Insulation behind drywall in a stone foundation basement

[Mark in Baltimore]

We are planning on finishing our basement, and I'm trying to figure out the best type of insulation for our budget. The foundation is stone (1930s rubblestone) and occasionally leaks water that very, very lightly trickles down the walls in three spots and into the French drain. The walls never have any sheen of water on it and is a minor issue.

We have closed cell spray foam on the band board, so the basement is relatively tight. Spray foam is too expensive for our budget and seems like overkill.

The plan is to have at least two inches of a gap between the stone and studs. Do I have the contractors go with foam boards cut and press-fit into place (recommended by one guy), unfaced fiberglass batts, foil faced fiberglass batts (recommended by another guy) and, if the last one, do the batts go on towards the stone wall or towards the back of the drywall?

We have CAC with the air handler in the basement. Because of this configuration, the basement is quite cold, so I'm concerned about condensation being created on the stone wall if cold air should hit it. I think I understand that having a proper vapor barrier is important to keep condensation down.

Am I overthinking this issue? Thoughts and guidance appreciated. Thanks!

 

[Moto]

I would think about building an addition (above ground), instead of finishing the basement.

 

[Hilltopmasonry]

If you have any moisture I wouldn’t put any sort of batts in there


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[finn]

I have a properly with a partial rubble stone foundation, partially insulated with bats.... It stinks.

If spray foam is out of budget, look into rigid foam panels.

Stay away from bats.

 

[tjpavlov]

Check out "insofast" panels. This old house used them on a rubble stone foundation with a special expanding glue.

I'd say run from any contractor who suggests batts.

 

[n20junkie]

Sealing in that sort of moisture between wood/drywall walls is asking for big problems.

Sure, people will say it can be done. But to properly finish a basement, water penetration needs to be handled before any framing goes up. Water that seems minimal in open air can be very problematic when sealed with wood.

 

[kelpaso1]

Ya that sort of situation is a hard one. The foundation would need to be sealed from the outside. Not sure how to do that with a rock foundation.

 

[matt_i]

Imo its very important to stop the water first. Entrapping it behind is a recipe for mold, as well as water getting into anything else "finished" like carpet, hardwood floors, drywall, etc.

I'm not an expert on how to fix this but I would think you have to go outside, carefully excavate, give a lower spot for groundwater to go and then be relieved, and then seal the outside of the foundation. The classic bituminous tarry stuff they use is probably an option as well as I think I saw a newer "membrane" product in the JLC recently. Im not sure how it would work with a cobbled wall though.

 

[Mark in Baltimore]

Thanks for all of the comments, everyone!

Building an addition on grade is not an option. We already have a family room addition, a sunroom addition and a pond/waterfall, meaning we simply don't have the room for another structure.

We have a full perimeter French drain, so what little (and I mean little) water is entering the property is, again, a non-issue since the three tiny trickles of water go directly into the drain tiles. I only mention the water since it means that there needs to be an air gap between the walls and the stone foundation. There is no way fiberglass should be in contact with the stone wall.

I forgot to mention that we have a radon remediation system. It runs 24/7, of course. The radon company owner (who also does mold remediation) said the system would help evacuate moist air behind the walls once we finish the basement.

I'm rethinking the idea of closed cell spray foam down there despite the much, much higher cost.

 

[TRWham]

ROCKWOOL is an option.

 

[schm1an]

I would first look at re-routing gutters or grading the earth against your house to divert any water coming toward your foundation. This solved a lot of problems for us before we finished our basement.

Second, use foam insulation against the walls as they are a vapor barrier when properly installed. I would stay away from anything that's not foam, including Roxul or any Rockwool insulation. Those will merely retain the moisture instead of blocking it.

I studied this for a couple of years before pulling the trigger and the way I decided to go about insulating our basement (1930s, but cinder block walls) was by first insulating the rim joists with 2" pink rigid foam insulation so no air could get in from the outside, with the edges sealed with expanding foam. Then, using 2" of pink rigid foam insulation glued to the block walls. The edges were taped to the wall and each other with overlapping layers of Tyvek tape and/or expanding foam for the tough areas. I then made the 2x4 stud walls and left them open, as the basement was night & day difference in climate after the insulation was up-- I went from feeling cold in the morning to feeling warm and cozy once it was up. Run electrical, hang drywall, mud/tape, paint, enjoy. I do keep a dehumidifier running 24/7 at 50% humidity in summer, just to be safe.

