foam thickness under heated slab

[PNWguy]

I'm pouring a slab with PEX for radiant heat, and have been assuming I'll use 2" XPS and a vapor barrier under the slab (and along the edges of course).

I am very well aware that "more insulation is better" but am curious if anybody has any calculations on how insulation relates to heating loads.

I'll need about 100 sheets, so going from 2" to 1" would save me about $1,500.

 

[Copymutt]

Did a google search, your area recommends 2”. 1” in southern states.
You might need to get creative to span the insulation between the floor top and top of stem wall. I have used nice cherry plank and mdf for attractive borders.

 

[kabinenroller]

I used a vapor barrier and two layers of 1” ridged foam crossed layers and tape the seams. I also used 2” of foam on the exterior of the foundation and 1” on the inside of the foundation so basically the floor floats between the foundation walls and does not contract thermo transfer from the outside. Going into my seventh winter with great results, hydronic Heat is the best.

 

[moserjj]

Skimp somewhere else if you must. Put in the 2" foam! I'm in WI and I often wish I'd done more than 2".

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[walrus]

You might think about 1 inch in the middle of slab and 2 inches near edge of slab.

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[Kaizen]

Using two inch will probably roi in less then ten years over one inch. The thinner you will be paying for every winter. Skimp on something else.


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[iowa4x4dieselman]

From all the research I did. Lowes around me had the best price for 2 inch. 25.52 per sheet normal price, and with their CC you get 5% off. I am a pro member which usually will gain you a better discount on larger purchases, but their profit margin was a little over $1 per board, so they were gonna try to offer a 4.97% discount, but the default is 5 on their CC, unless your a veteran, then I think you can get it doubled to 10%. I purchased 100 sheets also, If I was thinking ahead the pro gets me $20 delivery and it would've been worth it, but I was already there with truck and trailer to pick it up.

 

[Miss the Pontiacs]

I did a thickened edge slab. I have 2” foam on the outer vertical surface of the slab which is covered with tin. Then horizontally below this 2” foam 18” wide placed below the vertical insulation radiating away from the slab. Below the slab a foam/ poly blanket was placed right up to the foundation forms.
After the fact got thinking it should have been some thing more substantial. My carpenter friend/neighbour who specialized in commercial property said I would be fine. His father’s shop was done the same way and this worked well. Guess I will know when the first gas bill comes in this winter.
I do not have either vertical or horizontal where I have garage doors.

 

[tymbo]

Try researching reclaimed sheets of insulation. They get them from demolished industrial roofing. They might be a little beat up, but you can save some $$.

 

[PNWguy]

Thanks all.

 

[mike93lx]

Skimp In areas that can be upgraded later if you must. Exterior cladding, windows, interior trimmings, etc. You can never change the insulation under your slab.

 

[yeldogt]

When talking about an on grade slab -- it's best to err on the side of too much. Especially in a cold climate.

make sure you have a thermal separation at the edges.

There are two things at work -- one is the cold ground. the other is a separation between the ground and the slab so the heat goes up. Even in a basement 8' down you don't want to be heating the ground.

I wish there were more real world testing -- my guess is in a large building the need in the middle of the slab will be less. I wish someone tested two buildings -- one with 3" maybe 8' or 12' in and then 1" for the rest. And test this against 2" all over ...

The one thing that changes all of this is moisture -- wet or moisture is so important to have a good VB. Moisture will wick the heat away --

 

[mcbane]

It has been suggested you might use thinner foam towards the center of your slab. I wouldnt try that without some site-specific data or a consultation with a local hydronics contractor.

The problem is that soil a few ft down is not assured to be at 55 degrees nor is it guaranteed to be able to absorb and store heat. At the site of my future garage, I buried temperature probes at 18" and at 42" deep and covered the area with a layer of insulation. I recorded data all year long at 10 minute intervals. The 42" deep probe was between 33 and 34 degrees F all winter long. I believe the issue at my site is that there is very cold ground water that is near to the surface and flowing at a slow speed under the surface. That soil would never warm up to the point where I could skimp on insulation.