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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Another reason I like to stop at the one foot below grade is I avoid most of the exposure to leakage.  3' would be better for total r-value and it happens that half a sheet of rigid usually gets you into that range.

Groves are only necessary for actual water as moisture will stay within the concrete.  I suppose some might migrate down those grooves, but most would just move towards the dry concrete below and out into the basement air.

Patrick, your approach isn't bad, but sounds labor intensive and you still have some wood back there.

I promise I will come back and share if I discover a great way to address basements beyond current approaches, but I've been testing, watching, and reading for some time and it seems I'm farther from a solution than when I started.  To me, a basement is the worst place to convert to living space.  Even when everything is done perfectly, a pipe burst, a washing machine hose lets go, or a water heater decides to retire and you have a flood to clean up.  Oh ya, and it can always flood. 


At the end of the day, I am going to stick with my initial thought on all this, which is to really push hard that folks upgrade to HE mechanicals...seal the first floor plane and insulate the ceiling.

Having said this, what are your thoughts on a flash foam layer to the underside of the first floor (basement ceiling), and then installing R-19 (or so) un-faced FG batts? At present, I spot seal all penetrations through the first floor with a closed-cell foam, and then add faced FG batts.

I really do look forward to what you all have to say about converting basements to living spaces. I feel that Bud's thoughts on the matter may be as close to the real truth as it exists (currently) -- i.e. converting basements to living spaces is always going to be a risky move.

I'm on the same page with you. Presently I also spot seal and insulate w/ faced f.g. I would think that flash and batt would do a better air seal especially with those old floors that you can practically look through.

I def hear you on that one Paul. I too can typically see through the gaps in the subfloor panels used on older homes. Spot sealing those gaps is quite tedious, and inevitably there are some gaps that remain unsealed.

I have been tinkering on my house for a while (as well as my Mother's older house), and have seen some rather dramatic drops in whole-building air leakage. The next issues to be addressed are the leaks that take place through can light fixtures and other heat sources that penetrate through the attic floor plane. I did some IR-assisted blower door testing while up in the attic and saw some significant air leakage and heat loss taking place around and through the cans, etc.

I dropped the idea before of building gypsum boxes over can fixtures, and I think this will pay dividends. The  clearance required around range hood vent pipes is still a mystery to me as to how to address them without causing fire-related risks.

Also, let me just say that I know that the majority of measures I have proposed or am considering may not fly with code officials (yet). The gypsum boxed are not allowed in my area, yet I have seen proprietary covers for can fixtures that are pushed widely in some markets. Some folks may just say replace the non-IC rated cans with IC rated cans, but the cost of this can be a major obstacle (at least as I see it).

But, yes, flash and batt does appear to be the winner for addressing floors over basements. Anybody else want to weigh in on this?


I don't want to drift here, but there is another factor involved with sealing and insulating the basement ceiling, and that is a reduced stack effect.  One of the details I'm gaining an understanding on in my "The science of hot air rising" thread is how the pressures are created and distributed inside our homes.  Just a quick example.  At some temperature difference between inside and outside in a 2 story home on a basement there could be as much as an 8pa pressure difference created by stack effect.  But, the the neutral pressure plane divides that in half with 4pa near the ceiling and 4 pa at the basement floor, one pushing air out and the other pushing air in.  So what happens if we seal off the basement?  The resulting upper and lower vertical areas will now create their own stack effect based upon their height, thus the basement would have 2pa and the rest of the house 6pa and each of those would be divided by a neutral pressure plane leaving a much reduced pressure forcing air into or out of the basement and some reduction in the pressure forcing air in and out the top of the home.  The net benefit of this reduction is beyond my speculation, but it would be a benefit.

I have a web site for details like this and will be sure to put up an illustration so maybe someone smarter than I can point to a study providing the benefit numbers.


In PA, where basements are rarely cooler than 55 degrees, insulating basement ceilings is typically a waste of money and time.  The Delta T is small.  Insulating the ceilings of ventilated crawl spaces makes sense.

My Mother lives in PA (reading), and it made sense to seal and insulate the ducts in the basement, insulate the rim joists, and even insulate the basement ceiling. I think the main driver for this was the amount of windows in the basement. On average, I would say her basement is in the high 40s during the winter. Any thoughts about the affect of windows on basement temps?


Replacing basement windows made a big difference in air flow and basement temperatures in a couple rentals we reno'd.  Try replacement windows -- hopper style.  If they are that leaky, you're looking at reclaiming some basement use.  

Retaing the heat that is being lost through the boilers or furnaces seems to be the issue as far as energy savings goes, at least that is what I am getting here.


I work with one coating that insulates.  I have applied it to boilers and steam pipes in hosptals and dropped the skin temperature from 267F down to 155F with just a thick coat of paint, a 35% reduction in heat loss.  Others have had results on boilers in steam generation plants in Canada that dropped the skin temp from 600-f down to 370f.

We have also sprayed the coating to the underside of homes to retain the heat in the houe, no more cold tiles on the floor.  We have sprayed concrete walls and block walls in basements to stop heat loss and stop moisture problems.

Now let me ask this;  Are you all talking about insulating the entire underside of the home to stop the heat gain produced by the basement boiler / heater unit?  If you stop the heat loss from the boiler or furnace, will that solve the problem?

‘Insulating’ Paint Merchants Dupe Gullible Homeowners

"Every researcher who has examined these products has concluded that there is no such thing as insulating paint."

An ‘Insulating’ Paint Salesman Is Tripped Up By His Own Product

Scientific American:

What Are the Benefits of "Insulating" Paint?

“We haven’t seen any independent studies that can verify their insulating qualities.”

Robert, I too was a bit skeptical of such claims. I always have an open mind and am willing to research any new product, but I think you have just saved me a good deal of time.

I think some of these contractors are confusing reduced heat gain with improved insulation. I am aware of cool roof coatings that reflect heat away, and this makes sense. 

Have a good one Robert!



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