Concrete Floor Insulation

[mshedb]

I have been reading the archives and have a question about foam insulation under a concrete slab. My garage will have footings, compacted soil, crushed rock, a vapor barrier and insulation under the concrete floor (4000# mix/5" thick). A couple of posts in prior threads suggested foam insulation was a problem because its lack of compressive strength. Yet, lots of folks here seem to use it w/o a problem. Anything special that needs to be put in the specs due to the insulation?

Thanks.

 

[larry4406]

I am an engineer and spoke to other engineers regarding the foam under the slab. Your slab will be either a slab on grade or a structural slab.

A slab on grade means it relies on the available compressive strength of the soil/stone base being greater than the applied load. When this occurs, the subbase will not settle due to the weight of the slab and applied loads (cars, etc). Typically, you want a minimum bearing capacity of 2500 psi for the soil/stone base. If you have this and then put in foam which has a rating of about 100 psi, then the foam crushes under the weight, settling/cracking the slab, and the foam's insulating strength is probably reduced since it is compacted.

A structural slab uses grade beams, piers, extensive re-bar mats to distribute the slab and applied loads to the perimeter footers and perhaps interior piers. This design is used when the subbase is compromised or has insufficent bearing capacity. With this arrangement, the foam underneath will not be crushed. Structural slabs are considerably more $$ due to the extra concrete, signicantly more steel (rebar, not just wire mesh). Often structural slabs use a higher strength concrete as well.

In spite of the above, many people put the foam under the concrete and claim it works even without a stuctural slab. I did not put it in in my garage due to these concerns. The civil engineers I spoke to would not endorse it either.

 

[z28toz06]

I have been reading the archives and have a question about foam insulation under a concrete slab. My garage will have footings, compacted soil, crushed rock, a vapor barrier and insulation under the concrete floor (4000# mix/5" thick). A couple of posts in prior threads suggested foam insulation was a problem because its lack of compressive strength. Yet, lots of folks here seem to use it w/o a problem. Anything special that needs to be put in the specs due to the insulation?

Thanks.

2" blue board is what i am using. If doing radiant heat it is more important for bouncing the heat back than an actual large R factor.

http://www.teksupply.com/farm/supplies/prod1;Copy+of+Copy+of+10f1c57b;4ea1efa6.html Alternately you can use this stuff or both if you are in a very cold area! This is a good company for heating and industrial supplies and they sell to anyone.

 

[mshedb]

I am an engineer and spoke to other engineers regarding the foam under the slab. Your slab will be either a slab on grade or a structural slab.

. . .

In spite of the above, many people put the foam under the concrete and claim it works even without a stuctural slab. I did not put it in in my garage due to these concerns. The civil engineers I spoke to would not endorse it either.

Thanks for the quick reply. Did you just use a vapor barrier between the rock and the concrete? I'd also be intereseted in what kind of floor you went with - how strong was the concrete and how thick?

 

[CraigFL]

I am an engineer and spoke to other engineers regarding the foam under the slab. Your slab will be either a slab on grade or a structural slab.

A slab on grade means it relies on the available compressive strength of the soil/stone base being greater than the applied load. When this occurs, the subbase will not settle due to the weight of the slab and applied loads (cars, etc). Typically, you want a minimum bearing capacity of 2500 psi for the soil/stone base. If you have this and then put in foam which has a rating of about 100 psi, then the foam crushes under the weight, settling/cracking the slab, and the foam's insulating strength is probably reduced since it is compacted.

A structural slab uses grade beams, piers, extensive re-bar mats to distribute the slab and applied loads to the perimeter footers and perhaps interior piers. This design is used when the subbase is compromised or has insufficent bearing capacity. With this arrangement, the foam underneath will not be crushed. Structural slabs are considerably more $$ due to the extra concrete, signicantly more steel (rebar, not just wire mesh). Often structural slabs use a higher strength concrete as well.

In spite of the above, many people put the foam under the concrete and claim it works even without a stuctural slab. I did not put it in in my garage due to these concerns. The civil engineers I spoke to would not endorse it either.

I understand what you are saying but if you consider that concrete is 150#/cu.ft and say the slab is 6" thick, that's only 1/2psi pressure on the insulation board. If you apply a 1200LB point load, even that is only about an additional 50psi to the insulation board conservativley. I always considered that bearing capacities like the 2500psi you quoted had to do more with the soil/base prep to be sure it would be stable over time.

 

[ownsaglock]

Im an engineer too and see minimum gain for a lot of potential trouble using a foam barrier. Ive been around plenty of concrete,structural and SOG and have never used it/seen it used.

