We are in Boston MA and are getting things in place to partially renovate our existing basement. I'm trying to get a good strategy in place to insulate and deal with potential moisture. Here are the project details: Existing basement in Boston MA - 700 sf Approx. finished area - 350 sf (remainder to remain unfinished utility space) Foundation walls - stone and mortar with interior skim coat. Floors - existing concrete floor to be treated with self-leveler, seal and polish. Water - We have an existing pump. We occasionally (tropical storm, many days of heavy rain) get a little seepage through the WALLS over by the sump pump as well as further along that wall). This is on the opposite side of the basement than where we are finishing. We are just concerned about the what if of water after we finish the other side. Our strategy is: Install French drains along the side that we are finishing and connect to the sump pump on the opposite side. Enlarge the catch basin for the sump pit. Then, for insulation, because our foundation walls are stone and mortar and are not plump, we were looking in to closed cell spray foam with a drip plan type membrane behind it to connect to the french drain. Not sure if I need that? Obviously, budget is a concern and we want to make sure we spend the money in the most strategic way.

If you can't manage the bulk water issues on the exterior better to preclude seepage, you may have to do insulating "cavity wall" style using rigid foam, or use a dimple-mat against the wall rather than 2lb foam sprayed directly on the foundation. You'd still want to use spray foam to insulate and seal the band joist & foundation wall, even if you went with a rigid foam approach.

In the Boston area there are multiple vendors of reclaimed roofing foam (search craigslist for "rigid insulation", or check out the Insulation Depot in Framingham), which is a FAR cheaper way to insulate basements than closed cell spray foam, and can even be cheaper than high-density batts for $/R per square foot. With a fieldstone foundation you may have to shim out some furring or a studwall with 2x4s turned sideways (creating a 1.5" drainage cavity- in which case you wouldn't need the dimple mat) on which to mount the foam, but there's always a way. At 3" thickness roofing iso runs ~R18, EPS runs ~R12 (R16 @4") and either would be a pretty good solution. Sealing the joints with can foam or FrothPack works. Put an inch of XPS between the bottom plate of the studwall and the slab to avoid wicking up groundwater (or bulk-water, during those seepage events). You can seal the top of the foam to the foundation sill & band joist when you hit them with closed cell foam. If this is a DIY, you could also cut'n'cobble rigid foam to glue to the band joist & foundation sill with foam board adhesive, sealing the edges & seams with can foam or FrothPak.

Any time you close up a fieldstone foundation it's time to fix any mortar repair issues, and in a wall that occasionally weeps, a sacrificial parge of lime mortar is recommended.

Thanks for your response. Your advice is really appreciated.
The particular foundation wall that is getting finished doesn't get any seepage as long as we pay attention to our gutters, downspouts and leaders. It is just that I am nervous about any seepage that may happen in the future given that it will behind the wall. This was why we were looking in to the french drains. Do you think french drains along this particular wall are a waste of money? The room that we are making is really glorified storage for musical equipment. It will also be soundproof so that we can play music and not wake the kids. As such, the drywall will be sealed to the floor with some acoustic caulk. As for the foundation sill, we need to get 2 layers of gwb against the rim joist (sealed to the floor joists with acoustic sealant). We were then going to pack the joist cavities with batt. I am not sure what to do about the top of the foundation wall/batt.

I can't really comment on the cost effectiveness French drains without a site visit, but bulk-water incursions can do a lot of damage.

The drywall should be kept from any contact the floor, since it will wick moisture readily. Stopping an inch or two from the floor and using a non-wicking polymer kick-board works. You can still use the acoustic sealant.


I'm not sure what the function of putting doubled-up wallboard at the foundation sill & rim joist(?). They need to be air-sealed and insulated (it's a bigger leak in most houses than the combined leakage of all doors & windows) which can done just fine with 2" of closed cell foam. (Or a 1" shot of closed cell plus high density batt. If it's just a small section you can do the foam with a 12 board foot FrothPak from a box store as a DIY.) If you use high-density R15 rock wool trimmed for a snug fit it does OK performance-wise even without an interior side air-barrier, but you need at least the flash-inch of foam for condensation and wicking control.

I assume the batts are for soundproofing?

We are getting mixed advice about insulating our foundation walls and I was hoping to find clarity here. Our existing foundation walls are stone with a skim coat. They are fairly uneven. We are also building an acoustic, sound proof room. Our approach for this is mass-air-mass - which translates into a double wall construction. My thoughts were as follows, from exterior to interior: existing foundation wall, R-10 rigid insulation with furring (no penetrations in to the exterior wall), 1" air gap, 2x4 stud wall filled with R-13 batt insulation, vapor barrier and 2 layers of drywall. My contractor showed up with Dow Tuff R which is a foil faced foam board insulation, as opposed to the rigid polystyrene boards I expected. He also showed up with faced batt insulation, which I was also not expecting. I am concerned he isn't getting the right stuff - and more importantly, if so I want to stop him so we don't create a mold cavity in my wall. I would appreciate any help in sorting this out.

