Vapor barrier in tricky basement new construction

[ChuckD]

@Il Gattopardo
I’m curious, Is this a purely functional cellar or a fancy finished space? I’m looking at a more functional one myself in my new additions basement.

And i forgot my manners… welcome to WMT.

 

[Jovimaple]

Welcome to WMT!

 

[JTS84]

I live in the Midwest. I used to use vapor barriers, then ended up replacing some when I worked in construction due to the issues they have. I now will only use "smart membranes" such as Certainteed Membrain for an example.

https://buildingscience.com/documents/insights/bsi-010-wine-cellars
Here is a link to a free resource on building science for anyone. I think it is a fun read (I'm a mechanical engineer and have met the author) that contains great info on building science regarding vapor transmission. Can't remember if I have posted this link on this forum before or not.

 

[Il Gattopardo]

@Il Gattopardo
I’m curious, Is this a purely functional cellar or a fancy finished space? I’m looking at a more functional one myself in my new additions basement.

And i forgot my manners… welcome to WMT.

Functional! At least meaning it is a clean finished place to age wine and store what we will drink along the way.

 

[p_j_goetz]

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.

Greetings Il Gattopard:

For what it is worth:

You are correct that the vapor retarder is to be placed between the warmer humid air (higher vapor pressure) and the cooler environment (lower vapor pressure); that does mean that the vapor retarder is to be on the warm side of the insulation. If the plumbing and electrical are run such that you have 6" between the bottom of the joist (the cellar ceiling) and the penetrations my recommendation would be to install nailers between the joists to allow the installation of foil faced rigid insulation (Rmax is R6 per inch and the foil is a vapor barrier) between the joists with the foil facing up (toward the floor above). The tighter the butt joint the better. The joints between the joist and the rigid insulation can then be filled with expanding foam. Installing the pieces of insulation so the joints are staggered is recommended. As you are aware the hard part is the making sure the vapor barrier is continuous between walls and ceiling. When in doubt fill all voids with foam, that includes the channel space created at the corners where three studs are ganged together. Typical open cell foam is not a good vapor retarder unless you can get 4" of it installed. Closed cell foam at 1-1/2" is an air and vapor retarder.

If everything is tightly sealed you will not have to worry about the interstitial space between the upper floor and the foam boards as that space will be close to the same temp and humidity as that of the space above the vapor retarder provided there is not a vapor barrier on the bottom side of the rough floor above the wine room. If there is a vapor barrier in both locations then moisture that leaks into that area can be a problem as it never diffuses fast enough to prevent a problem in the long term.

To answer your question "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", yes, if you can make sure that all the joints are tight and sealed.
If the MEP is already in place you may end up with a lot of small pieces of foam board that will require lots of foam.

 

[ChuckD]

@p_j_goetz
Thanks for the explanation. My concern has always been that a wine cellar is indeed cooler (57F vs 70F) but much more humid than the house (70% vs 6%). with your vapor barrier on the warm side you allow that humid air into the insulation… will that not be a problem?

 

[p_j_goetz]

@p_j_goetz
Thanks for the explanation. My concern has always been that a wine cellar is indeed cooler (57F vs 70F) but much more humid than the house (70% vs 6%). with your vapor barrier on the warm side you allow that humid air into the insulation… will that not be a problem?

Chuck;
The keys are the direction the vapor travels and whether there is a condensing surface that is colder than the dew point in the path of travel. The second law of thermo-dynamics basically states that things move from more to less, from hot to cold, and from wet to dry ( learned this from also going to lectures by the Joe Lstriburek, the author of the link that was posted by JTS84. And since JTS84n shared he is a mechanical engineer he can probably explain this better than I can/did).
So the amount of moisture in the air in a cold room with high humidity is not typically greater than the amount of moisture in an interior habitable room, so the flow is into the cold room not into the warm room. The vapor retarder in the assembly is to stop the transfer/diffusion of moisture through the wall/ceiling components so that it cannot reach and therefore not condense on a colder surface in the wine/cold room. That interior cold surface would typically be the face of the drywall within the ceiling cavity. If the moisture in the air in the wine room was high enough to be a problem there would be mold beginning to form on the wine labels.

