Forgo slab insulation?

[SD929]

Hi fellow GJers,

I'm in Tennessee (in the mid north near Clarksville) and climate zone four. I'm working with a builder on a 60x40 pole barn that will be used primarily as a wood / metal shop. I plan on heating / cooling with mini-splits. While going through the contract w/ the builder (not yet signed) I noticed the description of the slab (4") had a vapor barrier but no insulation underneath. The builder says it's no big deal to go without and unless I'm laying on the slab, I wouldn't notice. It's an approximately a 6.5k add-on. Am I crazy to consider not doing it?

The building plan is a 60x40x14 post-frame with metal siding.

If it matters I'm torn between spray-foam and rockwool or a combo.

 

[ConCretin]

I don't think you'd notice the difference from a cost or comfort standpoint with or without it and you wouldn't recover the cost in your lifetime from increased efficiency. Personally, I'd forego it and put the money saved in my pocket.

 

[rjacobs]

If you were doing in floor radiant heat, yes. Without in floor radiant heat, nah. Spend the 6.5k elsewhere.

Spray foam it, just realize that your warranty on your metal panels will likely be void. The building company I am working with right now says they will warranty the panels, but they wont fix the spray foam, so a $50 panel replacement ends up costing you $1k to get the spray foam guy to come back out.

 

[ericm]

Insulating the perimeter of the slab is another option. Most of the heat loss is there.

 

[CraigStu]

For $6500 I'd skip it. Worse case you end up wearing shoes w/ thicker soles while working for 3-4 months of the year. I have some old work type shoes that I wear in my garage in the winter in SW Va. And that's only if I will be out there for > 1 hour.

 

[Snip]

Just my 2 cents here. I'm a little further south and east of you in Crossville Tn and did not insulate under or around my slab as "winter" here not as long or cold as other places. Shop is 66x40x14 with a 66x12 lean to, 5.5" concrete. I am heating with a center run radiant tube heater (75k, natural gas). As of now I have only insulated the ceiling (r38) and one end wall (r19). I am retired and don't spend every waking hour out there but do spend a fair amount of time in it even thru the winter. Heat is turned off when not in there set to about 60ish when in there. I think my highest bill this past winter was 80-85 dollars, separate meter than the house. I never did feel like my feet were cold when out there. For me I think investing in wall and ceiling insulation was a better return on my investment than the slab, YMMV. At my age I don't think I would have ever saved enough on natural gas to offset the cost of the insulation under the slab before I can not longer be trusted to be left unsupervised. Like as if I was, LOL.

 

[bannerd]

I would insulate, I don't think it would cost that much. If your into natural building you could lay plastic down and straw then plastic and pea stone on top of that. Shouldn't be more than 1K to get that done and local farmers can sell straw for cheap. I've seen people take cardboard and lay that between two layers.. it will rot eventually but it will be from the outside in and you'll not be around that long nor the next guy.

 

[Professor Fate]

I would insulate, I don't think it would cost that much. If your into natural building you could lay plastic down and straw then plastic and pea stone on top of that. Shouldn't be more than 1K to get that done and local farmers can sell straw for cheap. I've seen people take cardboard and lay that between two layers.. it will rot eventually but it will be from the outside in and you'll not be around that long nor the next guy.

As a Contractor I would not warranty that slab if you put that system in before I placed the concrete.

 

[racecougar]

I would insulate, I don't think it would cost that much. If your into natural building you could lay plastic down and straw then plastic and pea stone on top of that. Shouldn't be more than 1K to get that done and local farmers can sell straw for cheap. I've seen people take cardboard and lay that between two layers.. it will rot eventually but it will be from the outside in and you'll not be around that long nor the next guy.

You're advocating for placing organics under a slab?

As a Contractor I would not warranty that slab if you put that system in before I placed the concrete.

I agree.


To the OP, I'm about 200 miles North of you (and about 200 miles West). I did not insulate my slab. I used R-19 batts in my walls and ~R-55 worth of blown fiberglass in the ceiling. Heating/cooling with a 24K mini-split in a 30'x60'x13' building. It has been an incredibly efficient building to heat/cool, and I absolutely do not regret not insulating the slab, as it is an excellent heat sink / stabilizer. If you were up in Michigan or the like, I'd certainly consider it, but where you're at, I wouldn't give it a thought.

