Expanded or extruded

[wgarner1414]

Ok guys I have been doing my research on under slab insulation for my new shop build. I have seen everyone using the dow board which is an extruded product. However I have seen articles and people on here using the white expanded stuff as well. I have found a good deal on some used white 4" thick stuff that can be bought cheaper than I can buy 2" stuff from a local manufacturer. What are your thoughts?

 

[cidermill]

check with you concrete guy. They will tell you what psi rating to get for under the floor. Vertical foam(foundation) does not matter.

 

[pstnbly]

EPS is not for under slab applications and should not be used below grade unless treated with borate (insects love it) 2.5 rated XPS for under slab only.

 

[jvitez]

Here's an Alberta, Canada company that only makes EPS, and spec's it for under concrete radiant heat. By my understanding it's not EPS vs XPS that's important, but the compressive strength of the product.

http://www.plastifab.com/homeowners/buildinghome/plastispan_duro/radiant_floor.html

 

[Highbeam]

EPS is not for under slab applications and should not be used below grade unless treated with borate (insects love it) 2.5 rated XPS for under slab only.

False. EPS is for underslab applications. It's buried all the time. Sheesh.

jvitez has it right, you want to be sure that the compressive strength spec meets your needs. The EPS under my slab is 25 psi stuff, same as the blue/pink XPS at the hardware store.

There is no good reason to choose XPS over EPS under slab when both are the similarly strong.

 

[anthony666]

i like xps .. it's nicer to cut .. whichever you choose make sure you grade and compact the gravel flat like a pool table .. use vapor barrier, overlapped and taped to keep the ground water from wicking away your heat .. and tape the seams of the xps, any bigger gaps should get spray foam .. you only get to do under the slab one time, take your time & do it as right as you possibly can

 

[pearltsi]

XPS. I used formular 404 3" thick. Actually purchased mine used as well. Came is 2x8 sheets which took a little more work but it was fine in the end. And they came in dirty. But concrete doesnt know the difference.

 

[Diesel Dan]

Check out this barn build by luke7734,
45x60 Harn

He used closed cell spray foam under the slab and gives cost per sq/ft compared to board insulation.

 

[wgarner1414]

Thanks guys. I found a local place that says they sell the same board as the dow board. I may pass on the other stuff as I want it done right. I know what I get if I buy new.

 

[wgarner1414]

That's a nice building but that foam is outrageous. I can buy the foam board a lot cheaper than the spray foam.

 

[Diesel Dan]

Yes the board is cheaper and it was pointed out in that build thread. However as shown there is more to it than just the 4x8 sheets.