Conditioning strategy for newly encapsulated crawl / basement
I’m looking for advice on how to condition my crawl space and basement given my unique situation. I’m in zone 4a.
## Context
The main structure sits above a dirt crawl space with a 1.5 ft clearance. I plan to put a vapor barrier (poly) on the dirt. A 5th of it is dug out—with a concrete floor—to accommodate appliances. See attached pictures.
An extension that has about half the footprint has a basement instead of a crawl. The basement and crawl are somewhat connected.
I had originally planned to keep the crawl space and basement vented and unconditioned because that’s how it had been for a century (not without issues, though). I was planning to insulate the floors above those areas with batt insulation in the floor bays and then rigid foam board across the joists, taped at the seams to create an air seal. I conducted my manual Js and Ds according to this original plan, meaning I did not account for the crawl and basement loads. I had two slim ducted heat pumps installed in conditioned areas in drop-ceilings in the house: 1 for the 1st floor above the crawl and 1 for the extension above the basement. They were intentionally not oversized to maximize performance. They’re my only heating and cooling source, no backup.
My crawl/basement insulation plans have since changed. I got a bunch of advice on this site that it would be difficult and costly to air seal above my crawl and basement using the rigid board + taped seams approach—especially for an old leaky house like mine—that vented crawls / basements have lots of moisture problems, and pipes freeze (this was a real threat this past winter). The case for encapsulating and conditioning down there won me over: lower cost, easier to air seal and insulate the foundation perimeter, better moisture control, etc.
(Note: because I deferred insulating the floor, when I ran the heat pumps this winter without having insulated in the crawl area, I lost a lot heat through the uninsulated crawl and my heat pump couldn’t keep temp.)
## The ask
What strategy would you recommend for conditioning my crawl space and basement if they’re encapsulated?
Here are some options I can think of, which is best? Is there a better one?
a) Don’t bother with heating and cooling. Once encapsulation is done, the basement and crawl loads will be trivial. Maybe open some vents in the floors to get some warm air in there and just run a dehumidifier in warmer seasons. (I’m worried that my heat pumps don’t have enough capacity for this strategy).
b) Set up a propane heating solution in the crawl or cellar and duct it to both areas. (No gas infrastructure available). This will only solve heating, but it would provide a backup if the heat pumps can’t keep up during the coldest days. Run a dehumidifier during warmer seasons. I’m worried that the heating unit would be oversized (I’m assuming they don’t come in capacities as low as heat pumps).
c) Set up another heat pump in the basement or crawl, duct it to both areas. I could easily get a low load unit to not oversize. I’d not only get heating and some extra support for the first floor, but also cooling and dehumidification down their. The con is that I still have no non-electric backup.
d) Something else? (I can’t really run a duct from one of the existing heat pumps into the crawl)
What would you recommend? Thanks!
-Luke
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Replies
Air sealing and insulating basements and crawls in 100+ year old houses is very difficult, but improvements can certainly be made. The very first thing you should do is deal with the moisture issues in the basement. From the moisture patterns on the walls, it looks like your roof drains are dumping water at the corners. Install extensions or some other kind of runoff drains to move that water farther from the house. Make sure your grading slopes away from the house for at least 10 feet. Clean and paint the walls with a waterproofing paint. Do some searches on other techniques to address basement water issues. With that much moisture you have significant risk of moisture related damage once you insulate and allow the surfaces to become colder.
That dirt crawl is going to be a challenge. The "best" option is probably to dig out the dirt to the bottom of the wall (top of footing) if there is one. In houses that age, there may be no footings in in that case I like to keep the crawl floor 4"-6" above the bottom of the wall. Leveling the dirt is the primary goal here. How do you access the deeper section with the mechanicals? Hatch in the floor? Does it make sense to dig a deeper pathway between the two basements with masonry walls to hold back the dirt?
For moisture management on the floor, install a rugged plastic sheet. 10 mil poly is OK, but you might consider one of the reinforced products that will take more abuse. Some of the professionals use EPDM or TPO roofing membrane, with roof-type flashings and seals at penetrations. these give a nice finish but are pricey. Whatever membrane you choose should lap up the walls about 6" or so and be mechanically fastened with furring strips or termination bar and sealant between the wall and membrane.
As much as we try to minimize the use of spray foam insulation on this site, it might be your best option for insulating and air sealing, considering age and conditions. It's not cheap, but 1"-3" of closed cell spray foam would make a big difference. The foam would lap over the floor membrane by a couple of inches, extend up the wall, and cover the band joist area. If the house is balloon framed, you should install air dams in the stud cavities prior to spraying foam and extend your foam across the stud cavities to the adjacent subfloor.
The big benefit of doing an airtight and insulated basement and crawl is that the reduction in heat loss will probably more than make up for the increased conditioned area, and you may see that the heat pumps can now keep up with the loads. If necessary, a small electric heater would probably be the best approach in winter, just to keep the basement above 50 degrees or so but that might not be needed.
