Wanted to tap into the expertise of others as I am planning a small passive cellar build (in Chicago area).
I’ve read through Gold’s book and searched past posts on the board but still find myself questioning a few things.
Background: space will be 6’ wide by 9’ deep. Basement floor to bottom of joists is 104”. Two of the four walls will be exterior along foundation; one wall up against an already finished interior basement wall; last one is interior to an unfinished part of basement.
Questions:
Exterior walls: was planning a plastic vapor barrier against foundation wall from bottom of joists to floor then fiberglass in each stud cavity. Does faced or unfaced matter? Would closed cell spray foam + fiberglass be a better alternative than plastic + fiberglass?
Inside wall shared with finished basement: not easy/practical to put plastic in against finished basement drywall since it is already finished. What is my best vapor barrier/insulation option? Faced fiberglass with facing away from cellar? Spray foam with fiber to fill in stud cavities?
Ceiling: was planning an 8’ ceiling to simplify hanging the drywall. If I spray the joist cavities and then put fiberglass in them, is it ok to have an air space between bottom of joists and drywall ceiling?
My cellar is 7’3", would love 104". For 8’ run the rock horizontal, how the pros do it. A dropped acoustic panel ceiling allows access for wiring, vents, plumbing, etc. I furred the exterior for insulation 2x4, 2x6 for the interior walls and insulation board under the slab, passive for me and stays between 65f summer & 70f wither with AC in the rest of lower level living space.
I have a similar question on insulation. On three sides I’m using closed cell foam but on one side I’m looking at using fiberglass. Planning on using faced vs not. Does it still need a poly vapor barrier? My contractor is telling me the paper face acts as a vapor barrier.
Yes the ‘paper face’ act as a barrier, just red tape it first.
Do not let the studs of the cellar touch the foundation you need to let a 1/4 inch of space.
While we are at it, why don’t you guys just spray polyurethane? It has the best R-factor of all by inch of depth follow by the foamular pannel. That said I bet you wont need to insulate all the way down to the floor if you have a 10feet deep basement as the soil never freezes that deep in your neck of the wood.
I’ve never heard of the paper backing being used as the vapor barrier, and even if it is rated as such, how are you going to tape the seams and keep the VB on the outside of the wall where it belongs (unfaced part of the insulation facing the inside of the cellar)? I do know that, as mentioned here, it can cause problems if you have both the faced fiberglass and poly, on opposite sides, so clearly it works as a vapor retarder of some sort, but that’s not necessarily the same thing (you want a very low permeability for your vapor barrier). From a quick search the backing on fiberglass batts is not a vapor barrier (i.e., a Class I retarder).
You might consider reviewing the below thread and maybe contacting some of the people in it for a second opinion. I did that midway through my cellar construction and realized that it was going very wrong. Chris K gave me a lot of good advice and comfort when I wasn’t sure of what my contractor was telling me, and while it cost me a bit more money I saved a lot of problems and worry down the road.
The thing I realized towards the end of my construction and after a lot of fretting is that a lot of cellars, maybe most, are built sub-par. And a lot of those sub-par cellars don’t have issues that materialize during the time frame of the owner being there. Some climates and environments are more or less forgiving than others. I’ve seen cellars in dry climates with single-pane glass walls and 1/8" air gaps that operated effectively for a long time. But if and when problems happen they’re expensive and a pain to fix, so you’re best off just doing things right the first time versus cutting corners and hoping for the best.
okay i’m probably gonna get flamed for this but here goes: Passive cellar means means no air conditioning right?
why is even insulation needed?? this is a Chicago basement. fully undergound. What is maximum cold in the winter? 55 degrees?
vapor barrier? How much temperature difference do you think there will be on a PASSIVE cellar between the concrete wall and a heat conditioned finished basement area? 10 degrees maybe? And even besides that we are talking extremely small delta changes in temp between the most cold and the most warm, over a 6 month period.
In this passive situation, I think it’s because people want to leverage the fact the the cellar is well below ground and they want the floor temperature to essentially dictate the cellar temp. My Chicago basement will sit at around 67 otherwise, with a few degree differential floor-to-celing and depending on how I set my thermostat. With insulation in my cellar it ranges from 56-64 or so during a year, and with temp control I keep it at 56-58. You insulate the walls and ceiling and depend on the thermal mass and lower temperature in the ground. The vapor barrier comes into play if you have a significant enough delta between the cellar and house temps. But why build a cellar in the first place if you’re just looking for storage versus temp control?
56 degrees at the floor? okay that’s impressive… but how much of that actually makes it up 5 or 6 feet higher? How much of that temp of the slab gets into the air? Okay back to the vapor… so if target is 50s in the cold room. and maximum 70 in the warm conditioned next room? still need a vapor barrier? that’s gotta be overkill. (but alas we are wine people and do overkill i’m guilty of that).
I turn my cooling unit off in the high winter, and the three thermometers I have in the cellar (ranging from floor temp to eye-level) all still read no more than 58ish. If you have good insulation, the ground temp should essentially fully determine the cellar temp.
If you really want to do vapor barrier calculations I’ll refer you to http://www.dpcalc.org.
It’s a fairly permeable vapor barrier. You’re better off using poly plus unfaced, or of course spray foam. (BTW, I believe the term “barrier” is outdated - it’s a question of permeability and just degrees of it, but the facing is fairly permeable).
Kept the full ceiling height (104"). Ended up spray-foaming all walls and ceiling. Used ceramic tile on floor with no insulation - hoping for the temp to get down to 58-60 as colder weather is finally arriving. But, it’s dropping slowly given all the thermal mass of wine which was in high 60s when I moved it in.
A few pics below. Thanks to all those past and present who have posted on cellar threads!
Thank you for the insight. What exactly is red tape and how do you apply it?
Thanks!
it’s a variant of the tucktape ( normally red but can be blue also) VAPOUR BARRIER SHEATHING TAPE is the technical name . You simply tape the junction of your insulation foam (thta has a Vapor sheet on it) to seal the vapor barrier
Looks great, congratulations Chad! If it’s all below grade in Chicago your floor should make a nice heat sink. In the meantime high 60s is nothing to worry about.
