The Official Vapor Barrier Thread

I’ve been reviewing various cellar building threads in preparation for my own project, but I’m still not quite clear on ideal vapor barrier placement. Given this is such an important topic; I thought it might be best to start its own thread.

My project is ‘all go, no show’ – basically a large 2x4 framed box in the garage – so I won’t be dealing with sheetrock or other fancy finishes.

My insulation plan (from inside to exterior):
2” Polyisocyanurate Rigid Foam (R-13) Glued directly to the inside of the studs, foil facing out. >> 2x4 stud wall (2x6 for celling), filled with unfaced batt insulation (R-15) >> Plywood skin on outside of box.

Everything I’ve read says the vapor barrier should be on the warm side of the insulation, so that would seem to dictate that 6 mil plastic would be wrapped around the box before doing the final plywood skin on the outside. However, I’ve seen pictures where the barrier is actually wrapped around the studs, so the studs themselves are on the ‘warm side’ with insulation packed on the cold side. Is this type of vapor barrier placement ideal? Or is wrapping the studs only used when dealing with an existing wall?

Last question, if my construction is Stud Wall >> Barrier >> Plywood, does it make sense to try and offset the ply from the plastic using spacers to give moisture a path to escape?

So you are attaching the foam board on the front (inside of room) of the studs? The foam, if taped properly is a vapor seal. You could put the foam on the outside of the room. It would give you more space inside and have the vapor barrier on the outside. Obviously you can’t do that with existing exterior walls.

Thanks Eric - I had considered that, but getting buy-in from my wife requires installing shelving on the outside for better garage storage. Plus I’m concerned about the longevity of foam exposed on the outside. I have a plan to minimize abuse of exposed foam on the inside.

Or spray the entire joint with closed cell spray foam. Quick overview of 3 barrier options.


You are right you want the vapor barrier on the outside (warm) side of the cellar.

It’s not clear to me where you’re starting though, in terms of studs. Are you building all the walls and ceiling? If so, build the stud walls. Then put the vapor barrier around them on the outside and on the ceiling. Then install ceiling studs (or do that first with vapor barrier over it). Cover outside walls with plywood with plywood.

Then I would put the foam on the outside of the wall - against the plywood, then put in R-15 batts on inside. But I don’t think you want foil faced foam - that’s two vapor barriers, which is a no-no because moisture can get stuck between. Also, if you’re using only 2x4, you’re not going to be able to fit foam board plus R-15 without compressing the R-15 a lot, making it more like R-8. Once that’s all in, put up plywood on the inside as well. Don’t worry about the studs - they won’t transmit cold from the cellar out to the room - low conductivity and the insulation will keep them at a similar temperature gradient as the insulation, which is what you need.

Spray closed cell foam and know with 100% certainty you have a perfectly insulated, vapor barriered cellar. I thought about doing what you’re thinking about but ultimately went with spray foam - I instantly knew I made the right choice when I saw the dried foam.

Mike, one idea might be to use 2x6 studs every four foot or so. That way you could still put your 2" foam on the outside of the 2x4 studs, but between the 2x6s. The 2x6s would then be used as the support posts for your shelves. Foil face to the outside, of course, with a water proof, metallic HVAC tape over the joints. Alternatively use 2" pink or blue closed cell styrofoam from Lowes or Home Depot. This type of foam is it’s own vapor barrier but you would still have to tape the joints. A thin veneer of plywood applied to both sides of the studs should finish the job.

Thanks for the thoughts. The foam on the inside idea came from the company that does the CoolBot AC controller that I’ll be using.

I hadn’t thought about using rigid foam over the interior of the studs, which insulates well and is extremely cost effective with no need for green board. If you look at that guide, a large portion of their market is going to 42 degrees - and I would think a larger temp differential would mean more condensation.

Construction would be:
Build stud walls >> Glue Foam to interior of stud walls >> install unfaced insulation into stud wall cavities from outside >> Vapor barrier >> Plywood.

On the topic of spray foam - I’m looking at about $350 all-in for insulation with the method above. Professional application would be many times that, this DIY solution would be $650+ for R-21 (3" thick):

Last thought: remember I am in Northern California - ambient humidity is not a thing here. I’ve asked this in the past with no real reply… Is there really a massive risk in this part of the world, versus the South/Midwest/NE? All the vapor barrier disaster stories I hear about happen to originate in places which have a ton of summer humidity.

Much less. If the dew point of the surrounding air is less than your cellar temperature you shouldn’t have much to worry about. But if you live in a warm, humid climate, dew points can approach 70 or 75 (at least in non-conditioned space).

I’m still hoping one day to turn my casita into a walk-in cellar…but it’s a finished room (with built in standalone HVAC, so tempting) - how would one re-convert a room? Already has finished walls, insulated, but not standard for wine cellars, just residential homes (what is that standard, anyway?) and certainly no vapor barrier. How would one retrofit existing residential walls with vapor barrier…and sufficient insulation?

Strip the walls - get rid of the existing insulation and go with spray foam. Is the AC a mini-split/ductless? You could use a CoolBot to mod that to go down to cellar temp: (scroll down)

Hmmm… I’m watching videos on this site for DIY spray foam. More money, but less to worry about and more efficient. Plus I can be more creative on dimensions for the space - working with 4x8 sheets is limiting.

It’s not a wall unit - it’s a massive built-in standalone HVAC system, Japanese-style in-roof type.

To strip the walls makes it a LOT less appealing…maybe I could just cut holes in the walls and spray foam in there [snort.gif]

C’mon Todd. Perhaps someone should get you a tool belt for the holidays… [d_training.gif]

Bro, I turned my third car garage into a workshop. I ain’t afraid of working…it’s just tearing apart perfectly good walls only to put them back up - that’s a bit beyond the scope of logical for me…

Thanks for that video. I’m about to start convert a room in my basement and that video is a great way to kick start things.

Does anyone know the typical cost per square foot for spray foam insulation? A friend, who is an architect, expressed concern with moisture getting trapped in spray foam and leading to mold. I’m in Seattle, so we do get our share of moisture.

Brian - depends on the R-value you want. DIY solution here is $750 shipped for 1 inch coverage of 600 sqft - R-7 per inch of thickness. You could probably get away with R-14 in Seattle. In an uninsulated garage in Sonoma, I’m thinking R-21 is better - so $3.75/sqft for me.

Not bad pricing, but I’ve heard bad things about DIY spraying – problems with mixing and application. Still, I’ll give it a deeper read.
My room is 380 sqft and can get a bit warm during the summer (it gets into the 90s these days), so R-21 would be best for my external walls, too.

I hear ya - that is my understanding as well. That system tries to address the issue with the A and B parts being different colors, so the user can visually assess mixture by the final green color. Not perfect, but a step in the right direction. If I did this, I wouldn’t do green board or anything else over the insulation, so accidental over-application leading to the need to shave he foam would be a non-issue.

The closed foam from Lowes or HD will work. I built mine that way. I don’t know why people insist on using fiberglass and sheetrock. You’re not building a bedroom, you’re building a fridge. If you buy a commercial walk-in fridge, it’s closed foam covered with aluminum or sheet metal. The insulation factor is higher and the vapor barrier issue nonexistent. It comes in 2 foot widths, so use that between studs - no need to go 16 inches. And put a thin layer over the inside and outside running horizontally, so you get a perfect seal. It’s tongue and groove, so don’t worry about taping. If you want to sheetrock the outside, do that - use long screws to get into the studs. Or use wonderboard - it’s waterproof.