Vapor barrier in tricky basement new construction

[Il Gattopardo]

Greetings, all.

We're building a new house and have space dedicated for a storage cellar. It's under a conventionally-framed floor, with 14" deep joists. I know we need to put a vapor barrier on the warm side, so that means along the underside of the floor above. How the heck do you do that with joists coming down, and wires/fire sprinkler pipes running between them? I'm wondering if I can use R6 insulation board up against the subfloor and foam the edges to ensure a tight fit (then insulate the rest with R30 batts and seal up any possible air leaks. But the joists would still be exposed rather than wrapped with the plastic.

Thanks for any toughts on this.

 

[Rice_Guy]

welcome to Wine Making Talk

* you are saying vapor barrier. Is vapor important or insulation? On a cold AC surface I would be tempted to do both with one product and use expander polystyrene or urethane with a foil skin.
* your climate is different. (than Midwest) I would be applying a vapor barrier on the sub-living room floor and walls. Again Midwest we have lots of examples of 1900 structures where the joists have survived on a crawl space or a porous cream city brick. Modern standards, after 1950 had plastics so could have good vapor barriers. What building life are you designing for?
* a 14 inch joist is unusual. 12” is a common size. 12 also is available in treated. What kind of load is it designed for? Running LVL or steel I beams is more typical in high load situations.
* modern electrical is frequently buried in insulation. Water could be but in SanDiego could be in a crawl space.
* R30 wow, that is a lot. What are the building code standards?

 

[FlamingoEmporium]

Spray foam.

 

[Il Gattopardo]

welcome to Wine Making Talk

* you are saying vapor barrier. Is vapor important or insulation? On a cold AC surface I would be tempted to do both with one product and use expander polystyrene or urethane with a foil skin.
* your climate is different. (than Midwest) I would be applying a vapor barrier on the sub-living room floor and walls. Again Midwest we have lots of examples of 1900 structures where the joists have survived on a crawl space or a porous cream city brick. Modern standards, after 1950 had plastics so could have good vapor barriers. What building life are you designing for?
* a 14 inch joist is unusual. 12” is a common size. 12 also is available in treated. What kind of load is it designed for? Running LVL or steel I beams is more typical in high load situations.
* modern electrical is frequently buried in insulation. Water could be but in SanDiego could be in a crawl space.
* R30 wow, that is a lot. What are the building code standards?

Thanks for the reply.
- Vapor barrier and insulation. Foam board is a possibility, probably easier than stapling up plastic and then sealing all the seams with tape.
- Correct, 14" joists. They're common here, Boise Cascade I-joists. And there's plenty of LVL and steel holding those joists!
- R30...code doesn't call for any insulation there. But the wine cellar cooler dealers call for as much insulation as you can get.

@Flamingo, I'd love spray foam but it's crazy expensive. A small job like this would still be $2k and weeks to schedule We don't have time.

 

[ChuckD]

You don’t have to fill the void with foam… just a thin (couple inches) coat for the moisture seal. Then you can insulate with fiberglass bats.

I think you can rent the spray gun and buy the foam.

 

[Rice_Guy]

- Vapor barrier and insulation. Foam board is a possibility, probably easier than stapling up plastic and then sealing all the seams with tape.

For R30 and to make it look pretty as a finished room I would vision filling the 13.5” cavity with EPS (it is mold and moisture resistant) then corrugated steel or FRP panels as a ceiling (dairy board is a wash down grade surface with good resistance to cleaners). Fasteners should be moisture tight as neoprene roofing screws. A wine cellar presents as much moisture risk as condensation from an air conditioned zone. While visioning sloped floors with a drain or sump.

 

[ChuckD]

Do you need the full ceiling height? Could you could put vapor barrier on the bottom of the joists then insulation and a lower ceiling in the cellar? No worry about moisture if you have a ventilated air space above the cellar.

 

[Il Gattopardo]

You don’t have to fill the void with foam… just a thin (couple inches) coat for the moisture seal. Then you can insulate with fiberglass bats.

I think you can rent the spray gun and buy the foam.

You can't rent them here, but you can buy kits that get shipped. I worry about the mess and the chemicals.

I'm realizing, too, that I'll have to install a lot of blocking between the joists where they run through into the garage. And that, too, will need a vapor barrier. At least I have easy access to the outside of the walls, for plastic wrap and Tyvek tape to seal.

 

[vinny]

You don’t have to fill the void with foam… just a thin (couple inches) coat for the moisture seal. Then you can insulate with fiberglass bats.

