I may be asking this question to late cause I'm in process of having a 40x48 garage and had them install every thing I noticed they installed 1in foam board at only a 5r value they almost have all the pex ran so it's kinda to late to pull it up but I thought they were doing 2in I'm not afraid to spend extra now if it means it will heat better later
In floor heat is 1" foam board enough?
- Thread starter bmxer883
- Start date
WisJim
Well-known member
Two inches would be much better. 2" under the slab, 2 feet down around the edge, and then 2 feet out from the bottom of the 2 feet down is minimum herein Western Wisconsin.
Kaizen
Well-known member
Yea 2 esp in cold areas like you. Did they put two inch in contract? Is it the right foam?
Yea you will be paying more every year in heat. I’d make them redo it or slip more under.
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Yea you will be paying more every year in heat. I’d make them redo it or slip more under.
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No I don't think contract said I should have asked more about it figured they would know what was best they do them a lot. It's a pole building so idk where else you would put foam other than slab. Would it be okay to slip another 1in board under there concrete was figured for 6in didn't know if that would mess with it also I do think it's right stuff it's DOW stryofoam
Don’t sweat it. Doubling that R value doesn’t double your return on investment. Windows are less than R-3 & heat rises. Put your bucks in the walls and roof/ceiling. That said I have 3” polystyrene under 4” slab, full course 8x16 CMU and 60 yards of crusher fines, but I’m totally passive solar. I also have a 12x20 kickout room off my garage. Hydronic in floor. 1” under it since 1995. Natural gas water heater. Best heat system w/o going total solar. Runs less than a buck a day.
Black Oak
Well-known member
Figured they would know what's best ? At this stage I would do ALL I can to insulate the edge of the slab . Lot's of heat is lost there . Good luck on your build .
So how can I insulat edge of slap? It's post building so there's just a board running all way around bottom so should I insulat 6ins up on that? And yeah I figured they would know you should have two inch I didn't research till after they put it in guess that's my fault
Kaizen
Well-known member
Yes they can try and slip in more under. Depending on size of pad is how difficult it would be. They are saving cost in their side. They might need to move up the skirt board if the are using that for the outside form. You can insert. Two inch foam against that board. Screw it a little to keep it in place. You only get one chance at insulating the slab so imo I would ask them to do it. Even if some pex has to be reinstalled. I don’t know the return on investment over the life of the building but I know if you don’t you will always wonder how much you are wasting. With radiant you want as much as you can so you need minimal input to keep it warm.
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jvitez
Well-known member
Under slab insulation is less about energy ($) saving that about quicker response to changes in thermostat setting. The ground is not an infinite heat sink unless you have a running stream under your building. The biggest delta T is between the side of the heated slab and the outside winter air temp, so more insulation on the side is key. Search for "frost protected shallow foundation" for more details.
In the past HVAC guys would put rigid foam board only 4' in from the wall/grade beam and leave the rest. It will take more to heat the ground in the fall but once it reaches equilibrium it's no different than a fully insulated slab. You make up the extra heat in the spring when the ground gives up it's heat into the slab.
So 2" would have been better but no catastrophe. Put foam board 4' horizontally outside the walls butting up to the wall and cover with soil.
In the past HVAC guys would put rigid foam board only 4' in from the wall/grade beam and leave the rest. It will take more to heat the ground in the fall but once it reaches equilibrium it's no different than a fully insulated slab. You make up the extra heat in the spring when the ground gives up it's heat into the slab.
So 2" would have been better but no catastrophe. Put foam board 4' horizontally outside the walls butting up to the wall and cover with soil.
kj_mustang
Well-known member
Slab edge is important to insulate.
Thanks for the picture I'll definitely be doing that. I guess I'll go ahead and leave the 1inch stuff alone then I get free natural gas from a well so not like I'm paying for it but I like to be efficient. Thanks for the help
kj_mustang
Well-known member
If you are leaving your interior walls unfinished, the insulation attached to the bottom skirt board you can cut the top edge on a 45 degree angle so the concrete covers most of it and it looks nicer. My walls were finished so I didn't do that.