I am building a small utilitarian wine cellar in the basement of my Mid-Atlantic house and am looking for some help to figure out how I want to insulate it. It is going to be a box 6’ x 6’ with an 8’ tall ceiling and will be actively cooled. I am working with existing 2x4 framing on two walls and will be adding the two other walls. For insulation I was initially planning to use traditional poly vapor barrier (on the outside / warm side wrapped into the studs) with rockwool insulation and then drywall inside the cellar. After demoing the space I am seeing some potential difficulties in installing the poly vapor barrier in the ceiling, namely cross-bracing in the joists and lots of nails coming through the subfloor above that I am worried could penetrate my vapor barrier. I don’t think it is impossible to go this route, but not as straightforward as I initially hoped. As an alternative I am considering using rigid foam insulation (specifically polyiso) installed between the studs and then sealed to the studs on all edges with spray foam. Do I need a vapor barrier on the outside wall of the cellar if I go this route? Would the rigid foam at 3” depth and foamed be enough of a vapor retarder? When researching I am finding different opinions on the need for a vapor barrier when using rigid foam.
If the entire cellar was encapsulated on the outside in full sheets of rigid foam and taped I would not worry in the least. But whether sheets installed between the studs and sealed is adequate is an open question for me. An advantage of the rigid foam route is that I could also get more insulation into my walls.
And I know lots of people will say hire a spray foam contractor and be done with it and I am considering that route, but would rather not because of the cost.
My recommendation is just to use rigid foam on the ceiling, and if you don’t care too much about looks, everywhere. With 8 feet, you should have room for at least 2". I have a lower ceiling in DC and did R-19 fiberglass between ceiling joists and the 2" foam below to get the air seal. I sealed either with canned spray foam, tape, or both depending on gaps. The only penetrations are the coolbot controlled AC unit, the door, and a water line for the humidifier. Mine won’t win any beauty contests, but it works.
Sound like yours would be similar to mine. Mine is: 1 exterior north-facing wall 13" brick covered in 2" rigid foam, a 14" (i think) inch party wall covered over with rigid foam ( I did leave an air gap between the exterior wall and the foam). In the 2 interior walls, I put R13 fiberglass and 2" rigid foam or 4" rigid foam.
Thanks Eric. To make sure I understand you correctly, you installed rigid foam on the interior side of the cellar on top of the framing and taped the sheets together to form a continuous layer of foam? I considered this but was worried this would leave the vapor barrier on the wrong side. But with thick enough foam it probably doesn’t matter - you are still separating the outer warm humid air from the cooler cellar air. And I can see how you could get additional benefit by reducing thermal bridging through the framing.
What about installing the rigid foam between the framing and sealing to the framing with foam. I can’t understand, if done carefully, why it would be any different than spray foaming the whole thing. And why would the former require an additional poly vapor barrier while the latter clearly does not.
I went through the same assessment for my cellar (significantly larger than your project). I finally decided to go with closed cell icynene spray foam 360 degrees. The challenge was ensuring all potential air leakage was isolated (cooling unit, outlets etc.). It was worth the cost for me though a project of your size may be a tough decision.
Over Twenty some years ago I built a passive cellar in NJ, with one cement wall on the cold side of the house, and studs on 3. I used fiberglass batts in the walls, then 1” foam board over the studs (including the ceiling), then sheetrock and finish. Never had temperature or humidity issues. Temps maybe moved seasonally, slowly from mid 50’s to low 60’s. I honestly cannot remember what I did about the facings. Used extra-long sheet rock screws. Solid core door with no window.
I have a 4x8 passive cellar in Virginia. 7’ tall ceiling. Construction is 2x6’s filled with rockwool surrounded by 2" polyiso foil faced insulation, fully taped on all sides. Ceiling joist bays are fully stuffed with rockwool as well. Floor is untreated concrete. North wall of the basement.
Temps go from 54 in the winter to about 68 in the worst of the summer on a nice slow cycle. Humidity ranges between 50% in winter and 75% in the summer.
I framed in an opening for a cooling unit should I ever want one, but right now I’m pretty happy with it passive. It wasn’t worth it to go with better R value (doubling the R value would only get me about 2 degrees cooler in the summer. Based on my calculations if I did get a cooling unit, it would only cost me about $30 a year to run at 55F. Just no reason to spend thousands more on insulation to save a few dollars a year on cooling. But this is for a pretty simple, well-sealed, small space.
