Home>Discussions>INSULATION & HVAC>Can I Heat under the floor in a crawl space.
4 posts / 0 new
Last post
Can I Heat under the floor in a crawl space.

I have home built over a crawl space. I live in Western New York, cold winters. I hot water baseboard heat. I would like to install the heating under the floor in the kitchen using the between the joist heating type system. The floor in the kitchen is Slate/Quarts mix tile. I had a Heating contractor at the house, and he recommended we don't do it. That floor over the crawl space is insulated. My question is can I do this, are there issues with doing this? Any suggestions or answers would be greatly appreciated. :)

Re: Can I Heat under the floor in a crawl space.


Yes, there are several ways to do this, but could you post back to expalin exactly why you want this done---is the kitchen too cold in the winter now???

Or does the kitchen floor feel too cold because of the crawl space???

This is known as sub-floor radiant heat & can be built into the existing system---what is the square footage dimensions of the kitchen, ceiling height, & how many feet of baseboard are in the kitchen now???

What reason did the heating contractor give for not doing it???

Be thankful you have hot water heat--there are many ways that it can be modified at relatively low cost--do you intend to do this yourself, or hire a contractor???

Please post back.

Re: Can I Heat under the floor in a crawl space.

Thanks for getting back so quickly.
The floor is cold because of the crawl space. It is insulated. But the tile is very cold. The only heat in the kitchen is a Kick Heater under one of the cabinets.
The square footage is about 12 X 15, 8 foot ceilings.
Yes I would like to do it myself.
The contractor just said he would not recommend doing that.

Re: Can I Heat under the floor in a crawl space.


I'm still worried about what that contractor was thinking---I would get a few more estimates from several heating contractors as to how they would solve the "cold kitchen floor problem".

Do you have any photos of the crawl space & where is the boiler located now???

I can't see the crawl space in person, so I don't know if you could close it in, insulate it, maybe even put the boiler down there.

For example, I had one job several years ago where there was a cold crawl space & the boiler was crammed into another part of the building.

Some minor excavation & concrete work was done in the crawl space & the boiler was moved down there---bingo! no more cold crawl space---the heat from the boiler & hot pipe mains warmed the kitchen floor, AND the crawl space, which became a cellar.

I'm not saying this is YOUR solution, it's just to say other contractors that can physically look at it may recommend something other than under floor tubing---this doesn't mean you have to HIRE the contractor, just consider what he/she recommends as a solution.

You also want to make sure you have enough exterior wall insulation & heat in the kitchen itself--there should be R19 insulation in the exterior walls (can be blown in if there is none).

If there's a lot of windows they should be tight, double pane or storm windows--windows & doors let out a lot of heat.

For a cold area like western N.Y., the heat loss factor would be between 40 & 60 for a rudimentary HEAT LOSS CALCULATION---thus a kitchen 12' X 15' = 180 sq.ft. X 50 (heat factor) = 9,000 btu/hr to heat the kitchen.

By the way, you should do this for ALL the rooms, total them all up & compare it with the I D TAG on the boiler, which will have the boiler heating OUTPUT: and will say something like: OUTPUT 80,000 btu/hour.

Check the output label on the kickspace heater---most of them are rated at 4000 btu/hr & if so, it would be too small to heat the kitchen.

If heating the crawl space is impractical, then either PEX plastic or copper tubing can be run between the joists---the simplest method would be to make the tubing running under the kitchen floor PART OF THE BOILER'S MAIN SUPPLY PIPE RETURN before it returns back to the boiler.

In other words, the boiler heats the water inside the boiler---then a pump comes on to pump the 180 degree water out the MAIN SUPPLY PIPE (usually 3/4" to 1 1/4" at the top of the boiler---this MAIN SUPPLY PIPE delivers the hot water to each baseboard in sequence & by the time the loop returns to the MAIN RETURN PIPE at the side or bottom of the boiler the water has cooled to perhaps 140 degrees---the water is again heated to 180 degrees & the circulation continues.

Before the RETURN PIPE returns the water to the boiler you could have it go thru a loop of 220' of 3/4" tubing between each kitchen floor joist & then have it return back to the boiler.

This simple method would eliminate the need for fancy mixing valves or zone valves & associated hardware, etc, that would cost a lot more $$$.

I would mount the tubing onto the kitchen sub-floor with enough looseness that the pipe would have room to expand as it got hot to avoid pipe-banging or expansion noises.

Radiant staple-up procedures call for aluminum HEAT TRANSFER PLATES to be installed onto the pipe to reflect the heat up to the slate/quartz tile.

PEX plastic tubing with a temp rating of over 200 degrees costs a lot less than copper, but has to be installed using a "lazy susan" spinning holder to handle the PEX.

Almost always, there's a lot of nails sticking out of the underside of the kitchen floor---many of these down the center of the area where the PEX/copper is to go have to be removed with a circular saw fitted with a metal-cutting blade (always wear safety goggles when doing this)--this will allow the tubing and aluminum heat transfer plates to be installed.

There are still two problems with putting sub-floor radiant in a crawl space---if you get near zero temps regularly in your area, the sub-floor pipes in the crawl space could freeze if the boiler off-cycle is too long---also the return water from the piping would have to remain above 130 degrees to prevent the boiler from condensating which could cause eventual rust & corrosion & shorten the life of the boiler.

Thus, another solution would be to close in & insulate the crawl space as best you can & install baseboard on a separate boiler zone attached to the concrete walls of the foundation---this zone would have its own t-stat which would keep the crawl space temps in the low 50's to conserve heat & prevent freezing--an alternative scheme to this is to run the tubing between the joists & set this up as a separate zone (using a zone valve) with a t-stat in the crawl space set to anywhere from 50 degrees to 140 degrees--this would be done if there is a freeze danger to the crawl space piping in your area.

The "house needs" site below is an excellent starting point to get the basic info on how some of these things can be done---the end of the heating season is an excellent time to shut down the boiler for several days do the installation---the "house needs" site offers PEX packages for sale---check elsewhere & price-compare on the internet & at your local plumbing supply houses for best prices before you buy---I have no relationship with any of the sites below---they're listed for informational purposes only.

There are tons of radiant sites on the net you can access if you Google "under floor radiant heat" (with and without the quotes).

Also Google "heat loss calculator" to get a more accurate amount of heat the kitchen is losing each hour on a cold day---it should be somewhere in the neighorhood of 9,000 btu/hr---baseboard puts out 580 btu/hr per foot---thus 9,000 btu/580 = 15.5 feet of baseboard needed (without the kickspace heater).

Since baseboard would take up too much room in the kitchen, you would probably have to install two kickspace heaters at 4500 btu/hr each to keep the kitchen warm.

How many btu/hr you would get from the new sub-floor piping is usually calculated at 20 to 25 btu/sq/ft = 180 s.f. X 23 = 4140 btu/hr from the radiant portion---thus only one kickspace may be sufficient.

There is a book by John Siegenthaler in the public library "Modern Hydronic Heating" that is readable & very good--New York State has a state-wide system & Siegenthaler is from NY, so they should have it.

Other articles written by Siegenthaler: Google "A little floor warming please", "The Plain Vanilla System". "Dashed Expectations", "Underfloor Installation offers a retrofit solution", and "Da Fundamentals".


TV Listings

Find TV listings for This Old House and Ask This Old House in your area.