Hey folks,

I've got a 106 year old house with a field stone basement. I've wanted to finish this basement for years, but the cost to do so is just astronomical. Multiple bids at 70k plus. Had a new idea though. Thinking of just putting some type of flooring down that add a few R's, replace the windows and door with energy efficient ones, and spraying the ceiling white. I'd clean up the walls but leave the stone exposed. The stone is actually really cool looking, so I'd be ok with this.

My main concern with this plan is temperature. My current basement windows and door are awful. I'm wondering if I address those two, is it enough to make it habitable to hang out down there during the winter (I live in Massachusetts). I can also put some baseboard electric heaters in to supplement, but not sure if the efficiency would be so low that it wouldn't even be worth it?

Note: Moisture thankfully isn't an issue, and a perimeter drain is also in place. I also have a dehumidifier set to 65 (I believe) to keep humidity in check. No visible water anywhere though since drains were installed 5 years ago.

Thanks!

A stone foundation may look really cool but it's highly thermally conductive and leaks a lot of air. In most of MA it's safe legal to put 2" of closed cell foam on it and leave it exposed as long as it's sprayed with an intumescent paint. You can probably even get that at a subsidiized price through MassSave. The foam isn't exactly rugged enough to take the abuse of banging furniture or soccer balls into it on a regular basis, so if you really use the space it's better to build a non-structural partition wall in front of it as the finish space wall.

Code min for foundation walls in MA is R15, but that would take more than 2" of foam, which is the thickest layer that should ever be applied in a single pass. At the high price of isn't really going to be worth going for a second pass, but R12-R14 reallly is "good enough". Be sure to seal & insulate the foundation sill & band joist at the same time.

If it's a dirt floor rather than a slab, put down an EPDM vapor barrier extending up the wall a bit before spraying foam on the walls. If you're up for it, buy some reclaimed XPS or EPS (but not polyiso)foam, and put that under the vapor barrier, then pour a 2" non-structural rat-slab on top of the vapor barrier to protect it. Used or factory seconds foam is VERY cheap from sources such as Nationwide foam in Framingham, or Green Insulation Group in Worcester:

http://www.greeninsulationgroup.com/

http://nationwidefoam.com/

Is there a slab, or no?

How much head room do you have to work with?

Any signs of seasonal moisture seepage?

This is a handy guide for figuring out all sorts of details, appropriate for any US climate zone 5 location, not just MA:

http://buildingscience.com/file/3318/download?token=uSYaWpWh

Hi,

Thanks for the response! There is a slab with a center drain. I'm not really looking to truly "finish" the basement with this plan, so I don't think code of a finished basement is a concern. In fact, there might not even be anything to get inspected. Just want to make it more enjoyable to be down there when we are. For example, buddies come over and we need a place to hang, or kids want a spot to run around. That being said, just want it warm enough to do those things.

No signs of seepage at all. 7.5'-8' ceilings.

I've considered doing your suggestion of getting the spray foam with the fire retardant coating and just leaving it exposed, but was concerned of outgassing and health affects down the road. Especially of the children. Is there any concern there? Not the most attractive option anyway though, but if it's the only way to keep it warm, it would be an option.

Any idea though on how it would be if we left the stone exposed and just did floor and windows?

Thanks again

How many SQ/FT is the basement? I could build a nice little house for 70K I agree, those walls may look OK, but they will forever be a heatsync...

About 800 square feet or so. I budgeted 35k and thought I was being generous. -- I should add that it included a 1/2 bath and some plumbing rework, but still...

re the walls being a heat sync. Originally my above ground floors didn't have any insulation. Was like that for about 100 years until I had it blown in. But I would have though, especially with the wind and exposure to air, that those would be worse than the buried underground walls of the basement, no?

Insofast

You have to meet fire codes whether you're finishing the basement or not.

The Ri5 continuous foundation insulation requirement on new construction is for all foundations, not just basements built out into living space.

The laws of physics are self-enforcing, no code officials required.

Insulating just between the floor joists won't air-seal nearly as well as sealing the foundation walls, puts any plumbing, heating or other equipment outside of the conditioned space, and in MA would increase the moisture & mold content of the joist edges during the summer months.

A typical fieldstone foundation will have a U-factor of about 1 BTU per square foot per degree difference, about the same as a single-pane basement window. Assuming it roughly a square 25 x 32' footprint(?), that's 114' of perimeter, and assuming 2' of it is above grade, that's 228 square feet of U-1 foundation. When it's 10F outside 50F inside that's a 40F difference, and a heat load of 40F x U1 x 228' = 9120 BTU/hour just for the above-grade portion. With 2" of closed cell foam that drops to about 800 BTU/hour, and the basement would be more like 60-65F, and your floors will be 2-3F warmer. With a warmer basement and less air leakage the summertime "musty basement" mold factor plummets too.

If you don't want to risk the closed cell spray foam you can still get there building and air-sealing interior side insulated non-structural studwalls, but it's awkward. If taking that approach it's worth installing an EPDM vapor & air barrier on the interior side of the fieldstone foundation, sealed to the slab under the new studwall plate, and sealing it to the bottom edge of the foundation sill, not over the top. There are a gazillion details to get right to prevent condensation from accumulating between the EPDM and the insulated studwall, and I won't go into them without a lot more detail, and probably more expensive than a closed cell foam solution. A 2" foam-over + intumescent paint job runs about $2 -2.50 per square foot. Figuring an average wall + band joist height of 8', with a 114' foot perimeter is 900 square feet, call it 1000 square feet, times $2.50/ft would be $2500. An you can probably get more than half that back through MassSave, whereas a more complicated vapor barrier + insulated studwall approach would probably be more like $5-6K, none of which would be subsidised. With a bit of cleverness you can probably do it on the cheap as a DIY with some reclaimed roofing foam from GreenInsulationGroup (in Worcester) or Nationwide Foam (in Framingham) though.

smartwall: Insofast, really? On a fieldstone foundation? How's that supposed to work? On a CMU or poured concrete it's a credible approach, but still not as cheap & easy as with reclaimed roofing foam trapped to the foundation with furring or studwall. Working with flat goods against the highly irregular surface of a fieldstone foundation is a bit messier.

Thanks @Dana1. Yeah, it became a curiosity thing once I remembered that my exterior walls weren't insulated (until 5 years ago), but was more than habitable. But the science behind it is obviously very complex, so I'm fine with just chalking it up to "Yeah, that's true, but it's different for a basement. You may not need insulation on the above ground floors to live in it, but you need it in the basement walls for sure". Called MassSave. They actually don't support spray foam insulation for basement walls as part of their program. I had gotten a quote a couple of years ago and I think I was at around 3.5k. There's challenges we have with it though due to the basement stairs being grandfathered in at 30", and when you add sprayfoam to the wall next to it, and then dry wall on top, now it's really pushing the limit. Will need to talk to the inspector. Thanks again for the help.

re: insofast. Called them and asked how it might work on a field stone wall. It can be done, but it's not really the application it was designed/recommended for and the level of effort will quickly go up to get it right.

Received an Insofast email about a field stone basement remodel. And there it was. It's probably on their web site

If you really want to be able to see the fieldstone, build a wall inside like Dana suggests as plan B, but leave space in it for a big double-pane low-e picture window that you can see the stone through.