I have a couple of options for insulating under a slab. This is a heated garage/shop.
Option one is to put 2″ rigid foam insulation under the entire slab. About 750 SF at .81 per SF, or $600+
Option two is to insulate the top 12″ of the inside of the stemwall (insulation installed vertically), and then insulate the first 24″ around the perimeter of the slab (insulation installed horizontally). This would take 320 SF, and about $260.
What’s the payoff on the extra $340 in insulation if I go that route? The climate here is fairly mild and the slab is going on top of a couple of feet of compacted sand.
Since the BI will approve either installation, I’m wondering if there’s only marginal added benefit in the extra insulation. My instinct is to always insulate as much as possible, but I thought I’d see what folks here know.
Replies
David,
I suggest you download the 'Builder's Foundation Handbook' from Oak Ridge Nat'l Labs at
http://www.ornl.gov/sci/roofs+walls/ facts/foundation/foundation.pdf
They have various foundation constructions illustrated, and based on your climatic weather and energy costs, they have recommended insulation levels and schemes.
i like the full slab... the comfort level will be easier to maintain.. and the $400 difference could be offset in a single year if we have big fuel spikes
Mike,Do you really think that $400 could be recouped in a years time? That seems like an awful lot to me, even if you were taking some hyperbolic liberties.The way I see it is once you're 2-4 feet in, your ambient ground temp is somewhere 55°. So if you keep your t-stat at 70° then you have a 15° temp. gradient.I would assume that the ground retains heat much better than the air that surrounds the rest of the structure, so your losses would be less than if you were comparing a 15° gradient at the wall section.Now I'm like David in that my inclination is to add more insulation, but I wonder if the money would be better spent elsewhere. I've been struggling with this detail for a while and this thread brings it all to the forefront.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
jon.. i always assume ambient ground temp is 45..
so ... 5 month heating season.. 70 - 45 = 25 deg Delta-T
and a cold floor to boot..
if all it costs is $400.. i'm inMike Smith Rhode Island : Design / Build / Repair / Restore
everybody alway think of insulation in a payback modes. Hell $400 aint nothing, go ahead and put it in. My floors are cold right now.. 2+3=7
I'm with Mike--put er in--sounds like a shakey bet to me.
I bet "x" dollers (the cost of the slab or the cost of the extra insulation or the extra cost of heat) that I can spend 400 better somewhere else.
Ya a gamblin' man---Even if you saved 100 bucks a year you would have to enjoy the shop for only 4 years--I have a bias opinion cause my shop is the garage and the slab is cold and gets to me if I have been off the floor pads for a extended period of time.
IAC your milder climite may trump my opinion so I guess it is worth what cha paid for it.
Mike
"Even a blind squirrel finds a nut once in a while" Mitremike c. 1990
" I reject your reality and substitute my own"
Adam Savage---Mythbusters
The building department here is very keen on their energy stuff. It seems to matter to them more than the structural details, the setbacks, etc. So, when he told me I could do the 2' perimeter I wondered if it was a wash compared to the full slab being insulated. Anyway, I bought 22 sheets and they're going in tomorrow. The stuff is unbelievably expensive here. What would a sheet of 2" XPS cost you guys?
2" XPS cost you guys?Do you mean expanded or Extruded--we use extruded--pink or blue etc and not the expanded--usually white--the ball stuff. YuckIAC I think 2" is around 24 bucks a sheet--I could check but I think it is in that ball park--IIRC the extruded is the only stuff rated for weight bearing--like under a slab. So I hope it goes well and I have a feeling you'll be happy you did it when your slab is air temp and not ground temp--those 15 or 20 degrees will make your feet happy--Plus you will cut down the amount of vapor coming thru your slab, A problem I have to live with in a 50 yr. old slab--Sealing the cracks has helped and I am considering epoxing the slab to make it moreso and make it easier to clean up.Mike"Even a blind squirrel finds a nut once in a while" Mitremike c. 1990" I reject your reality and substitute my own"
Adam Savage---Mythbusters
2" XPS cost you guys?