I am quite tempted to go the route of polyiso within the stud bays sealed to the bays with foam. I think it should be good enough of a vapor barrier. I don’t understand why a poly sheet barrier would be needed here. If I want to be extra careful I can tape the sheets that are in each bay together on the outside of the stud bays, at least in the walls which I can access to seal things up even more. The way I see it, even closed cell spray foam doesn’t result in a perfectly vapor and air sealed space, there is still infiltration through gaps in framing. But it works well, so 100% perfect air sealing isn’t a necessity. But I may be mistaken in my understanding. For my application, I think rigid foam will be safer and easier than a poly sheet which could be difficult to install properly in my space. And I can appreciate where a bad poly vapor barrier installation leaking humid air into a cool cellar would be a recipe for disaster.
The wood can breathe and let vapor through. Worry is that water will condense on the wood and cause mold and rot. That is, when more humid air, say outside in summer, has a path in. The moisture from the warm humid air has to condense as it cools in the cellar walls. But your setup might be good enough. Maybe just plastic on the outside facing walls. My cellar is foil foam outside the studs. That with the tape is a near perfect vapor barrier.
It’s all tradeoffs. Ideal is 2x12s with spray foam but not many have room or money for that. Some folks just put a cooling unit in a closet and that works too.
I dont think it would be different. The issue is the joists (or an wood with insulation between) will act as a thermal bridge and reduce the overall R value. My feeling there is no real way to avoid condensation on the warm side. The main thing is to ensure that the condensation can be taken away by the air. In my setup, from inside the cellar, it goes:
(1) 2" foam screwed to joists (2) R19 between the joists (3) air gap (4) subfloor for 1st floor. The foam (taped up) is the air/vapor barrier to avoid mass transfer. It helps that my house is old, so my subfloor is planks with substantial spacing between them, not plywood, so you get more airflow. The air gap mentioned in (3) has unobstructed air flow to the basement so (cross fingers) I wont have any rot in the joists under my living room.
@Ken Leiter, I assume you have determined your floor doesn’t need to be insulated? If the floor is concrete on dirt, then in the summer, if earth temp is lower than desired cellar temp, it will act as a heat sink to keep your cellar cool. On the other hand, if uninsulated and given that concrete has virtually no insulation value, if earth temp is greater than cellar temp. it will allow heat to enter the cellar at an alarmingly high rate!
It is concrete on dirt. I have read on other threads that since it is well below ground, it should not be too big of a deal in my climate. There is a chart of ground temperatures for different locations and its variation over the year in an old thread. I am considering using Wedi panels as a tile backer for the flooring which has some r-value and could help with mediating some of the heat transfer.
Cutting sheets of rigid foam to fit between wood framing is called “cut and cobble” and it works but is also a PITA. It also does nothing to stop the thermal bridging through the framing. While closed cell spray foam (CCSF) does an excellent job of stopping vapor/air transmission and has a higher r-value than batt insulation, it too does not address the thermal bridging so it usually is not cost effective when it comes to insulating walls. The best place to use CCSF in is the rim joist or the rafters of cathedral ceilings.
For my current cellar, I decided to build a room that had the air, vapor and thermal control layers on the inside. I used XPS on the wood frame walls and EPS on the exterior concrete walls (InsoFast). Foil faced polyiso would have been better than the XPS because of the higher r-value and lower impact on the environment, but there was a supply issue with iso at the time. The stud bays are filled with Roxul. My ceiling has 12" of dense pack cellulose and a vapor control membrane that allows moisture to travel into the room at a rate of 4 perms, but only out of the room at a rate of 1 perm. Since my first condensing surface is the subfloor above and cellulose does a god job of absorbing moisture in times of high humidity, thereby protecting the subfloor, I was comfortable using this design. If you don’t use CCSF in the ceiling, you could use Roxul batts and cover it with poly or a smart membrane like Membraine. I built a grid in the ceiling for all the electrical so that I did not have to penetrate my control layers. All of the seams are taped with a vapor closed tape. Since this was new construction, I also added 4" of EPS below the slab. I used quite a bit of Wedi Board in the showers throughout the house and, while it is expensive, for a 6x6 room, it won’t add that much if you want to insulate your slab a bit. Superior is another manufacturer of these boards and they are cheaper but perform just as well. In my area I can get both at The Tile Shop. The drywall is attached to 2x3 furring strips that are secured to the studs with 4" FastenMaster HeadLok screws. I used Vintage View 3 deep metal racks and I wanted to attach them directly to wood, so that is why I added the furring strips instead of just attaching the drywall to the studs with long screws.
While we all trying to minimize heat transfer when building a cellar, we also need to avoid creating a path for condensation. I built my first cellar using the poly on the warm side design. It worked great while I kept the cellar cold. However, I stopped using it after I built a bigger cellar next to it (this time with CCSF). About a year later I walked into the old cellar and got hit with a whiff of mold. What I think happened was in the winter time the now warm and moist air in the cellar was hitting the cold poly that I placed on the face of the studs closest to the exterior walls and condensation was forming. In the summertime, that face of the stud was the warm side, but in the wintertime it became the cold side and the temperature differential became big enough to allow condensation. I had to demo the entire cellar which was not fun.
