Insulating existing partial basement with no exterior insulation or vapor barrier
I’m working on a full gut and renovation of a 1954 home in Kansas – climate zone 4. The home is largely slab on grade, however, there is a partial basement that is also a walk-out basement. There are 4 main insulation topics that I’m having a tough time making decisions on.
1. Basement insulation.
There is no exterior basement insulation and if there is an vapor barrier, it is not working very well. We’re working on exterior drainage and will be making improvements there but during a big rain, the CMU block does show a bit of moisture. We’ve taken quite a few steps to mitigate this from the outside but worst case, I’d like to assume this issue will persist.
I’d like to insulate the basement. The portion of the basement that is up against the rest of the slab-on-grade home will remain exposed as it is in a mechanical room and I’d like wall area to be able to get air for drying. With the furnace in that room, I don’t imagine that condensation from inside will ever be an issue. Wicked moisture would have the opportunity to dry to the inside along this wall.
Given this open wall, I’ve considered leaving an air gap behind the other two walls. The basement is rectangular. One short end of the rectangle (mechanical room) butts up to the rest of the home/slab and the other short end is the walkout portion and is essentially all glass. So there are only the two long walls to insulate.
First thought wast to use rigid-foam with taped seams on the back of the stud wall. Stand up the wall and set it 3/4″ off of the foundation wall. The HVAC room would allow air to move from that room into the space behind the stud wall aiding with drying if damp and also warming the foundation to prevent condensation. The idea with the foam is that we are also creating a moisture barrier between the wall the studs.
That said, I’m worried about the condensation portion. Is that a good approach here? I’ve read a lot about rigid foam adhered right to the walls but given the moisture I’ve observed, I’m concerned about that drying out and would like to allow it to dry to the inside. Or… given all this should I simply not insulate at all (don’t love this)?
1a. Similar with the basement floor. I’ve not really observed the floor wicking up any moisture. However, if I insulate, should I be using something like Dri-core where it can vent? Or am I ok to just lay down XPS and subfloor on top? There is no sump pump as water table issues are generally not a concern where the home is.
2. A similar situation exists on the main level. There is a solid brick wall that I’d like to fur out. If I adhere foam directly to the brick, then stud wall, is there any concern with condensation on the inside of the brick?
3. Same question for the main floor slab. I’d like to add some insulation as I do not believe the slab to be insulated from below. Similar with vapor barrier – if it does exist, it’s likely failing. I have seen a small amount of moisture coming up from the slab/first floor portion of the home. Very little and in select areas but it does exist. Should a product with an air gap be used – dri-core or similar (dimpleboard/foam/decking)? Or should I go right to the slab with foam.
Thank you so much for your help!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I am partial to lime wash on CMU.
https://www.limeworks.us/
this coating will mitigate the dampness from the grade. not a panacea and won't do much for an active water leak. However, it's a good start so that you can address internal sources of humidity. likely the air space will not totally work to prevent the wall from reaching dew-point.
A supply duct for upstairs rooms will run over both air gaps. I thought about adding the smallest little take-off just to move a few CFM in the air gap, effectively pressurizing that cavity and pushing conditioned air into the cavity that would empty at the opening at the mechanical room. ¯\_(ツ)_/¯
If you have bulk water issues that you can't solve from the exterior, what I've done in the past is to put in an interior perimeter drain and a dimple mat tight to the wall. I run rigid foam right over the dimple mat. The dimple may provides about a 3/16-1/4" or so drainage plain against the wall, the perimeter drain then carries away any runoff. I seal the top edge (and sides) of the dimple mat with polyurethane sealant so that it doubles as a vapor/air barrier against the wall. Once you've done that, you effectively have a "dry" basement wall that can be insulated in the normal way with rigid foam panels tight to the wall.
I don't ever build out regular studwalls against the walls, and I never use batts of any kind in basements. If I need room for electrical lines, I run 2x3s "on the flat" against the foam, then use 1.5" deep 4" square electrical boxes and mudrings to bring the devices out flush with the drywall surface. This works well, and uses standard parts that are easy to get and to work with.
If you have bulk water issues with the floor, dricore is a good option, but it will need a way for the floor to drain, so you need floor drains or a sump pump in the slab. If you don't have bulk water issues, and are only concerned with moisture coming in, there are epoxy floor coatings that can act as a vapor barrier.
For a brick wall, if it's uneven, spray foam is the only really safe option since it forms a fully adhered layer (no gaps for condensation to form), but it can put the brick at risk in areas with freeze/thaw cycling. Better is to leave an air gap between the brick and the structure if at all possible.
Bill