Conditioned crawlspace w/radiant heat

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headrc

Member
Mar 28, 2008
152
MidEast Tennessee
Hi all ....does anyone have experience/input on utilizing a conditioned crawlspace with radiant floor heat versus bat insulation underneath the radiant piping? I want to try and decide if it is worth the extra expense ....is it better? Is there any problems with this concept? Thx, RH
 

begreen

Mooderator
Staff member
Nov 18, 2005
79,590
South Puget Sound, WA
I don't have radiant heat, but can say that changing to a conditioned crawlspace for our old house was a really good move. I will never worry about frozen pipes down there and our floors are way warmer.
 

headrc

Member
Mar 28, 2008
152
MidEast Tennessee
The research I have done on conditioned crawlspace (where heavy mill plastic or other even other heavier mil material is used on the ground and sealed around any supports ...then foam insulation is blown against the walls) states that it actually reduces mold and mildew ...and then equalizes the temperature so that the temperature below the radiant floor is not so cold with the result of it drawing or reducing the heat from the radiant tubing. I intend to install a metal heat reflector under the tubing (my thoughts are flashing at this point) to direct the heat upwards from the pex. So the question is whether the conditioned crawl space with its insulation approach is better than the bat insulation under the radiant floor pex installed as I just indicated. To my knowledge the foam insulation is superior to bat insulation ....but more expensive of course. I also do not like the idea of the foam being sprayed between the floor joists under the pex .....and actually it is cheaper to create the whole conditioned crawl space as opposed to this. The issue is how much heat will transfer from the pex and the heat reflector to the conditioned crawl space vs. having bat insulation underneath it with whatever heat loss happens through it if any. Looking for anyone with experience with the concept of a conditioned crawlspace as described. Thx, RH
 

Willman

Minister of Fire
Jan 15, 2008
670
Sabattus Maine
I used foil face polyiso foam board attached just below my underfloor tubing.It really forces heat up. It comes in various thicknesses. Prolly get away with inch or so down there. Use foam board around the inside of your foundation. Dig down some and use soil to bank board up against foundation. Remember heat always seeks out cold.
Will
 

begreen

Mooderator
Staff member
Nov 18, 2005
79,590
South Puget Sound, WA
That's what I did, rigid foam board on the walls and fiberglass on the sills and rim joists.
 
I think that I understand this. Basically you have staple up and if you leave it uninsulated you will heat the crawl space also.
The staple up tubing needs 3 times the insulation below than above, to make the heat go up.
If there is 1 inch of wood above, R1, then you will need at least R3 below.
 

Ncountry

Feeling the Heat
Feb 11, 2008
283
northern NY
Jersey Bill said:
I think that I understand this. Basically you have staple up and if you leave it uninsulated you will heat the crawl space also.
The staple up tubing needs 3 times the insulation below than above, to make the heat go up.
If there is 1 inch of wood above, R1, then you will need at least R3 below.
Admitting that I do not know all of the science behind it, this is how I understand it.... Radiant heat using pex under the floor does not rely upon the transfer of heat through the air to the floor above.Instead it transfers heat directly from the actual contact of the the pipe surface and the sub-floor. That is why using the plates to hold the pipe and distribute the heat is more efficient than the staple up method. Also making the insulation value under the bays less important. For example, hold your hand 1" away from 100 degree pipe and then touch it , big difference in heat transferred. All this being said the pipe still does radiate heat. Even though most of your heat will go to the floor above , the bays should be insulated to keep the most heat up where you want it.
 

begreen

Mooderator
Staff member
Nov 18, 2005
79,590
South Puget Sound, WA
One thing we found in our old house that sold me on switching to a conditioned crawlspace was air infiltration. We we replaced the flooring we found they used the poorest wood for the sub-floor. There were major gaps, knot holes throughout. We were getting a lot of cold air coming up through the floors. With a tightly sealed crawlspace now we get none and the coldest I've measured down there is 61 degrees.

This isn't for everyone, but we have the right conditions to take advantage. That is our soil is very sandy, we have large overhangs and new gutters and the crawlspace is bone dry. For lots more info on this subject and on homes in general here is a link to a great site: www.buildingscience.com . They breakdown designs by climate regions. Lots of great reading here.

For conditioned crawlspace info in New England (figure 5):
http://tinyurl.com/6kq95z

For heatflow and controlling losses:
http://www.buildingscience.com/buildingphysics/thermalcontrol/heatflowbasics/
 
Ncountry said:
Jersey Bill said:
I think that I understand this. Basically you have staple up and if you leave it uninsulated you will heat the crawl space also.
The staple up tubing needs 3 times the insulation below than above, to make the heat go up.
If there is 1 inch of wood above, R1, then you will need at least R3 below.
Admitting that I do not know all of the science behind it, this is how I understand it.... Radiant heat using pex under the floor does not rely upon the transfer of heat through the air to the floor above.Instead it transfers heat directly from the actual contact of the the pipe surface and the sub-floor. That is why using the plates to hold the pipe and distribute the heat is more efficient than the staple up method. Also making the insulation value under the bays less important. For example, hold your hand 1" away from 100 degree pipe and then touch it , big difference in heat transferred. All this being said the pipe still does radiate heat. Even though most of your heat will go to the floor above , the bays should be insulated to keep the most heat up where you want it.
I dont believe that the insulation is ever less important. The plates help distribute heat from the source. Its the laws of physics that dictate which way the heat will go. Heat doesnt go up just because it would be easier to install staple up tubing. Heat will travel form high to low, all the time.
Take an example like a tube, heat plate, and a wood floor surface above, concrete floor surface below. The heat travels from the water, through the tube wall, into the heat plate, and out to the perimeters. So far this is typical of heat conduction. From here the heat could travel in 2 directions, up & down. On top the heat sees 1" wood , an insulation value of R1. But there is also some mass, so heat by conduction heats the wood mass. The surface temperature is slightly below the water temperature, maybe 115. Heat will also go down from the bottom of the floor, radiantly. The floor above is maybe 112, the concrete floor below is 50. Thats a 63 deg delta. Similar heat flow would be from a radiant panel 63 deg over room temp, or 133 deg.
Without insulation below there is a huge loss of btu's delivered to the conditioned space.
 

headrc

Member
Mar 28, 2008
152
MidEast Tennessee
What about using foil backed bubble insulation under the pex ....would that work as well as regular flashing type material for reflection the heat ....and then provide the needed insulation to prevent heat loss into the conditioned crawl space? Thx again, RH
 
I have done that in some parts of my house. Its OK, but foil faced 4" batts in an 8" joist is better. 6" batts are better still.
With R1, U=1 above and R10, U=0.1 below 90% of the energy will go up.
 

headrc

Member
Mar 28, 2008
152
MidEast Tennessee
Did you use the foil backed bubble insulation with a conditioned crawlspace ....or just as the total insulation solution it self in the areas you used it? Thx, RH
 
i have used it only above conditioned spaces in this application.
 
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