 

[bannerd]

You're advocating for placing organics under a slab?

There's nothing wrong with doing it this way, straw is an amazing product. Laying plastic down as a vapor barrier is pretty common, putting 5-6mm of straw on top of that followed by another sheet of plastic and compacted p-stone is cheaper alternative than laying foam and or rockwool/floatstone/etc on the substrate. As more people are being focused on natural living/building, few of my clients request this which consist of pea-stone, 10mil plastic, straw and another 10mil of permeable plastic to create a unique vapor barrier and drain for water above grade. You can also treat straw with borax to increase stability and becomes unpalatable to bugs, fungus and organic decomposing. To keep run off and Impermeability at a minimum I would bury a drain pipe socked around that pole barn and away. Since you're using 5-6 mill of straw settlement is virtually non-existent and should be compacted regardless. Like all slabs... R-value is dependant on ground moisture. Even foam fails here, if it's wet the R value tanks.. which leads to long decay.

No different than taking down an 1890s house in 2021 to find straw under a portland slab some rotted, some not..and tearing down a new construction to find foam falling apart and loaded with bugs. If doing a full basement with a finished floor I would look at crumble from hempcrete or nexcem. Just a suggestion to the OP, choices is theirs.

 

[racecougar]

There's nothing wrong with doing it this way, straw is an amazing product. Laying plastic down as a vapor barrier is pretty common, putting 5-6mm of straw on top of that followed by another sheet of plastic and compacted p-stone is cheaper alternative than laying foam and or rockwool/floatstone/etc on the substrate. As more people are being focused on natural living/building, few of my clients request this which consist of pea-stone, 10mil plastic, straw and another 10mil of permeable plastic to create a unique vapor barrier and drain for water above grade. You can also treat straw with borax to increase stability and becomes unpalatable to bugs, fungus and organic decomposing. To keep run off and Impermeability at a minimum I would bury a drain pipe socked around that pole barn and away. Since you're using 5-6 mill of straw settlement is virtually non-existent and should be compacted regardless. Like all slabs... R-value is dependant on ground moisture. Even foam fails here, if it's wet the R value tanks.. which leads to long decay.

No different than taking down an 1890s house in 2021 to find straw under a portland slab some rotted, some not..and tearing down a new construction to find foam falling apart and loaded with bugs. If doing a full basement with a finished floor I would look at crumble from hempcrete or nexcem. Just a suggestion to the OP, choices is theirs.

What R-value are you expecting from 5mm-6mm of "compacted" straw?

 

[u2slow]

No slab insulation or hydronic tubing here. The money bleed had to stop somewhere. Slowly put in roxul by my own hand, long after I dismissed the GC.

As it turns out, 12 years later I'm not ready to heat or cool the building yet.

 

[bannerd]

What R-value are you expecting from 5mm-6mm of "compacted" straw?

Dry straw, compacted/compressed at 5-6mm you're looking around 1.7-2 per inch. Something much thicker 25mm (1") R4-6 easy. I personally wouldn't do that much because you will always get some rot and you'll get some settle. If you run a 6" slab with 1/2" stick then I wouldn't mind it as much.

 

[bannerd]

No slab insulation or hydronic tubing here. The money bleed had to stop somewhere. Slowly put in roxul by my own hand, long after I dismissed the GC.

As it turns out, 12 years later I'm not ready to heat or cool the building yet.

Happens, my own house I started laying rock wool for the full basement and at $100 a bag easy 10K. I'm grateful I bought about 2000ft of oxygen pex years ago, on demand heater and other things. Hoping to finish this year!

2018 rockwool per bag was $28! Check different states.. you might find an 8 hour drive from NY to West Virgina is worth your time. I was down there last year and rockwool was CHEAP and I kick myself for not bringing a trailer.

 

[u2slow]

2018 rockwool per bag was $28! Check different states.. you might find an 8 hour drive from NY to West Virgina is worth your time. I was down there last year and rockwool was CHEAP and I kick myself for not bringing a trailer.

Hah! Back then it was $70/bag. Now it's $90ish. Not buying it all at once has been easier on my finances.

Across the continent is too far a drive for me.