 

[bmwpower]

Wow, this is the first time I'm hearing that foam is bad. I've seen a lot of floors on here installed with foam. Maybe they could post their experiences?

 

[ownsaglock]

I'd also be intereseted in what kind of floor you went with - how strong was the concrete and how thick?

3500 psi is a standard mix in this part illinois. It is used a lot in road and bridge construction. A common spec is a concrete test cylinder should reach the compressive strength of 3500psi in 14days of curing. That is plenty for residental construction. Most residental stuff here is 4" thickness. If youre building a shop and might put in a lift the thicker floor could give you peace of mind. It isnt required by lift specs that Ive researched but if you dont mind spending the extra money what the heck.

 

[Lu47Dan]

mshedb , Be sure to insulate the perimeter of the foundation , I posted in one of these threads on insulating a floor slab about studys of cost to benefit ratio of perimeter insulation or full slab insulation all of them I have read shows that it is more cost effective to insulate the foundation perimeter than to insulate under the slab . Make sure you have good drainage , a stone capillary break and a vapor barrier . An uninsulated foundation allows cold to creep into the building . No stone under the slab allows water to wick up and rob heat from the slab , and no foundation drains traps the water inside the foundation . I you want to insulate go with a thinner insulating board than the 2" stuff 1/2 " would probably be thick enough for a thermal break , Remember heat rises and cold sinks . Compact your back fill in lifts , six inch lifts work wit what I use for back filling ,but I will be going with 4" lifts on the shop I intend to build this summer . Remove all topsoil and organic matter , they don't compact well and can cause settling at a later date . The better your base is the longer your slab with last and the happier you willl be . Dan

 

[Junkman]

I installed 2" blue foam board under the garage floor back in 1982 and have never had any cracking or other problems. It is 4500 pound mix 4" thick with wire. My garage floor is always warm and there is no moisture under the floor. I have perimeter drains around the whole foundation on both sides of the foundation walls, including the garage. The only concrete work that has given me any problems is one outside approach apron that heaves in the winter, and settles back down in the summer. I am toying with the idea of breaking it up and replacing it, but I don't think that I will be able to resolve the water issue that is getting under it. For this reason, I and still thinking about it...

 

[Lu47Dan]

Not To Hijack Thread , Junkman . If you are having heaving problem with your concrete aprons , and know it is a problem with water laying under them then breaking them up and placing drain tile and stone under the new aprons should cure your problem . If you can run the tile ,with 1/8 to 1/4" of pitch to daylight your water problem will disappear . But you could still have problems with it heaving if you have organic material (topsoil) under the slabs as it doesn't dry out as well as stone . If you have a surface water problem that is saturating ground under the aprons than you would need to remediate this problem also . I have a gravel driveway here and every spring it would break up because it could not drain . I dug about 400 feet of ditch and put in tile to drain the water out of the ground I had to pitch the tile two ways to make it drain correctly but it does drain well , as it has not heaved and broken up since . I remove all the top soil , subsoil and clay and back filled the tile with 18" of 2-B stone then covered that with what we call " bank run " gravel here . I left two places where the 2-B stone comes to the surface to help remove the surface water quickly . Dan

 

[JohnZ]

Mine was built seven years ago - 44' x 58', 4500# mix, 4"-5" thick, wire-reinforced, over 5/8" thick 4' x 8' sheets of high-density closed-cell foam with heavy foil on both sides, over a 10-mil poly moisture barrier, over crushed stone, on mechanically-tamped soil. Relief cuts in 8' squares, cured for 4 months, steel shot-blasted, 2 coats of solvent-based industrial epoxy resin.

Not a single crack anywhere, zero moisture migration, floor is warm even in below-zero weather.

 

[Junkman]

Not To Hijack Thread , Junkman . If you are having heaving problem with your concrete aprons , and know it is a problem with water laying under them then breaking them up and placing drain tile and stone under the new aprons should cure your problem . If you can run the tile ,with 1/8 to 1/4" of pitch to daylight your water problem will disappear . But you could still have problems with it heaving if you have organic material (topsoil) under the slabs as it doesn't dry out as well as stone . If you have a surface water problem that is saturating ground under the aprons than you would need to remediate this problem also . I have a gravel driveway here and every spring it would break up because it could not drain . I dug about 400 feet of ditch and put in tile to drain the water out of the ground I had to pitch the tile two ways to make it drain correctly but it does drain well , as it has not heaved and broken up since . I remove all the top soil , subsoil and clay and back filled the tile with 18" of 2-B stone then covered that with what we call " bank run " gravel here . I left two places where the 2-B stone comes to the surface to help remove the surface water quickly . Dan

The water comes off of a hill and down onto the driveway. At the apron it gets under it and heaves it. Because of New England ledge, there is no way to correct this situation. It isn't that bad that I can't live with it, and is only an issue when I bump it with the snowblower, so it will probably stay. Unfortunately, there is no way to drain the water away in this one location.