This is a relatively big and messy job but the results are generally worth it.
Thanks, Pete. The dugout portion of the crawl area is accessible via bilco doors and steps from the outside. I'm pretty sure there's no footing for the crawl brick foundation. I applied cut and cobble EPS to the rim joists per previous recs because the sill is directly on the foundation and did my best to seal the balloon-framed bays to the floor above using fireblocking and foam around it. I was hoping I could get away with rigid foam on those inner crawl walls + the poly lapping up the wall, but maybe spray foam is massively better?
The first issue I see is signs of bulk water issues in that basement area. I'd work on fixing that issue FIRST. I see two ways to do that:
1- Grade the external ground away from the house, and add gutters if you don't already have them. This would be the preferable option if it's possible and is sufficient to solve the problem.
2- Put dimple mat against the walls, sealed at the top, and put a perimeter drain in around the bottom to collect runoff from the dimple mats and the floor. You'll likely have to add a sump pump to pump out the collected water from the perimeter drain.
You can insulate the basement with rigid foam after you deal with the bulk water problem. I prefer polyiso for this application, and I'd leave a 1" to 2" gap on along the bottom edge to make sure the polyiso is never sitting in water.
That crawlspace is going to be a pain to insulate, due to the tight access. I would use EPS here, AND A SKIRT due to the limited height of those crawlspace walls. Basically since you can't insulate the walls down to frost depth, you insulate the entire wall and then "continue" that insulation inwards along the floor to form a skirt, which is helpful in this situation. You could insulate the entire floor if you want, but that's a lot of material and work, especially since it doesn't appear to be a flat floor. Hopefully you don't have any bulk water issues to deal with in the crawlspace.
I would insulate the exterior wall of the dug out area the same way as the basement, and probably wouldn't bother with the interior "walls" (are they even walls on the interior side, or are they earthen mounds?).
The rim joists are probably best insulated with spray foam here due to the difficult accessing them for any more labor intensive methods. I'd use a 10+ mil vapor barrier on the earth floor of the crawlspace. Thicker is better here to keep it from getting damaged in the future. Sealing that into the floor of the dug out area is going to be tricky if you don't have interior walls to make nice surfaces.
Bill
Thanks, Bill. Since I'm small, I can and have navigated the entire dirt crawl. I applied cut and cobble EPS to the rim joists per previous recs because the sill is directly on the foundation.
The dugout portion of the crawl interior side has interior brick walls. I'm assuming the poly on the dirt would end there, right? (a), (b), (c) or (d)?
Lastly, what would you recommend for conditioning the space after all this is said and done?
I'd probably continue the polyethylene down those interior walls then insulate them with polyiso panels, although with interior walls the insulation isn't as necassary -- maybe just use some thinner panels to save some money.
You might not need to condition the space, although you are supposed to have some air movement through it per code.
Bill
I like Bill's approach above, but will add a few notes:
- drainage at the base of an old brick wall is problematic. The conventional treatment is to put the drains below or adjacent to the footing but you can't do that in walls without footings - the water flow can move enough soil to undermine the foundations. Probably not an issue with the block walls in the basement, but with old houses you never know.
-I don't like the idea of a skirt of foam extending into the crawl space across the floor. This reduces heat flow from the house that is currently probably keeping the soil under the walls from freezing. With no footings and an insulation skirt, the risk of frost heave increases. With frost depth footings, I have no problem with the skirt, but not for your walls.
-As far as EPS vs. spray foam, either one will work. Spray foam is a better vapor retarder and will keep the crawl air a bit dryer. The EPS is a bit "greener" but is a little bit hard to attach to brick. The surfaces are dirty and uneven, making adhesive difficult, and it can be tough to get any of the easier masonry anchors to really bite in. Since you still need a spray foam guy for the band joist areas, I generally find it's easier to just spray the whole thing. Do what makes the most sense for you.
If you are insulating your crawl space with rigid foam you need an ignition barrier on top or you need to use a foam approved to be left exposed. I did see that rockwool just came out with a product that is integrated with intello, designed for basically this, so that could be an option.
As far as conditioning you should just follow code which requires 1 cfm supply per 50 sf of crawl, with a return path. Alternatively you can use a dedicated exhaust fan at the same rate. The last option would be a dehumidifier sized for the space.
I would definitely not condition with gas, if you can’t use central ac I would do either heat pump or resistance electric.
One way of achieving the dedicated exhaust requirement without taking as large of an energy penalty would be to have a branch of erv exhaust connected to the space.
The ignition barrier is a good point and few spray foam contractors willingly talk about it. Some spray foams are rated for exposed use in crawl spaces. If the foam is just used in the band joists, there is an exception that allows you to skip the ignition barrier. Some local inspectors extend that exception to all of the crawl space walls but that is a bit of a stretch. I don't know of any EPS products that are rated for use without ignition barriers. Dow Thermax is a rigid foam with a thin aluminum facing that is ratetd for exposed use in basements and crawls, even in finished spaces. But Thermax is spendy.