Insulation - foil under cement board?

I have a tongue and groove wood floor over a dirt filled crawl space and I don't want to rip up the floor to insulate. I am going to put cement baord down and tile the top with granite using an underfloor heating mat since there's no heat in that part of the house.
Can I put some kinds of foil between the wood and the cement board to keep the heat in the house? Any ideas? I have seen 2 kinds of foil one that looks like bubble wrap and another that's like think kitchen foil. I was thinking the bubble wrap would cause too much give and make the tile crack?
Would the foil cause any kind of moisture problems that would rot out the wood?
Help!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
foil works by reflecting radiant heat, and i think it requires an airspace to have any value at all. so the answer is that the foil would have little value IMO. do you have the room to add a layer of rigid? if you did, you could even add a layer of 2" extruded polystyrene (dow blue or similar) with a layer of plywood over it. expensive, yes, but then pumping heat into a crawlspace isn't exacty cheap either. otherwise, i think you are going to regret not tearing up the subfloor and insulating between the joists.
Mulan wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
2.5" is a lot of height to lose and would mean a step into and out of the room :-( Do you think the bubble wrap style of foil would have too much "give" under cement board? That would give me air space?
Or maybe the roll of white polystyrene that you put under wood floors then the foil on top and then the cement board. Would that thin poly be enough air? I SO don't want to pull up the tongue and groove.
marson wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Try Googling "Delta-FL"

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

i'm really skeptical about any thin layer. you might call the company that makes the insulation product you are considering and ask them to tell you the R value in your situation. also, when you find out the R value, call and ask the manufacturer of the radiant heat system if their product works with whatever R value you come up with underneath it. i also think anything resilient directly under the cement board is asking for cracked tile.
so ignore my advice if you want, but you are risking cracked tiles and a radiant heat system that is too cold to be effective...after going through all the labor of installing tile.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
you want the easy mat http://www.custombuildingproducts.com/welcome.aspx

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.