 

[larry4406]

Thanks for the quick reply. Did you just use a vapor barrier between the rock and the concrete?

I used a 6 mil polyetylene plastic between the #57 gravel and my concrete

I'd also be intereseted in what kind of floor you went with - how strong was the concrete and how thick?[

My slab is about 5 to 6 inches inches thick, has piers for a future lift, but is basically a slab on grade. If I recall correctly, my concrete contractor used 3500 psi concrete with both 6x6 welded wire mesh and rebars. he works for me so i got a good deal.
/QUOTE]

 

[BowtieNut]

Mine is 1.5 years old, no cracking or issues yet. I was told by my heating supplier that the Type 150 foam may compress, so use the Type 250 2" foam. My garage has no base other than the natural sandy soil, then the 2" foam, and then 4" fiberglass reinforced concrete (no wire). Again, this is just coming from my heating supplier, but they say most people in our area do it this way, and so far mine seems to be working good.

 

[PAToyota]

I've got 2" of DOW SM under my slabs (24'x40' with one expansion joint down the middle) - no settling, no cracking, and over ten years old. We do it all the time - actually in up to 50' squares.

 

[Kapt]

I'm in the process of prepping my slab and will be insulating it with extruded polystyrene foam, 2" in the field and 2" on the inside of the wall. I checked with my radiant guy after reading this thread about the structural integrity of the foam and he said it's no problem, they do it all the time. Good enough for me.

By the way, does anyone have a good supplier? Do any of the big box stores carry it?

 

[ownsaglock]

I still wouldnt do it myself but its nice to know you guys are having good luck with it.

 

[IDASHO]

By the way, does anyone have a good supplier? Do any of the big box stores carry it?

I work at a Do it Best building supply store.

We carry everything from styrofoam, to foil faced rigid foam, to 2500 psi foam.

Most people insulate under their garage floor slabs around here.

The 2500 PSI foam (normally white) is what you want.

 

[Kapt]

I work at a Do it Best building supply store.

We carry everything from styrofoam, to foil faced rigid foam, to 2500 psi foam.

Most people insulate under their garage floor slabs around here.

The 2500 PSI foam (normally white) is what you want.

I've never heard of 2500 psi foam before. Do you know who makes it and how much it costs?
Tom.

 

[bmwpower]

I work at a Do it Best building supply store.

We carry everything from styrofoam, to foil faced rigid foam, to 2500 psi foam.

Most people insulate under their garage floor slabs around here.

The 2500 PSI foam (normally white) is what you want.

2500 psi foam? So I can drive my car over it and not dent it?

 

[IDASHO]

I've never heard of 2500 psi foam before. Do you know who makes it and how much it costs?
Tom.

er... that was a typo. It is 25 psi.

The stuff we stock is a STYROFOAM™ Brand called "Scoreboard". 1.5 and 2 inches thick, comes in 4x8 sheets, and is 'scored' to break into smaller pieces.

Just be sure it IS the 25 PSI stuff.

STYROFOAM makes 2" Scoreboard in 15 and 25 PSI sheets. And the ONLY marking Ive ever seen to distinguish between the two is on the wrapper they put over the entire unit. Once the wrapper is broken, and the sheets are sold in singles, all bets are off. This is one small problem that has created unneeded havoc.

 

[IDASHO]

here is a link I found showing product info:

http://www.dow.com/styrofoam/na/res-us/products/STYROFOAM_SCOREBOARD.htm

 

[HoosierBuddy]

Gee there sure is a lot of "I'm an Engineer-itis" going around this post, like that qualifies us for a special deal at Waffle House or something.

http://www.dow.com/styrofoam/na/res-canada/projects/insulating_under_slab.htm

And I quote:

"It is important to insulate slabs to reduce the potential for condensation (or sweating) on the slab and to reduce energy loss and increase comfort. And, it is critical to insulate slabs that are going to have floor coverings installed over them to reduce the potential for condensation and mold. It is easy to insulate under the slab before the concrete is poured (placed). STYROFOAM™ extruded polystyrene insulation offers high moisture resistance, compressive strength and R-value and is ideal for this application"

The product is rated at 30 psi. As a mechanical engineer (damn...it must be catching) I take solace in the fact the product manufacturer recommends it for the application in question. I had no qualms in using it in my garage and wouldn't consider building another without the same product or similar under the slab.