And this works provided that there are no holes in the assembly that allow for bulk air/moisture to move into the insulation packed ceiling from the warm side of the wine room. How could warm are leak into the insulated cavity? Heat ducts, fart fans, etc., if they are within the cavity and have leaky joints can dump lots and lots of air into a ceiling.

Hope this helps.

 

[Il Gattopardo]

@p_j_goetz, this does indeed help. Thanks for the detailed info. I suspect you've helped many more than just me here.

Our various penetrations through the joists are less than 6" from the bottom. I'm more confident I can get two layers of rigid foam up tight against the subfloor (no vapor barrier there).

I'm going to get a bid for spray foaming both the underside of the subfloor and the blocking/rim joist that goes around the walls. If as outrageous as I fear, then I'll do the foam boards and be diligent about wrapping a barrier around the walls from the outside (leftover heavy plastic from our slab foundation barrier).

I'm actually glad our space is small now!

 

[ChuckD]

Chuck;
The keys are the direction the vapor travels and whether there is a condensing surface that is colder than the dew point in the path of travel. The second law of thermo-dynamics basically states that things move from more to less, from hot to cold, and from wet to dry ( learned this from also going to lectures by the Joe Lstriburek, the author of the link that was posted by JTS84. And since JTS84n shared he is a mechanical engineer he can probably explain this better than I can/did).
So the amount of moisture in the air in a cold room with high humidity is not typically greater than the amount of moisture in an interior habitable room, so the flow is into the cold room not into the warm room. The vapor retarder in the assembly is to stop the transfer/diffusion of moisture through the wall/ceiling components so that it cannot reach and therefore not condense on a colder surface in the wine/cold room. That interior cold surface would typically be the face of the drywall within the ceiling cavity. If the moisture in the air in the wine room was high enough to be a problem there would be mold beginning to form on the wine labels.

And this works provided that there are no holes in the assembly that allow for bulk air/moisture to move into the insulation packed ceiling from the warm side of the wine room. How could warm are leak into the insulated cavity? Heat ducts, fart fans, etc., if they are within the cavity and have leaky joints can dump lots and lots of air into a ceiling.

Hope this helps.

Thanks. This makes sense. I’m in the process of building an addition and the basement space is for wine and food processing. I plan on having a processing area with commercial sink and a corner room for bulk aging and wine storage. I was wondering if leaving the two concrete walls of the cellar bare with a well insulated ceiling and walls would be best. They are insulated on the outside with 1.5” poly but I was hoping they would help moderate temp and humidity. I was hoping to forego the cellar AC unit.

 

[JTS84]

P_j_goetz explained it well. The cellar will be the cool, so you want to keep any warm humid air from moving into that space and contacting the cool backside of you finish. Make sure that you seal any holes from electrical, HVAC, plumbing, etc. That is key to keeping air out of that space.

In mine, I have two layers of 2" XPS with seams tarred against the bottom of the floor joists. (Done by previous owner) I have had to access it once for a plumbing issue and found no issues inside there. I'm now thinking of turning that room into a fermentation chamber and making a new storage area somewhere else....

At work our original coolers were insulated with layers of cork tarred together. Knowing that the previous owner of my house worked for the same agency, it doesn't surprise me he used tar instead of spay foam.

 

[wood1954]

Thanks. This makes sense. I’m in the process of building an addition and the basement space is for wine and food processing. I plan on having a processing area with commercial sink and a corner room for bulk aging and wine storage. I was wondering if leaving the two concrete walls of the cellar bare with a well insulated ceiling and walls would be best. They are insulated on the outside with 1.5” poly but I was hoping they would help moderate temp and humidity. I was hoping to forego the cellar AC unit.

With the insulation and the earth the outside walls will stay pretty cool. The floor will also be a big heat sink. My house in Appleton was like that, it stayed pretty cool, so much so that I put a fireplace in the basement.

 

[Rice_Guy]

Functional! At least meaning it is a clean finished place to age wine and store what we will drink along the way.

Hungary was a learning event. A traditional cellar was considered non-functional if it had not developed black mold. ,,, In the scheme of things mold requires humidity to grow. Having the black mold was a low technology way to measure humidity.