I think you can rent the spray gun and buy the foam.

You can get kits in Canada that have the gun and 2 propane style tanks with the foam in them. I was thinking i would either spray foam the bottom of the floor, but why not insulate the floor with batts, seal off the bottom of the joists and then use foam board to cover that. Should stop any condensing from the cold side and as chuck just mentioned in his last post the floor should have enough ventilation to not be a concern.

 

[Il Gattopardo]

I do need it. It's already about 7-10". I'd be barely 7-2" after dropping the lid and insulating enough.

 

[Il Gattopardo]

You just raised a good point....the floor. We're on a 5" basement slab. I may have to seal that, too?

 

[ChuckD]

When they insulate the inside of concrete basement walls in cold climates you seal the wall and use poly board with sealed seams then studs with regular insulation and no vapor barrier on that inside wall. You don’t want to trap moisture in the walls. You allow it to migrate into the basement and use a dehumidifier.

You need to do this in reverse. Like @vinny said, loose batt insulation between the joists then vapor seal the ceiling of the cellar and allow the minimal moisture loss to migrate through the floor.

 

[Il Gattopardo]

I may be confused. We have all 4 walls framed, no concrete walls (but a concrete floor). The vapor barrier should go outside all of the insulation, on the "warm" side. So that's outside the walls. And for the ceiling, up against the subfloor above, either only sealing off that subfloor or, as I have seen some pics on cellar sites, using plastic to cover even the joists, weaving up and down them.

 

[Il Gattopardo]

And....it all needs to be essentially an airtight capsule, otherwise the cooling unit will be constantly dehumidifying, and generating buckets of water.

 

[ChuckD]

I may be confused. We have all 4 walls framed, no concrete walls (but a concrete floor). The vapor barrier should go outside all of the insulation, on the "warm" side. So that's outside the walls. And for the ceiling, up against the subfloor above, either only sealing off that subfloor or, as I have seen some pics on cellar sites, using plastic to cover even the joists, weaving up and down them.

I was thinking basement=concrete walls. And with concrete in cold climates it’s more of a damp side/dry side issue. You minimize moisture intrusion and dehumidify the living space.

 

[vinny]

I may be confused. We have all 4 walls framed, no concrete walls (but a concrete floor). The vapor barrier should go outside all of the insulation, on the "warm" side. So that's outside the walls. And for the ceiling, up against the subfloor above, either only sealing off that subfloor or, as I have seen some pics on cellar sites, using plastic to cover even the joists, weaving up and down them.

In my house the walls are siding, plywood, 2x6 batt insulated, vapor barrier, drywall. The crawl space is a gravel floor with vapor barrier. The bottom side of the house is breathable membrane (poly board), 2x12, 2 layers of batt insulation. No vapor barrier to the floor. The intent is to keep the crawl space ventilated enough to keep moisture at relative humidity and the floor space can breath.

In your case you want to make the cellar moist, so you need to leave your floor space breathable to the outside of the cellar walls and seal off the moisture from entering that area within the room. The two options are spray foam on the upper floor and seal the floor to the outside walls, or cut off the moisture from the floor area with the batts, barrier, and foam board would be an additional barrier to condensation.

This is leaving your floor space ventilated, thus eliminating the warm side.

 

[Il Gattopardo]

Gottcha. You know, some of this consider San Diego to be a cold climate!

 

[Il Gattopardo]

Vinny, can you explain what you mean by this part? "you need to leave your floor space breathable to the outside of the cellar walls"

 

[vinny]

I agree with Chuck. This is a wet side/dry side thought process. If you seal off the outside walls to a moist room the moisture within the cellar will rot your wall, you want vapor barrier in the cold room and your walls breathing to the outside. This is the thought process with vapor barriers in the wall. If you seal the outside wall it can't breath so the seal is between the drywall and outside environment and your walls breath to the outside. If you seal the outside your walls can't breath to expel the moisture and then rot, mold, etc.

 

[vinny]

Vinny, can you explain what you mean by this part? "you need to leave your floor space breathable to the outside of the cellar walls"

Frame up to the joists, but don't seal the space between the joists leaving airflow between the joists to the outside of the walls of the cellar. Then seal the moisture into you room with poly under the joists. Your floor can't rot because the space above the poly is breathing.

The other option is seal the joist space at the outside of your walls making the floor space part of the cold room, but then you need to protect the joists and floor from the moisture as it is not a breathable space, you need to spray foam the joists and ceiling to protect from moisture.