Craigslist deal on Insulation

[bimmer1980]

Here's a deal I got on craigslist this weekend! Some insulation for super cheap. It's about 80 sheets of 3 1/2" high density foam. I plan to use this under the slab of my new garage. It does have some tar paper on both sides so it looks a little dark.

I hauled part of it home with my bmw and trailer and the guy was nice enough to deliver the remainder of it!

 

[baugie]

what's the "R" value of the board?

 

[Piper]

if it's like the stryofoam sm it's R5 per inch

 

[senlow]

From the photos it appears to be polyisocyanurate. If so, it has an R value of 7.2 per inch thickness.

 

[Nutty 5.0]

That's what I used for my ceiling and its great stuff and I got mine cheap too.

 

[bimmer1980]

just curious, but what did some of you pay for your insulation?

I pad about $450 for 80+ sheets.............

I figured retail would be at least $2500 for 75 sheets of 2"...............

 

[Piper]

2" 2x8 sheets where I am in Canada go for $25 per.

 

[GSSFC]

You may or may not want to use that under your slab. In general you would want a higher density foam for that purpose. The weight of the concrete can be substantial. You want to consider the compressive strength of the insulation you propose to use.

Tim

 

[Nutty 5.0]

I got mine from a construction auction for the ceiling like what OP got for a few bucks a sheet. But figure ~$25 for the foil faced stuff at 2" around here for a 4x8 sheet.

 

[bimmer1980]

GSSFC--- I did consider the density of the foam. THat was one of the key items I was reviewing prior to purchasing. In looking at it and compressing it, it is every bit as dense as the "Dow Hi-density" foam at lowes or home depot....

 

[malodin]

i may be wrong and please clue me in if i am but it just seems to me that after a few years of driving on it and the weight of the concrete it would settle or squish flatter thus messing up your concrete floor?

 

[bimmer1980]

perhaps you may see a little bit of settling over time... but Radiant Floor Heat 101 uses foam under slabs all the time. Granted the system has to be engineered for everything to work right. The concrete takes a point load--think forklift, equipment, etc and spreads it out over a wide area. That's why you need the slab to be engineered with rebar to spread those loads out. Concrete is good in compression but not in tension. Hence the rebar--steel is good in tension and compression. The bottom of the slab is generally in tension as loads are dispersed.

 

[malodin]

ahh, so you definately have to use rebar if your going to use a foam underlayment. cool, it just seemed a little odd to me is all

 

[GSSFC]

GSSFC--- I did consider the density of the foam. THat was one of the key items I was reviewing prior to purchasing. In looking at it and compressing it, it is every bit as dense as the "Dow Hi-density" foam at lowes or home depot....

3.5" is not what is typically used though. We use a 3# foam in cases like that. You won't find that in sheets at that thickness readily available.

Tim

 

[walrus]

Poly iso will absorb water if submerged, it will dry out though. It loses R value when wet.

 

[Possum]

I don't know how I missed this thread when I posted this one -

http://www.garagejournal.com/forum/showthread.php?t=39830

I found some cheap craigslist insulation too. Still seems to be 50/50 on the yes/no.