Radiant in 3" 1/2 slab

bobFlor

Pouring slab over an old slab . new sill plates all around. Going to insulate the side with some rigid board insulation, 3" 1/2 inches high. I thought I should use a foil barrier over the floor  because we have never had any vapor barrier or insulation under our feet . is this going to be enough insulation and is 3 inches of concrete plenty . 

bobFlor

This is going to be 1/2 inch pex in slab . what does one use to hold the pex in place

bobFlor

I managed to get 4 " border around the perimeter . I saw some forum talk where 4 " of concrete was sufficient . I hope so. I am still not sure if foil insulation will be enough to reflect heat .or if rigid is nessecary

bobFlor

Alan (California Radiant) Forbes

Foil is not enough; you've got to get more separation. One inch rigid insulation should be enough, but if you're in a colder climate than me, more may be needed.

As far as slab thickness, you want at least 2" of concrete cover over the top of your tubing. A four inch slab should be enough, but I'm no engineer.

bobFlor

Thanks . it looks like I'm going to put down 1" rigid polystyrene then I'll have three inches of concrete over 1/2 inch pex. I can just staple the pex to foam.

Alan (California Radiant) Forbes

We usually put down 6" x 6" 10 gauge welded wire mesh, secured by long Tapcon screws and fender washers into the slab and then tie the tubing to the wire. The rolled wire mesh is difficult to work with and never straightens out, so we buy the flat kind.

bobFlor

Alan Thanks I appreciate the feedback. .So, I have a couple days to get this as great as i can before I call for concrete. Its an old slab .never had insulation under our feet . winter months have been like being in a cave . I took up tiles , linoleum , wood floor , got it down to just slab . I put the rigid around the will plates to insulate the sides . now I have 4 inches of depth . is mesh necessary  in this application. I don't see the reason for mesh here .is there any reason why I can't go without it. Also I read in building science that a vapor barrier is best if put on top of the insulation and not directly on the slab. Anymore help would be great . I'm finding so many different ways to do things.

bobFlor

I  see a lot of jobs on you tube like Marshall remodel laid some plastic down and then rigid XPS and call it a day on a huge $$$$ job . so its all very confusing. 

Alan (California Radiant) Forbes

We use the 6-6-10 wire mesh as a substrate to attach the tubing as I don't know of a good way to make the attachment to foam insulation.

GroundUp

Foil is useless. 1" rigid would certainly be better than nothing, but 2" would be twice as good as 1". Contrary to popular belief, heat doesn't rise- you NEED a good thermal break to keep the new slab's warmth from transferring into the old slab and the earth below. 2" is the "right" answer, and if you are confident that the old slab won't shift, 2" of concrete over the top of the 2" foam would probably be adequate.

Solid_Fuel_Man

I've always put 6 mil poly over an existing slab, then 2" of XPS "blueboard" over that, then as others have said sheets wire mesh and zip tie your pex to that. 

I know it doesn't seem like it, but you want everything as rugged as you can so that nothing goes wrong when the concrete trucks are sitting there. The pex can float, the foam can move....we seen it all. Do it once...do it right. 

It sounds like you've done a lot of work to get your house to this stage, finish the job right. I always and I mean always, use fiber in the mix, it costs $10 a yard here, and is cheap insurance. Concrete is a funny and wonderful thing. But when it's coming down the shoot, you dont want anything going wrong. 

hot_rod

I would go with more insulation thickness and less concrete. 2” foam with a foam stapler is an option for the tube install.
concrete with a smaller gravel could be poured. Around here pea gravel is considered a 3/8” aggregate. Ideally you want 3 times the aggregate size over the tube, or 1-1/8”.  So with a pea gravel mix you could get by with a 2” pour, even 1-1/2” if you add fiber to the mix.
 They usually double the amount of fiber in thin pours also.
3/8” Pex would buy you another 1/8” and is super easy to install. Use an A Pex as it straightens out better.

archibald tuttle

there is such a thing as 1 and 1/2" XPS. harder to find and sometimes available in weird double shiplap configuration meant for installing over sleepers.

as far as mesh goes, it's convenient to attach the tubing although there are plastic solutions for going directly to the rigid foam. buehler . . . anyone got a link for that (see hotrod mentioned the system i'm thinking of)? i've never used them but i know i've seen them. i often don't have height for another pour so i put ridgid foam routed with a 1/2" radius core box bit and press 3/8" pex tube into that with an industrial (6 times as heavy) aluminum foil to and then go over with 1/2" hardi backer for mass (trying to remember where the tubes are! only hit one once) and float a dense imitation laminate floor over that.

what is your plan for flooring. plus, mesh will not really prevent cracking. you could score the pour strategically. over wood subfloor i put 2x3's on their side every 16 or 24 inches which basically takes care of expansion joints and you have built in screeds and something to attach flooring (use lightweight concrete unless i'm over a slab. With long tapcons you could do the same over the ridgid foam (light weight concrete. little bit of heat transfer on the screws but i've never really noticed that extent of performance difference. i bet they would show up in infrared though. but with floating floors that is skipable if you can screed the pour well enough for whatever your finish solution is.

the more ridgid foam the better but at least 1" gives a thermal break. i defer to those with higher thermal engineering skills to say here but i have done 1 and 1/2 inch slabs by the method i mentioned above. you would get 2 inches with 2 inches of foam. you'd have to rip your sleepers if you used 'em. depending if you have any walls in mind i recommend sleepers even if not on frequency for floor nailing. but you put em where your wall is going and you got something to attach it to without hitting pipes. i sometimes stagger off end arounds or i've notched the sleepers and had a metal shop bend up some 3/32" steel into a U channel that I cut short lengths from for nail guard in the notch.

you probably won't be able to read all that in two days , nevermind get it installed for concrete but that's my 2 ¢ or more like a quarter.

bobFlor

I want to thank everyone in their regard to the help. Alan, Hot Rod, Archibald Tuttle, Solid Fuel Man , Ground Up. All this info has come together to give me all the confidence I need to put it together and get it done . thank you. B

bobFlor

Yes you are correct .I've seen the huge staplegun  with staples that look like three quarters of a rectangular gear , 

hot_rod

plenty of places rent foam staple guns, do a Google search. Buy one and sell it on FB marketplace or your local online classifieds, after you are done. Might even find a used one on Craigslist or FB marketplace

archibald tuttle

@bobFlor BTW, i think you were originally just talking about that small air pocket foil stuff and not like 1/2" isocynoate with foil, but make sure you are using XPS insulation. density is negotiable and the residential standard is fine for these applications. But I would advise against isocyonate. Too permeable. (And not EPS either).