 

[jetnow1]

I would use metal studs and never use batts where water is present. That tiny trickle may
just increase if you get a week of wet weather, or over time wear a path for larger amounts to come in, and you would not see it till the damage is done.

 

[Mark in Baltimore]

Thanks, all. I'm doing more research, and fiberglass batting is out. Rock wool aka Roxul is great stuff and is supposedly excellent for sound absorption, but, if wet, still needs to dry out.

I briefly considered having the stone walls sprayed with closed cell spray foam but have ruled that out since there is no drying path for any water that will inevitably hit the exterior stone. Digging up the foundation and laying dimple mat all around the walls is not in our budget nor necessary.

Regrading the front of the house where we see two trickles of water is a good idea and makes total sense.

We were planning on using pressure-treated bottom plates and synthetic trim. My fiancee used metal studs at her old place and had the contractor hang the drywall 1.5" off of the floor.

Here in Maryland, we have gone through the wettest twelve months or so in memory. Some of you may have seen the terrible flash flooding in historic Ellicott City from this year as well as a year ago, but, if you missed it, here it is:

With all of this rain, last year and this year, we have had only three or so tiny trickles of water. In fact, we have been currently going through over a week of fairly steady rain with constant flash flood warnings. The Zoeller sump pump is running on a regular basis, so it's earning its keep as it relieves the hydrostatic pressure against the exterior walls. I'm confident that the water issue is minor since I've lived with the house for four years and through very atypically wet conditions.

Really appreciate the comments.

 

[Jackfre]

The very limited water intrusion is not a problem currently, but perhaps, that is because it is ventilated well and dissipates into the space before it can become a problem. I would not use drywall in that area. If you do enclose it use removable panels that are light enough to allow inspection.

 

[Frost]

I briefly considered having the stone walls sprayed with closed cell spray foam but have ruled that out since there is no drying path for any water that will inevitably hit the exterior stone. Digging up the foundation and laying dimple mat all around the walls is not in our budget nor necessary.
...
With all of this rain, last year and this year, we have had only three or so tiny trickles of water.

If there is not a large amount of water coming through the wall in weather this warm, why rule out spray foam?

Any type of insulation will have the same problem with being unable to dry unless you address the water on the outside (sealing, dimple board, etc).

At least the spray foam will not absorb water over time like fibre (Roxul, etc) insulation would.

 

[Firebrick43]

Spray foam, even closed cell, will absorb water. Ever remove the flotation foam out of an old boat? It can be significantly water logged after 15 years or so.

I would use insofast panels.

 

[Mark in Baltimore]

Thanks, everyone. The more I read about spray foam for stone foundations, the more I'm convinced that adding a layer of foam is the wrong way to approach this for the reason I previously stated.

 

[biggziff]

Sealing in that sort of moisture between wood/drywall walls is asking for big problems.

Sure, people will say it can be done. But to properly finish a basement, water penetration needs to be handled before any framing goes up. Water that seems minimal in open air can be very problematic when sealed with wood.

This ^^^ is your answer. You seal moisture behind anything and you'll end up with a big mess in short order. You either fix the water issues (and moisture) or leave it open to breathe.

 

[johnnyradiant]

The french drain is just a way of dealing with the symptom.
Go outside dig down to the footing and put a new cement wall in to just above ground level. It doesn't have to be thick and massive and you only need to form up the outside of the wall. That will get you a non rubble wall that can be sealed. I'd then dampproof the wall and put the plastic dimpled membrane up. Make sure you have a properly functioning perimeter drain. Back fill with mostly gravel and then move inside and use rigid insulation.

Then we cross our fingers that your water incursion is only the walls and not the floor. It would actually be cheaper to deal with the floor at the beginning or in conjunction with the foundation walls to avoid doing some things twice.