Don't install any type of vapor barrier on your basement walls. In the event anything gets wet or damp, it needs to be able to dry to the interior. The foam, being next to the foundation wall will be OK.

the polyiso is fine as long as it is in a dry location; it has a higher R value and lower carbon footprint than the styrofoam.

I don't understand how or what the foam is fastened to and what the furring is fastened to if there are no penetrations into the foundation wall.?

Thanks for your response Bob.
My plan was the provide furring at the foundation wall to attach the rigid insulation to. I was thinking of ripping a 2X4 in half and using it as a sill and attaching to the floor. Then, there would be a ripped 2X4 attached to the underside off the above joists. There would be 2X4 turned on their sides, or 1x3 wood stripes running vertically. We would attach the rigid to those, as opposed to fastening them directly to the foundation wall. The foundation wall is stone - and is very irregular with large bulges etc. It's an over 100 year old house.
As for the polyiso...it is a basement, so I just feel like there is more moisture down there and there is a chance it would be exposed to moisture or humidity. I am also concerned about the foil and creating a second vapor barrier. Foil just seemed wrong to me.
I was putting the vapor barrier on the warm side of my stud wall, directly behind the 2 layers of drywall to trap condensation as it moves from the warm side of the wall (the interior) towards the cold side (which is the mostly underground foundation wall). We're in Massachusetts. The vapor barrier will trap the moisture before it has a chance to get in to the wall cavity. If it got in to the wall cavity, the moisture would try to get back to the warm side when the house starts to dry out. But it would get trapped in the batt insulation and that's not good. No? If I trap it before it gets in to the cavity then it can more easily get back out and dry up....
Then there is the facing on the batt insulation in the stud cavity...is that the right product?
Thanks so much in advance for your help....

The majority of the moisture - water vapor as opposed to liquid water (hopefully) - will be/is coming in through the basement wall into the house. It does not penatrate polyiso or extruded polystyrene. The problem here is that it will be hard to get the foam tight enough to keep the moisture out of the basement; that's the big advantage of spray foam in this application. The ideal insulation against the stone is a dimple mat with closed cell spray foam, but separate the mat from the wood sill so you aren't driving moisture into the sill. With a few inches of spray foam you could paint it with an intumescent (fire retardant) paint and use that as your finish wall.

Since you can be getting moisture from the outside as well as the inside, you do not want a vapor barrier on the interior. All of these foam products are vapor barriers, but they are next to the stone wall so will help keep the exterior moisture out. Putting a VB on the interior risks trapping moisture and growing a mold farm.

I see what you are saying....I hope it is ok if I ask/talk about a couple more points...
We decided on the rigid polystyrene because it is a lot denser than spray foam and more cost effective. We are trying to make this is a sound room...so we really need density and the double walls separated by an air gap. We patched the wall and applied ardex.
I guess I am wondering if we can still proceed with our original solution and either eliminate the vapor barrier or relocate it within the cavity?

The foam is the vapor barrier, so you do have one; just do not put it on the warm side of the insulation. Foam sheets will work as long as they are air sealed - top, bottom & sides. You can install a stud wall and fill it with insulation - rock wool is ideal since it isn't affected by water or moisture.

2X4 in half and using it as a sill and attaching to the floor

Be sure to use treated wood. If you want to go all out with a design that won't have moisture problems, you can build a room within a room and then air seal and dehumidify the gap between the sound room and the foundation walls.

We renovate many basements here in Minneapolis and usually start with demo of the existing floor followed by plumbing, drain tile/radon, vapor barrier, PEX and concrete. The walls are then furred out and 2# foam sprayed between outer wall and framed. Electrical and water piping and ventilation proceed in a typical manner and everybody happy.

I stand corrected - that would be an "all out" design :-).

I know, I get carried away. Watch your toes my friend:-).

Thanks so much for all your help. I really appreciate it!
We are building a room within a room as it is an acoustic sound room. The inner walls will all be comprised of 2 layers of MR drywall, with sound dampening compound between and all seams and corners caulked with acoustic caulk. One of the "outer" walls is our foundation wall - the other 3 are wood stud walls with batt insulation and 2 layers of drywall similarly caulked, etc.
Pressure treated wood will be used for all sill plates - I think that is building code here for basement construction anyways.
With regards to insulating the existing foundation wall, we wanted to frame out the foundation wall with the 2X2 and then use the extruded polystyrene. I wasn't going to completely seal it up because I didn't want to create a situation with a double vapor barrier. I am totally confused about the placement of the vapor barrier. I really thought that in colder climates like Massachusetts the vapor barrier is supposed to go on the warm side of the insulation. I have been reading stuff and they say warm side???
http://www.energybooks.com/pdf/D1142.pdf
http://epb.apogee.net/res/reinvap.asp