 

[racecougar]

Dry straw, compacted/compressed at 5-6mm you're looking around 1.7-2 per inch. Something much thicker 25mm (1") R4-6 easy. I personally wouldn't do that much because you will always get some rot and you'll get some settle. If you run a 6" slab with 1/2" stick then I wouldn't mind it as much.

Seems like a fair bit of work and a fair chance of causing future issues for a R0.3-R0.5 value.

 

[manwithtools]

OP, I'm just east of Nashville and I chose no insulation or radiant heat. I'm planning on heating and cooling with a mini-split like @racecougar. I believe that $6.5k is better spent elsewhere. I also would in no way fart around with straw under a slab and I doubt any concrete contractor will tell you they would mess with it either.

 

[andyvh1959]

MY 24x28 shop in Green Bay WI is on a 4" slab, poured over in floor heating tubes (still have to finish that system). Slab is on 2" stiff pink foamboard sheets, on top of 6mil plastic, on top of screeded/compacted asphalt grindings, on top of leveled/compacted sandy soil. I needed to come up anyway for the grade I wanted, so the grindings and insulation helped get there too. Slab was poured late Oct 2019, very rainy. Not one crack or fault has shown anywhere on the slab since.

 

[Hank11]

I don’t think I have ever seen an insulated slab in Tennessee, with the exception of in floor heating, which is pretty rare in a shop here. Usually in a house over a crawl space or basement.

 

[theoldwizard1]

Vapor barrier is more important than slab insulation.

 

[SD929]

Thanks for all the thoughts and suggestions. I appreciate even the non-traditional ones like straw, that's not something I had ever heard of before, I'm not entirely sure the risk of organic material for the rvalue is worth it but perhaps I'm just not capable of thinking outside the box. For now it will be vapor barrier alone. Now getting second bids for spray foam, first one for 2" closed cell everywhere was 15k and change.

 

[manwithtools]

Now getting second bids for spray foam, first one for 2" closed cell everywhere was 15k and change.

That does not sound terrible. I'm at $11.5k for a 32 x 48 x 12. Closed cell - 3" on roof and 2" on walls.

 

[carcruse]

Insulating the perimeter of the slab is another option. Most of the heat loss is there.

That is what I did on my 40x60. Only went 4 feet in all the way around.

 

[Hank11]

Thanks for all the thoughts and suggestions. I appreciate even the non-traditional ones like straw, that's not something I had ever heard of before, I'm not entirely sure the risk of organic material for the rvalue is worth it but perhaps I'm just not capable of thinking outside the box. For now it will be vapor barrier alone. Now getting second bids for spray foam, first one for 2" closed cell everywhere was 15k and change.

Think about open cell for part of your job. It will save you money.

 

[Skooterj]

Doing the math, 60x40=2400 SF. 32 SF per sheet. 75 sheets. $50 a sheet. $3750. If I was doing it, I would do it myself and save $3000 in labor for a couple hours of work. Even taping it isn't worth $3000 to me. But I don't think I'd worry about it in Tennessee.

 

[mike93lx]

Think about open cell for part of your job. It will save you money.

Its cheaper, with lower r value. You can also save money by just putting less closed cell in

 

[SD929]

Doing the math, 60x40=2400 SF. 32 SF per sheet. 75 sheets. $50 a sheet. $3750. If I was doing it, I would do it myself and save $3000 in labor for a couple hours of work. Even taping it isn't worth $3000 to me. But I don't think I'd worry about it in Tennessee.

I looked at a few places it was all around $10 more a sheet (between 59 and 62) - mostly big box stores and a few builder supply places. I didn't look super hard since most were similarly priced.

 

[Skooterj]

I looked at a few places it was all around $10 more a sheet (between 59 and 62) - mostly big box stores and a few builder supply places. I didn't look super hard since most were similarly priced.

2 inch? Formular 250? Hmmm, around here $51.

 

[Todd Baumann]

Mine is the same size and I literally just bought the foam this afternoon, like the others said above 75 sheets 2" cost me $2810 I bought it from an insulation supplier up here in Michigan

 

[firebirdparts]

The earth is 56 degrees in Tennessee. This is great for comfort in my opinion. FWIW.

Insulate around the edge. If you are going to heat to 75 all winter insulate it all, I guess.

