes they are kind of pricing but you are eliminating framing costs, use of mechanical fasteners ( which just creates more avenues for moisture)  the ease of sheetrocking. no worries whether there is moisture build up behind the panels.

And of course if a high efficiency sealed combustion  unit was to be installed, I'll insulate the ceiling, which was the case with the last two jobs.

Your thoughts?



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