When I insulated my cellar 20 years ago (it’s actually part of a former garage inside the house), I used rigid polyurethane foam which I stuck to the walls and ceilings. The racking is freestanding to avoid any thermal bridges. The door is like that of a fridge and there is a Fondis Winemaster cooling unit which has insulated pipes going through the walls, outside. I have no condensation problems of any kind and the whole thing works well at a minimum cost - as I discovered recently when the cooler broke down, apart perhaps from high summer, I could easily get by without the unit.
In order to make the foam look less ugly, I covered it with a sort of thick plaster, which I then painted, so it looks a bit like stone. If anyone is interested I’ll post a photo. I’m really happy with the whole thing.
I’d insulate between the studs with bat insulation the best you can, apply poly over the studs, then run 2" furring studs perpendicular to the wall studs. Rip/cut 2" rigid foam to fit snuggly between the horizontal boards. You can then attach sheetrock or some other sheeting to the horizontals. I did something similar to this in a room I had in my basement. I used 3/4 T&G clear cedar instead of sheetrock. Left is natural, uncoated. Same stuff I installed in a sauna.
100% agree with you. The only way to reduce the thermal bridging is to install the foam in a continuous layer over top of the studs. And it is also true that CCSF sprayed between the studs does not reduce thermal bridging.
Cut and cobble will be a PITA, but I have a small space so I think it is doable. I considered adding a continuous layer of foam on the inside wall of the cellar to reduce the thermal bridging but am worried about attaching drywall through it. I also considered other wall finishes.
One thing I don’t 100% understand is why CCSF in cellars eliminates the path for condensation to form. Is it the fact that the room is now sufficiently air and vapor tight that there is no ability for warm air to infiltrate and condense on a cold surface? I would think I can achieve similar tightness with cut and cobble, but maybe I am overconfident and foolish. I don’t see why an additional poly barrier would be needed in the cut and cobble case, assuming I do a good job sealing edges to framing. Especially if I can tape the rigid foam boards together, across the studs, using 4” tape on the outside walls (not possible in the ceiling). And I think I am likely to do a better job with the rigid foam than I would installing a poly vapor barrier in my space.
I would not be concerned about placing the drywall directly on top of the foam, and you will get a much bigger bang for you insulation buck by reducing the thermal bridging. The foam will not compress and the drywall is not going to fall down. You want to use long enough screws to get 1" embedment into the studs.
CCSF is vapor closed, it has the ability to get into every nook and cranny so it does and excellent job of air sealing, and it has a high r-value. Those 3 things combined make it very unlikely that warm, moist air will ever come in contact with a cold condensing surface on the cellar side of the insulation. You can get there will cut n cobble, but I think it is much more work than attaching rigid sheet to the stud face. If you are packing the stud bays with Roxul, than you should be fine with 1" rigid foam. You can then attach virtually any kind of finished paneling that you want directly over the foam, such as T&G, using 3" finish nails.
Attached is picture of my previous cellar. This one had T&G cypress which you could use in the scenario with the 1" foam attached to the studs.
The photo isn’t great, but it’ll give you an idea. The rigid foam is 10 cm thick and yes, it made a sort of large fridge, 4m x 5m and 2.20m high, holding around 4 000 bottles. Putting the rough plaster on top, painted sand colour, made it look OK although the cellar is meant to be functional rather than aesthetic. It wasn’t hard to do and cost around 5 000 euros, including the freestanding cooling unit (which is in the photo). The photo just shows part of the cellar, the rest being on the left.
If I may ask, did you put rigid foam board over the existing walls? Then plastered over it or did you play another layer of drywall over the foam board? Was the rigid foam board acting as the vapor barrier?
The rigid foam boards are interlocking. I just placed them next to the walls, leaving a very small space between the wall and the boards. On the ceiling, I used the wall-side boards as support and stuck them in the middle of the ceiling using a very strong type of glue. There is no vapor barrier but I suppose the boards do act as one. I have no condensation at all, but the room is naturally cold already and thanks to the cooling unit, the air is constantly circulating.
In fact I was talking rubbish about the plastering! I remembered last night that in fact, all I did was paint over the foam boards with a very thick masonry paint, which is designed to cover over cracks in walls (so it contains large bits of sand). It was very easy and gives the vague impression of sandstone, which is what I wanted.
I’m always in awe of the tremendous work done by others to create such beautiful cellars - mine is just a very cost-effective and efficient storage space, with the cellaring conditions I wanted. 20 years on, it all still works fine and I can highly recommend the Fondis cooling unit.
