Insulating Crawl under heated floor

[NWOhioChevyGuy]

In my build I have two rooms that are living space along the house. A 12x12 bathroom and a 14x12 mud room/entry.

Mudroom will be heated with forced air from existing furnace.

Bathroom will be heated with electric floor heat.

These rooms are over a crawl that has a dirt / sand floor. How would you insulate the floor? Simple unfaced fiberglass? Rockwool?

https://www.menards.com/main/building-materials/insulation/insulation-rolls-batts/johns-manville-tempcontrol-reg-r-23-mineral-wool-insulation-batt-15-25-x-47-24-9-sq-ft/90021182/p-1568096959764-c-5780.htm?tid=-8864047586184085383&ipos=1

Once plumbing and insulation is complete the crawl floor will get a vapor barrier laid down.

 

[NWOhioChevyGuy]

FYI: Crawl space walls have 2" rigid foam boards installed already & is an unvented area.

 

[billconner]

Crawl spaces should be conditioned and a part of the living space. Insulation on the walls and a robust vapor barrier on the "floor" like Stego Crawl. Any vents or opening to the outside should be sealed. I think trying to insulate the floor over a crawl space will inevitably lead to moisture problems.

 

[NWOhioChevyGuy]

Crawl spaces should be conditioned and a part of the living space. Insulation on the walls and a robust vapor barrier on the "floor" like Stego Crawl. Any vents or opening to the outside should be sealed. I think trying to insulate the floor over a crawl space will inevitably lead to moisture problems.

1860's brick farmhouse, completely sealing the "Michigan basement" is not really feasible. Plus that would add around 1800 square feet of conditioned space that would require a larger furnace and ductwork upgrades. Looked into that and way outside budget.

 

[jack stand]

I'm with Bill as for a moisture concern. Maybe others might know, but I'm wondering (if it's not too late) if 2" of foam and a serious plastic barrier under a crude "rat slab" would fight off any moisture issues?

 

[NWOhioChevyGuy]

Dehumidifier is run in the basement year round to combat any moisture.
Crawl has a full footer foundation and 48" poured walls w/ 2" rigid foam from footer to floor joist.

I can honestly say this crawl will be the driest part of the basement.

I understand the concerns, but there is not really an easy way (or in budget) to condition the whole basement.

 

[billconner]

If I had to do this, probably rigid foam attached to underside of floor joists. At least 2", 3 or 4 if possible. Fill seams with foam-in-a-can.

 

[NWOhioChevyGuy]

If I had to do this, probably rigid foam attached to underside of floor joists. At least 2", 3 or 4 if possible. Fill seams with foam-in-a-can.

The access is not large enough to get sheets of rigid into the crawl space.
Foundation walls were done before framing.

Plus all I have read, you want anything you have to breath to the unheated space. Rigid would be more likely to trap moisture, would it not?

 

[mustangfan]

There are a couple options.
1. Staple up or unfaced insulation and add a housewrap to the bottom of the joists. This will help seal the floor and insulate it from the cold crawl space.

2. Spray foam the floor from the crawl space area. This will seal the floor from all moister/air penetration. It is just expensive and not that easy.

I have a 1850's farm house like yours with the kitchen/dining/living room over a crawl space. That crawl space is not damp! It is usually to dry!
What I did was run water tubes for heated floor over the existing floor. I have the kitchen floor insulated from the bottom, but the living/dining room is not insluated. The floor is still warm because of the infloor heat.
Before I put the infloor heat, the dining/living room floor would read 50' with 72' air temp in the room.
Now you can lay on the floor and it feels nice.

 

[PoorUB]

I think the safest would be unfaced fiberglass bats, just because of the potential moisture problem. Let it all breathe.

Heat rises so not much will be going down any way, and the little that does go down will help temper the crawl a bit.

I wonder how much difference it will make between uninsulated versus insulated? You say the crawl is tight, so no air movement, plus the walls are insulated.

If the floor joists are some "normal" distance apart you can just used insulation supports. Basically long wires you jamb between the joists.

https://www.strongtie.com/miscellaneousconnectors_woodconnectors/is_support/p/is

 

[Mzungu]

Air seal the subfloor and rim joists, you can even picture frame the cavities between the joist with polyurethane or canned spray foam. Install unfaced fiberglass batts or rockwool insulation in the cavities. Make sure the insulation is tight to the subfloor, you don't want any gaps. Use the insulation supports as previously suggested. I would go with at least a minimum of r22

 

[NWOhioChevyGuy]

Unfaced R23 Rockwool is now the plan with the wire supports, "tight" to the bottom of the floor.
Joist are 2x10's 16 on center, so should be pretty straight forward.
I will insert pieces of the rockwool around the rim joist to insulate that cavity fully.

Then the crawlspace floor will see a heavy vapor barrier.