Thanks for your help - this is so confusing.

missmoo, Do not run the French drain discharge INTO the sump pump in the house. Keep the outside water outside! You would be amazed at how much additional water would be going into the sump pump during and after a rain event. If the French drain cannot be discharged to daylight don't install one. Instead, dig around the foundation and waterproof it from the outside. I have done this many times in upstate NY and it always worked. If you rip 2x4's and use them for studs be aware that they will want to twist way more than a full 2x4. For the size room you are building the cost difference will be minimal. I used wool batts in a bathroom remodel in my own home and was very impressed with how it fit irregular spaces so well. Nothing like fighting with fiberglass batts. And, BTW, don't "stuff" fiberglass batts if you do use them. They must be "fluffed not stuffed" to obtain their claimed R-value. I had an insulating contractor foam the band joist around the perimeter of my basement, 180 ft., and it was far less $$ than I could have done it myself with a Froth Pack. And what an instant difference it made in the basement comfort! With no walls insulated! Best of luck with your project and make some sweet music! Richm

In subgrade apps you have both ground moisture drives and winter condensation issues to be concerned about. If you throw up a studwall and put an interior vapor barrier on it you trap ground moisture in the studwall. If you throw up a studwall and DON'T put up an interior vapor retarder, you get condensation inside the studwall in the winter.

The solution is to put up sufficient R in air-impermeable foam between the foundation and studwall that the cold side of the fiber insulation has an average wintertime temp above the dew point of the conditioned space air, and leave the warm side of the studwall relatively vapor-open (to water vapor), but make it air tight. In a MA location it doesn't take much- R5 of foam for a 2x4 studwall is enough, and that's fine for either above-grade insulated sheathing behind the siding, or in below grade apps between a studwall and the foundation. The vapor-retardency of the foam becomes somewhat moot once you've hit this level- ANY foam is at least somewhat vapor retardent, but it needn't be fully vapor-tight, just air-tight.

By designing the stackup to be somewhat vapor-open to the interior the assembly becomes more resilient, so long as you have sufficient exterior R to keep the exterior stud edges and/or wooden sheathing warm enough to keep from accumulating too much moisture in winter. True vapor barriers are a very sharp double-edged sword, and will CAUSE as many problems as the prevent in a MA type climate. Designing the stackup to have reasonable drying rates in one or both directions without loading up with wintertime moisture isn't too tough or expensive in a MA climate, and is preferable to the interior side vapor barrier approach that became the standard in the 1980s (and the cause of many "sick building" mold issues.)

See:

http://www.greenbuildingadvisor.com/blogs/dept/musings/how-insulate-basement-wall

http://www.buildingscience.com/documents/reports/rr-0202-basement-insulation-systems

http://www.buildingscience.com/documents/reports/rr-1003-building-america-high-r-foundations-case-study-analysis

Extruded polystyrene uses HFC134a as the blowing agent, with ~1400x CO2 global warming potential. By contrast, polyiso and EPS are blown with pentane at only 7x CO2 GWP. If you have a choice (and you usually do), going with the lower GWP foam is far greener. Any of them will work.

One of the "outer" walls is our foundation wall -

This is the wall where you might get moisture problems. Consider making it like the others, even if only set in by a few inches.

Thanks for all the responses. I guess this is complicated because we are trying to insulate a sound room. Thus all the materials are serving 2 purposes.
1. The outer wall is the foundation wall and we need to insulate that. We can't add another wall inside of that - thus having 3 walls along that side - because that would make a triple leaf for the sound waves. As a result, the room would actually be less sound proof with three walls than 2 walls.
2. We are using extruded polystyrene because of its density. We need this for the soundproofing.
3. We aren't doing the french drains at the interior. We worked to resolve the water at the exterior of the foundation, and then also painted the interior of the foundation wall with Ardex.
4. In order to achieve the soundproofing, we are using double stud wall construction with a 1" air gap in between. If all goes ok, it will be totally sealed for air...The wall assembly is as follows: exterior foundation wall, 2X2 stud wall within which we will put the extruded polystyrene R-10, 1" air gap and then a 2X4 wood stud wall with batt insulation, then the vapor barrier and then the 2 layers of drywall. The rim joists are insulated with a layer of drywall and then a layer of rigid polystyrene.
Given the assembly I am describing and using these materials should I not use a vapor barrier at all? Should I put it on the warm side of the inner stud wall?

I don't know anything about it, but it looks like the triple leaf effect only occurs with small (eg, 1/2") cavities. The gap from the foundation wall could be "large" (as in 6-12 inches).