Do you mean expanded or Extruded--we use extruded--pink or blue etc and not the expanded--usually white--the ball stuff. Yuck
XPS is extruded (blue/pink). EPS is expanded (white). You can get hi-load XPS for just about any application.jt8
"Take your life in your own hands, and what happens? A terrible thing: no one to blame." -- Erica Mann Jong
Yeah---I had that in my brain but couldn't come up with the initials.Was ordering materials today and asked about the current price around here--2" XPS load rated is 19.80 a sheet---Mike"Even a blind squirrel finds a nut once in a while" Mitremike c. 1990" I reject your reality and substitute my own"
Adam Savage---Mythbusters
David, is your slab isolated from the stem wall?
EPS foam with borate to keep the bugs out is rated for this use. Lot cheaper than Dow board.
http://www.r-control.com/eps.asp
No idea on current price, bought a bunch of it a few years ago. They'll deliver if you're close to one of their manufacturing plants.
There's one brand here... Owens Corning Foamular 150. According to the web it's rated for under slabs in residential and light commercial. ~ $25/sheet.
Edited 2/22/2006 10:53 am by davidmeiland
EPS foam with borate to keep the bugs out is rated for this use. Lot cheaper than Dow board.
Just checked both, and Dow t/g extruded was a bit cheaper than the R-Control expanded, about .60 less per sheet of 2"
Is load-rated different from the "regular" XPS? And shoudl the regular stuff not be used under the slab?
Edited 12/12/2007 11:43 am ET by westmich
Can't say for certain but my understanding is that the expanded product will absorb water . The extruded will not. Best to use one that will not absorb any moisture at all if it is ground contact application.
They can't get your Goat if you don't tell them where it is hidden.
A number of years ago I picked up a piece of blue extruded foam that had been sitting in a wet location. At first I was thinking it must be some high-density foam, as it was much heavier than I expected.
I stuck it away for some period of time, and later found it had dropped down to a more normal weight. Must have absorbed a fair amount of water to have been so noticeably heavy.
In a vapor situation, I'd guess that the extruded doesn't pick up much water, but in an immersed setting, I doubt I'd trust either the extruded or expanded.
I used expanded under the slab in my garage a couple years ago - a friend of mine owns a company which manufactures it. My layout is very well drained, so I wasn't concerned.
Don
Extruded is rated for only a couple of percent water absorbtion.I have a dock that is floating on it for 15 years without sinking.And my old dock was was much older and the foam was not water soaked when I removed it.Now expanded will get water soaked under pressure after a couple while and it get VERY, VERY, VERY HEAVY..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Bill, I have a question concerning Poly isocanurate . I was told when it first came out it would absorb water. Now I hear people talk as if it does not. I looked at web pages for manufacturers and they seem to indicate that water can enter it , but drains quickly, Therefore it doesn't really "absorb" water, in the sense it will hold it but that it will allow water to pass into it and then drain out again. What, if any, is your understanding.
They can't get your Goat if you don't tell them where it is hidden.
I really don't know.I know that I have some under my slab that was built in 79..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Prices have changed, or maybe where you are. Used to be other way round.
EPS is rated for PSI, but a 4" slab doesn't weight much.
Joe H
Put the foam under all of the slab AND install some radiant heat in the slab and your feet will really be happy. So will the rest of you while working in there when it is cold outside.
I'd insulate at least to 6' of the perimeter. Then insulate the foundation on the exterior side for as much as possible (ideally to the mudsill). Then cover the insulation above ground with something durable.
The edges and the walls is where you'll have your greatest losses, unless your local water table is really high and you have a lot of water movement below. Insulating the whole floor will make the slab more responsive if you try to heat that space (and RFH is the best way to do just that).
I insulated the exterior foundation walls and the slab with 2" of XPS in our home.
I cut 12" rips x 96" long with a 45 bevel on the edge. Those are on the inside. Then I'll by laying down material flat under the rest of the slab. It is not typical to insulate the exterior of concrete here, I've never seen it done.
To me it is a no-brainer. I'd put XPS under the slab while I had the chance. Whether you add the other insulation is up to you.
jt8
"Take your life in your own hands, and what happens? A terrible thing: no one to blame." -- Erica Mann Jong
No-brainers are a great thing, eh?!