I DID remove the foam in 4' X 4' squares under my lift columns (2 post 10,000 pound). That meant I ended up with 2" more concrete under those sitting right on the vapor barrier. The rest of the slab was 4" to 5" thick (sloping to the floor drains) with heavy wire grid reinforcement (designed for the hydronic tubing) AND fiber reinforcement. The fiber reinforcement was only $7 per yard extra...so I said "what the hey".

The slab is 1 year old and the only imperfection on it is where I dropped one of the lift columns while unloading it.

Let's think about it...30 psi compressive rating means 10% deformation at a loading of 4320 pounds per square foot. That's dynamic load. Your concrete is actually going to float any applied load across an area much larger then 1 square foot, because it can't deform as much as the styrofoam.

I would not use styrofoam under a machine base, or something like that...but under a garage floor it's a GREAT solution to an important problem with slab on grade construction, namely breaking the thermal barrier to the ground, so that indoor comfort can be obtained with some efficiency.

That's my take on it.

Phil

 

[mshedb]

Interesting reading.

http://www.dow.com/styrofoam/na/res-us/installations/blueguard.htm

 

[Ryan Wilke]

Fellas,

1) When you guys refer to placing polypropylene sheeting down for a vapor barrier,,,do you somehow tape or seal the seams, or do you just overlap the edges, or ???

2) Same type of question regarding the sheet styrofoam, do you simply bunt the edges or do you tape or bond the edges,,, or ????

Thanks!
RWilke

 

[Junkman]

I butted the edges together, and then covered the entire floor with 6 mil black poly. I started in one corner, and worked my way across the floor and to the other end. The blue Styrofoam that I was using is tongue and groove on all 4 sides. If I had found that it wasn't holding together, I could have used some duct tape to hold the pieces, but it wasn't necessary. I installed the poly on top of the insulation, because I wanted to keep as much of the moisture in the concrete for as long as possible. The longer that concrete stays moist during the curing period, the stronger it is eventually. I also asked the contractor not to thin the mix, but use it the way that it came from the mixing plant. If you add too much water to the mix, you will weaken the mix. It is a fine line between not enough water, and too much water.

 

[IDASHO]

I also asked the contractor not to thin the mix, but use it the way that it came from the mixing plant.

They still haul wet mix where you live?

It has been a LONG time since I have seen a mud truck rolling wet.

All the trucks I see now-days roll the mix dry to the jobsite, then add water and mix on site. The advantages are pretty obvious.

Just imagine if the truck got stuck in traffic behind a wreck.

That would be one HUGE paperweight

 

[kxmotox247]

Maybe I need to re-read these posts but is the simple thin foil from the Tek-supply place enough for slab insulation? I'm in central illinois and will be pouring a slab on grade pretty soon here. Very good reading here. Thanks!

 

[CraigFL]

Maybe I need to re-read these posts but is the simple thin foil from the Tek-supply place enough for slab insulation? I'm in central illinois and will be pouring a slab on grade pretty soon here. Very good reading here. Thanks!

That sounds like it's for reflection only which is only a small part of the heat loss from a SOG. If you are heating the structure inside the slab then you probably want insulation under the slab to keep it warmer.

 

[kxmotox247]

Thanks Craig... I was kinda hoping that it was a cheaper alternative to the foamboard insulation. With 2000 sq.ft., it's going to be expensive.

 

[JohnZ]

My foam sheets (described in post #12) were $8.00 per 4'x8' sheet seven years ago; don't know what they are now, but with 10-mil poly under them and foil on both sides of the foam, it's made a 100% effective moisture barrier and insulation under the slab.

 

[kxmotox247]

The description says that it replaces up to 2" of foam insulation?

http://www.teksupply.com/farm/supplies/prod1;Copy+of+Copy+of+10f1c57b;7fa5ed7b_100779.html

If I need to buy about 65 sheets of that foam... $8 I can handle, but $30 each will break my budget.

 

[BowtieNut]

When I got mine 2 years ago, it was around $20 per 4x8 sheet of the 2" Type 250 foam at the local Menard's. Mine took about 50 sheets, so yeah, that was $1000 that I hadn't really planned ahead for. It hurt at the time, but I'm sooooo glad I did it now.