Concrete question

[Reit38]

I'm currently building a 30x40 pole building. I don't plan on pouring concrete till next spring but I'm trying to figure garage doors and a walk in door height going off my finished concrete in the future..

Now to my question. The building will be heated. Do I just want a vapor barrier below my concrete or would having 2in Foam board benefit me more. In floor heat is out of question. So I'm not sure if the foam board would really help or not in this case

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[machsnell]

You want 2 inch rigid insulation board for sure as well as minimum 6 mil plastic vapor barrier.

Keeps the floor warm and it won't sweat. Keeps transmission of moisture through slab if you ever decide to coat. A dry room temperature slab is a wonderful thing.

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[coleman1495]

Just to be clear, wouldn't a person want the rigid foam board around the perimeter only? Lets say four feet in from the sides?

 

[TractorJeff]

In answer to post 3, no, you want to foam the whole floor. There are a lot of Threads on here about heating the floor and cold transmission into the slab. Even if you don't heat it, by insulating you will improve its Thermal capacity.

 

[coleman1495]

Okay. My understanding is that heat is lost as it conducts through the concrete and under the walls. The further from the walls you move the less heat is lost (as the heat has more ground to travel through). You would reach a point of diminishing return with insulating under the slab. The ground underneath the slab will simply warm up.

Furthermore insulating under the slab makes it more susceptible to frost heave. The ground temperature underneath your foam will vary with outside temperatures. This means the ground goes through freeze-thaw cycles hence the frost heave. You want to make sure the ground underneath your concrete does not go through freeze thaw cycles.

I am under the impression that insulating around the exterior perimeter is not only the most cost effective, but also the best structural option.

This is only my understanding of this.

 

[Reit38]

Would 1in f250 foamboard work also. The way the building was built i will only have room for 1in foamboard if I bring in 2in of scalpings

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[rebelranger]

My understanding is you need to do both. A. Rigid insulation around perimeter to frost depth and b. Foam under the entire floor. 1in is r5 2in is r10. The us code is r13 for floors. Being a shop you make the call.

The perimeter foam stops the temperature transfer which risks frost heave.

 

[coleman1495]

Like this:

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This is from the National Association of Home Builders guide for frost protected shallow foundations.

 

[rebelranger]

I agree with that but insulation under the concrete slows the heat escape out of the floor thereby improving building thermal envelope.

 

[coleman1495]

Right but the heat loss is through the perimeter and not the ground. At the very least use minimal insulation under the floor and maximize insulation around the perimeter. The floor may be a little warmer with insulation underneath it but its a fairly major expenditure. If a very comfortable floor is a factor then I guess insulating it is. Otherwise it isn't going to help the thermal efficiency of the floor.

Poly should still be put underneath the floor. I think gravel also makes a good substitute and adds to the structural integrity depending on the soil conditions.

 

[Reit38]

Issue is I have a pole building. Would make it tough to do the perimeter

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[coleman1495]

Yeah makes sense. Personally I would put down poly underneath the slab, and put foam board about 4 feet in around the interior perimeter.

 

[lakeroadster]

Maybe tell us more about the planned use of the barn, and its construction....

Traditional pole barn construction produces structures that are inherently "leaky sons-a-*******".

My point? Unless you are planning on spray foaming all of the seams in the siding and adding a lot of insulation you may be throwing money down a hole following all the "foam and membrane" advice you have received here.

 

[Reit38]

Building is tyvek on walls. Plan on foaming bottom rib along the base board to stop infiltration. Walls will have fiberglass bats. Ceiling will have blown in fiberglass. Inside walls will be sheeted

Not sure on heat source right now

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[Reit38]

Bump

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[Reit38]

Has anyone used Reflextix under their slab? Looking at this today and it advertises R-21

https://m.lowes.com/pd/Reflectix-400-sq-ft-Reflective-Roll-Insulation-48-in-W-x-100-ft-L/3775461

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[Firebrick43]

I don't know how they get away with such false advertising. It slightly better than nothing but that's it. Reflective products don't work unless they have an airspace adjacent to reflect into.

There are several test that show it