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travellerw
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# Posted: 24 Mar 2025 11:42am - Edited by: travellerw
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I thought I would ask here as there is a ton of great experience among the members.
We recently bought a new house and have decided to put in floor heating in the main floor since the basement was already plumbed for it. We are doing simple "Staple Up" PEX. We are just about at the point where I need to insulate below the PEX and I'm trying to figure out the best route.
Research says it pretty common to do bubble foil (below the PEX), backed by fiberglass, but I'm not sold on that. Bubble foil is stupid expensive for essentially a reflective surface as the R-Value is pretty much nothing.
Does anyone else have experience or thoughts on how to insulate for the best value.
EXTRA INFO FOR THOSE THAT LIKE DETAILS.
I would spend a little more money for something easier to install. To make things more complicated the joist bays are 19" wide, so standard Batt rolls would probably need 2" cut off as waste.
Once this is complete a drop ceiling will be installed under it all to complete the basement finishing. This is why we are insulating. Otherwise a bunch of the heat would be wasted in the cavity between the drop ceiling and the PEX. Essentially, we are trying to push the heat up to heat the main floor.
Rodents or critters are not going to be an issue as this is inside a finished walk out basement.
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gcrank1
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# Posted: 24 Mar 2025 12:56pm - Edited by: gcrank1
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Regarding the bat insulation, think Id just squeeze the 'extra' inches into the bay.
Btw, what does your local code have to say about having a moisture barrier for such? It goes to the warm side.
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DRP
Member
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# Posted: 24 Mar 2025 01:27pm
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Foil faced polyiso foam sheets cut to fit with great stuff around the edges. Also look at aluminum heat transfer plates.
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ICC
Member
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# Posted: 24 Mar 2025 09:11pm
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I have a neighbor who put radiant into an existing wood framed floor similar to what you have. He used a 1" foil faced EPS foam sheet, cut and sealed much like DRP suggested. Then he placed fiberglass batting under the foam. The foil facing on the EPS faced up and there was about a 1" airspace between that and the upper subfloor.
I think that as time passes and the foil face gets dusty some of the reflectiveness is lost. His floor is nice and warm in cold weather.
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travellerw
Member
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# Posted: 25 Mar 2025 06:37pm
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So far, for cost, 3/4" foam backed by pink is winning. But I'm not sure I want to play the itching game for days.
Terrafoam Type2 is winning in my mind. I can get a 4'X8' by 4" sheet for $54. It has an R-Value of 16@ 4".
However with the joists being 19" in between that means I will need to cut the sheets at 4' x 19" to avoid waste. If I cut them at 8' x 19" I will be left with a 10" piece.
I guess I could glue sheets together using spray adhesive and tuck tape, but that seems like a big faff.
Still trying to figure out which way to go. I will let you know once I pull the trigger.
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Brettny
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# Posted: 26 Mar 2025 07:10am
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Theres really no easy way to insulate between bays if your not spraying foam on it, but that's expensive.
Most bays "should" be the same dimention. The problem you may have is how to keep the foam board up there before the foam dryed. For my cabin I used a custom foot spacer on my trim nail gun so I could put nails in the bottom of the subfloor that dosnt go in all the way from the bottom. You cant do this with radian tube space needed.
Have you thought about just putting foam board on the bottom of the floor joists? Way less cutting and I assume they make a drop ceiling hanger that can go through 1-2in foam board.
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travellerw
Member
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# Posted: 26 Mar 2025 11:18am
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Quoting: Brettny Have you thought about just putting foam board on the bottom of the floor joists? Way less cutting and I assume they make a drop ceiling hanger that can go through 1-2in foam board.
I thought about that, but not keen losing 4-5 inches of headroom on top of the drop ceiling.
After talking with some people who have done this, they indicated that you don't need to "seal" things. Its not like insulating under an open structure. The goal is to create a dead air pocket with some breathing to provide movement as things heat up and cool down. None of them sealed the edges of the foam with expandable .
I should have mentioned it in the earlier post, but my joists are I-beam. The top runner of the i-beam is thick enough that foam placed against it will make a 2" deep pocket to the subfloor above (hope that makes sense). I now plan on using a little glue and some cleats brad nailed to the i-beam web to hold up the foam.
In a fortuitous coincidence, a pallet of 3" thick X 20" wide rockwool just got listed at a local auction. If I can win it for a decent deal, then I will be using 3/4" foam backed by this. My fingers are crossed as I really want the sound deadening this would provide.
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travellerw
Member
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# Posted: 26 Mar 2025 11:37am
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Ok.. I drew a quick cross section to make things clearer (it helps me visualize it as well).
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Brettny
Member
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# Posted: 26 Mar 2025 03:07pm
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O. just screw foam board on then.
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travellerw
Member
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# Posted: 26 Mar 2025 03:46pm - Edited by: travellerw
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Quoting: Brettny O. just screw foam board on then.
I had an engineer out for some other changes I'm making and he informed me I cannot screw to the runners. Nothing more than a brad nail is allowed in the runners. Building inspectors are SUPER strict about it too.
I can screw all I want to the web though. Thus the use of cleats as I can brad either to the web or up through the foam into the runner.
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Brettny
Member
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# Posted: 26 Mar 2025 06:29pm
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Brad nail into the runner. Push the foam up against it and do 2 dots of spray foam on each side. When its dry it will be stuck there and the nails covered.
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gcrank1
Member
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# Posted: 27 Mar 2025 10:45pm
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You might want to have the brad nails exposed so an inspector doesn't make you pull the foam off to see if you used 'more than brads'.
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