bloomingtonmike
Well-known member
I found some used 1/2 XPS Blue foam. Can I stack it to match my 1.5" XPS foamular and use it under slab for radiant floor heat?
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1/2 XPS - Can I stack it and use it under slab?
- Thread starter bloomingtonmike
- Start date
OP
bloomingtonmike
Well-known member
So don't stack/adhere three to make similar 4'X8'X1.5" panels to match the 1.5 I already own?
So don't stack/adhere three to make similar 4'X8'X1.5" panels to match the 1.5 I already own?
That's fine as long as it's the same PSI rating. No glue is necessary. Just stagger the joints on the thinner sheets. The more you stagger the joints the more the insulation will act as one continuous layer. So for two layers the best stagger is by 1/2 the panel size, three layers 1/3 of the panel size, four layers 1/4 etc... If you can afford it you may even want to get another full set of sheets so that all the joints are staggered instead of just the thin ones. The reason for staggering the joints is to disrupt the air flow between the sheets of insulation. But DEnd you say it's underground there is no air flow. B.S. How the heck do you think plants are able to grow. One of the main nutrients taken up by plants' roots is nitrogen, one of the ways it gets there is through rain fall and snow, but quite a bit is through air exchange with atmospheric air. Dirt has holes and spaces and where there are holes and spaces air flows, and water too.
Zeke
Well-known member
I see what he's thinking. The 1.5 sheets are not staggered in layers so why do the thin ones?
Because they are thin and because you can.
Because they are thin and because you can.
rburke65
Well-known member
What would be the result if the sheets were not the same psi rating? Explain please.
What would be the result if the sheets were not the same psi rating? Explain please.
Ideally the slab will act as one piece with no deflection. It will therefore exert the same pressure across its entire bearing surface. Think of the foam as a sort of spring, and its PSI rating as its spring rate. The lower a spring rate is the more the spring compresses. So if you place different spring rates under equal pressure they will compress different amounts. If those springs are under a slab then since they compress different amounts the slab would then tilt, and possibly fail due to unequal loading. This is also a reason to stagger the joints as no two pieces of foam will ever be exactly identical. By staggering the joints we make the entire sub slab foam act closer to one piece. It's not a huge concern with the same psi rating, and depending on the soil even different PSI rates MAY be ok, but without doing the engineering work it is better not to risk it, especially as the slab is probably the most expensive part of a garage build.
Use the thicker stuff around the permitter -- and place the other on the inside. It is the permitter that makes the most difference. Be careful with the base as the thinner stuff will snap when walked on.
Also -- the vapor barrier goes on top. I see people placing the vapor B and then foam and then sand. Crushed stone -- foam -- vapor barrier. The concrete guys love the sand.
You want to protect the slab form moisture -- you don't want to increase the mass.
Also -- the vapor barrier goes on top. I see people placing the vapor B and then foam and then sand. Crushed stone -- foam -- vapor barrier. The concrete guys love the sand.
You want to protect the slab form moisture -- you don't want to increase the mass.
OP
bloomingtonmike
Well-known member
I planned to do stone, vapor, foam, mesh, pex, then pour - Is that not right??
OP
bloomingtonmike
Well-known member
The 1/2" blue foam is Dow Residential Sheathing 15psi extruded polystyrene R3. I bought all the sheets. The other foam I have is 1.5" Foamular 250 R7.5 pink.
I would not use the 15 psi stuff.
How much of each do you have? You could fill in the perimeter with the formular and then evenly stagger some of it in the field filling in the rest of the field with the 15 psi Dow stuff. You may need to add rebar to the slab to do it that way. If you have enough formular to do that then run it by a structural engineer first. I imagine how well it works, and the design will depend on what your site soil conditions are.
How much of each do you have? You could fill in the perimeter with the formular and then evenly stagger some of it in the field filling in the rest of the field with the 15 psi Dow stuff. You may need to add rebar to the slab to do it that way. If you have enough formular to do that then run it by a structural engineer first. I imagine how well it works, and the design will depend on what your site soil conditions are.