 

[Dagny]

I have dug up water lines with straw put in on top in the 50s that the straw looked like it was put in yesterday 8 feet down though.

 

[Bwestfla]

Hi fellow GJers,

I'm in Tennessee (in the mid north near Clarksville) and climate zone four. I'm working with a builder on a 60x40 pole barn that will be used primarily as a wood / metal shop. I plan on heating / cooling with mini-splits. While going through the contract w/ the builder (not yet signed) I noticed the description of the slab (4") had a vapor barrier but no insulation underneath. The builder says it's no big deal to go without and unless I'm laying on the slab, I wouldn't notice. It's an approximately a 6.5k add-on. Am I crazy to consider not doing it?

The building plan is a 60x40x14 post-frame with metal siding.

If it matters I'm torn between spray-foam and rockwool or a combo.

If you were to consider insulation, go with 25psi EPS rather than XPS. Its better for that application and cheaper. Also, an added benefit of underslab insulation is stabilization. I live in Crossville, TN and the back half of my shop settled and I have significant cracking. Wish I had gone with it.

 

[SD929]

If you were to consider insulation, go with 25psi EPS rather than XPS. Its better for that application and cheaper. Also, an added benefit of underslab insulation is stabilization. I live in Crossville, TN and the back half of my shop settled and I have significant cracking. Wish I had gone with it.

I'm still new to a lot of this - I thought that insulation was just that, it's dense enough to save off cracking of concrete due to settling? Also can you describe what your base was like underneath the concrete?

 

[ConCretin]

I'm still new to a lot of this - I thought that insulation was just that, it's dense enough to save off cracking of concrete due to settling? Also can you describe what your base was like underneath the concrete?

It's very unlikely that the presence or absence of under slab insulation would have any effect on a slab cracking. Most cracking is the result of shrinkage and the rest is the result of inadequate base construction.

 

[1redTA]

I used to work at Ft Campbell and while looking at the sprinkler system and fire alarm panel on a new building in winter I was shocked at how warm my boots were and how cold arm felt he environment was even with a roll up door open. Come to find out it had in floor heat. Even still, if I was working on my TA on the floor in Clarksville ( had a carpet remnant under the whole car) it was comfortable to work in shorts and a t shirt with a Mr buddy heater after it had ran a little while

 

[manwithtools]

I live in Crossville, TN and the back half of my shop settled and I have significant cracking. Wish I had gone with it.

Pretty doubtful that any form of insulation would prevent settling and cracking of a floor. Proper base construction is far more important.

 

[75gmck25]

Based on many years of outdoor activities, when you are on top of a large uninsulated slab there is a lot of cold radiated up from the ground. You can overcome it with heaters, but that all that dirt and concrete is still really big and really cold. If you can keep that cold out it will make it much easier to heat the building.

Considering your location and no plan for floor heat, I would insulate the slab and put a very thick vapor/water barrier, but not get carried away. Keep all the moisture and most of the cold out of your building. However, I'm surprised it would cost $6,500 to the price of the building. That seems like a lot.

 

[dcg9381]

Pretty doubtful that any form of insulation would prevent settling and cracking of a floor. Proper base construction is far more important.

I've done 3 engineered foundations. They ALL crack to some degree (not structural). You live with it. We could blame this on my work and my contractors, but I see the same thing in multi-million dollar homes that are owned buy builders.

Base construction is critical. As is getting a perc test and knowing what's down there so you can build the right base (per foundation engineer). Our garage got 16 x 10' piers drilled because I built it over some "old road spoils".

Here the temp of the ground is "advantageous" (we have no real frost line), so all we do is a vapor/water barrier. People don't know what "floor heat" is here.

 

[racecougar]

Based on many years of outdoor activities, when you are on top of a large uninsulated slab there is a lot of cold radiated up from the ground. You can overcome it with heaters, but that all that dirt and concrete is still really big and really cold. If you can keep that cold out it will make it much easier to heat the building.

Considering your location and no plan for floor heat, I would insulate the slab and put a very thick vapor/water barrier, but not get carried away. Keep all the moisture and most of the cold out of your building. However, I'm surprised it would cost $6,500 to the price of the building. That seems like a lot.

The heat sink capability of the pad is a good thing in warm climates / in the Summer.