Unvented Crawlspace Insulation and Vapor Barrier
Building somewhat of a “pretty good house” and I have an unusually tall crawl space due to the slope of my property. It is 6ft tall on one end and about 4ft tall on the other. Because of this, I’ve decided to convert the previously vented crawl space to an unvented and conditioned “basement”.
Property is located in climate zone 3C, sunny southern California and I have had no problems with water intrusion into the crawl space, even with historically heavy rains these past few months. It is clean and dry.
My plan is to pour a 2″ concrete slab in the crawl in order to use it as a nice clean storage space. I’d like to confirm the best way to go about insulating the area and what issues I should look out for. I happen to have more Rockwool Comfortboard 80 than I know what to do with so I intend to use it as exterior insulation around the entire house, sub slab insulation in the crawl space, as well as potentially using it on the interior foundation walls if it makes sense.
Main questions:
1. Would the correct order of a crawl space concrete slab assembly be: Dirt > Comfortboard 80 > Vapor Barrier > Concrete Slab ?
Or should the vapor barrier go underneath the insulation?
2. Are there any potential problems with using the mineral wool insulation on both the interior and exterior of the foundation? Should I just put more insulation on the exterior of the foundation and leave the interior foundation walls bare?
3. Is a water proof coating on the exterior or interior foundation wall necessary, given the relatively dry climate and nice temperatures of Los Angeles?
4. Will my hair turn grey before I finish my remodel?
Thank you for any advice you can offer.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
1) You have the order correct, vapor barrier goes over the insulation otherwise the insulation could float to the top of the wet concrete mix. I am not sure if ComfortBoard is rated for underground use, but you can confirm.
2) There is no perfect amount to divide exterior and interior insulation, some people avoid exterior insulation under grade (like Jake Bruton) because they claim critters will eat away and ruin the integrity of the insulation. My counter point to that is this is your only chance to install exterior insulation so go ahead and do it while you can cheaply, and then you can always add interior insulation at a future date if needed. But you won’t have another good chance to install exterior insulation.
3) A good vapor barrier of some kind should be included whenever you are below grade. I know you haven’t had problems yet but you should design for worse case scenarios. On the exterior you can use a cheaper asphaltic-based waterproofing product and so that is usually what is recommended. Interior waterproofing products are usually reserved for remodels and they will be facing “negative” pressure (as opposed to positive hydrostatic pressure) so the deeper penetrating membranes are more recommended. Peter Yost just mentioned this on his Build Show Podcast interview recently. I treat this like the interior vs exterior insulation question, this is the best possible time to apply waterproofing to the outside of the crawlspace walls so go ahead and do that now, and then later if necessary you can always apply waterproofing to the inside.
Matthew,
Thank you for the detailed and thoughtful response. Because this is a remodel and not a new build, the exterior foundation walls are all below grade. Due to the slope of the property this is somewhat of a "walk out" crawl space where the 6ft deep side is below grade and the 4ft tall side is above grade. Because of this, insulating the exterior foundation walls would require me to dig down and expose the walls. It is something I am considering doing but it will be quite a bit of effort for the 6ft tall side.
If I go ahead with insulating the exterior foundation walls, I know I will just put the comfortboard 80 right on to the surface after coating the concrete with a waterproofing product.
If I have the exterior insulated as such, and I have left over comfortboard 80 to use on the interior, is it acceptable to just put the comfortboard directly on the interior foundation concrete? I've read that it can lead to moisture problems due to allowing the interior concrete to stay cool and potentially make the interior humid air condensate on the surface when it comes in to contact because the comfortboard is vapor permeable.
Would it be advisable to use a similar assembly order as used for the slab? - (from exterior to interior) concrete foundation wall > comfortboard 80 > plastic vapor barrier
This assembly makes sense in my mind to keep the interior humid air from coming in to contact with the concrete foundation walls, and as a result avoid condensation issues. I've just read so much conflicting information and seen videos (https://constructioninstruction.com/video/how-to-insulate-your-basement-wall-comfortboard-80/) which advocate putting the vapor barrier directly on the foundation walls so it's very difficult to determine the best practice.
I dont think I would turn my crawlspace into unvented in LA. pour your slab and keep it vented and store stuff in there, your temperature swings are extremely mild. if you add a vapor barrier you can decrease the needed ventilation ratio to 1/1500, basically like having a cracked window.
One of the main reasons for converting the crawl to conditioned space is so that we can have our hvac ducting and air handler as well as our heat pump water heater in conditioned space and not in our living space. The plans are already approved and I don't want to give LADBS any more money to make changes ;)
I'm in LA, had one project with conditioned crawlspace for an addition because we had trouble getting proper cross ventilation, but the GC was so unused to that he just talked to the inspector and vented it. I suspect 99% of contractors will be the same. Also with California Title 24 energy calcs, I've found the software doesn't recognize conditioned crawlspaces, and you have to insulate the raised floor, or take a penalty and make it up in other ways. Curious how your CF1R addresses this?
Stego has a post about if insulation should be behind or in front of the vapor barrier in a crawl. Arguably, no one knows vapor barriers better than they do.
Since Stego says it's a personal decision, rigid foam is ground contact rated (like NGX), and it is not all together clear that water can't come up through the footer and above the recommended 18" up wall and then onto the rigid foam to roll down inside of the vapor barrier...
Onto rockwool 80, Rockwool residential instructions have the 80 directly on the concrete wall then a vapor barrier stapled to studs over it. I pointed this out, was told it was wrong, and do agree now that studs in this approach...how would they dry trapped in this humid location if the concrete wall takes water.
I think you have to decide if your are doing Comfortboard 80 only.
It's interior rated. It is NOT formaldehyde free. The product might test at natural level but it + natural level is greater than 0?
The other concern is I noticed Thermafiber throws a great deal of crap into the air when I just put it in. I assume that is 100% done with once not disturbed. My Rockwool 80 in my garage waiting for the same thing you are doing is much more connected to itself.
It is perfectly acceptable to use Tyvek inside, with instructions for it. If you decide to put the vapor barrier up first then 80, you could use Tyvek as a mineral wool fiber stopper.
I think the question on rigid foam behind or in front of the wall vapor barrier still isn't answered. I think the Rockwool probably should be interior of the vapor barrier, but that means the concrete wall and Rockwool 80 needs rigid between it.
Now on to vapor barrier companies, almost all (and you can confirm yourself) show their work with the vapor barrier up all the way over the rigid foam.
So that's 3 competing ideas from each area most in the know.
But in your warm area, maybe directly on the concrete does actually make sense with the vapor barrier covering it.
Good luck.
Thanks for all the info. I'm still a bit unsure of what makes the most sense but I think putting the Comfortboard 80 directly on the foundation walls and then covering with a vapor barrier makes the most sense. That order would keep humid interior air from being able to condensate on the cold concrete walls. And for a mild climate crawl space, maybe it's not all that important in the first place.