Posted By zygote on 01 Mar 2012 10:02 AM
Guys,

I live in nyc and I notice there are mold build up on the foundation walls. I'm pretty sure the wall is coming from the outside. What are my options in insulating and not have to worry about moisture coming from outside?

The moisture can enter the foundation in many ways- capillary action from the footing, rain wetting of the exterior,  infiltration of humid summertime air, etc.

Is there any efflorescent on the foundation wall, or moist areas after rainy periods?  What are the exterior overhangs/drip-edges & drainage conditions?  Is the floor ever damp near the foundation walls? Is this a solid wall masonry building? Cavity wall? Other?  If masonry, is the exterior in good shape, and sealed with a masonry sealer at least every decade? Handling the bulk moisture issues are of primary importance.

When you have that under control, vapor control & air-sealing comes next, since the humid NYC summer air has dew points higher than the subsoil temps, leading to high interior relative humidity in the basement with any air infiltration.  If you can make it air-tight to the exterior and seal the floor & foundation walls with masonry sealer the basement air's dew point will track that of the air-conditioned floor above. 

At that point you can go ahead and insulate without much of a mold concern.

Posted By Dana1 on 06 Feb 2012 06:55 PM

FWIW: I did my basement in central MA with reclaimed 3" fiber-faced iso @ $20 for a 4x8 sheet (63 cents/ foot for R19, or a bit more than 3 cents/R-foot), and I've seen it cheaper since.  It reduced my heating bill by more than 15%.  YMMV.

Dana,

I am not very familiar with iso, but when you say fiber-faced, should I assume that you mean fiber-glass faced?  From what I understand, iso tends to char rather than burn like XPS or EPS, but to rigorously meet building code (in Middlefield, Connecticut in my case), would you need to put gypsum over the fiber-glass faced iso?  I am looking at prices for foam to put up in a basement, and the fireproofing is probably a significant effort and cost consideration.  It looks like DOW Thermax can be installed without a fire wall, but it is apparently iso covered with an aluminum skin, and seems to be expensive compared with other rigid foam.  I assume the practical thing is to use iso (or XPS or EPS) and use furring strips to help anchor the foam and to provide something to attach the gypsum to (plus the glue as you mentioned, initially to hold the foam).  I found some sources in Mass. for the reclaimed foam board as you had suggested.   

Yes, fiberglass or rock-wool reinforced goods is what I'm talking about. Steer clear of asphalted paper facers or asphalted perlite composite facers unless you can get a spec on it- some of those are very vapor tight. Most of the fiberglass faced goods are rated sub 1-perm, but they're usually more vapor open than 2" XPS (~0.5 perms)

To meet code with fiber-faced iso you'd need to install an ignition barrier (half-inch gypsum/OSB/ply). Fire-rated Thermax can be applied without the ignition barrier, but unless there is a membrane type or metal capillary break between the foundation & foundation sill (possible to retrofit and in some houses it's not as bad as it might seem) you could be looking at sill-rot ten years down the road. Using reclaimed iso the material cost of the solution is still less than fire-rated Thermax, but it's more labor intensive.

I did my house with using furring as the mechanical anchor for the foam and gypsum. If you go thicker than 2.5" on the iso you'll need Tapcons longer than what most local box-stores carry, so it's good to line up your fasteners in advance.

When using iso on foundation walls it's prudent either leave a gap or put a capillary break at the bottom edge between the slab and the iso, since iso is hygroscopic and will wick & retain water over time, if given a constant source such as capillary-drawn groundwater through a slab poured without a poly or membrane vapor barrier. A half-inch of air or half-inch XPS, or 10mil-poly- just about anything works, so long as it is vapor & wicking-retardent enough that the drying capacity of the iso facer is an order of magnitude better than the wicking-rate. My slab is less than 12" from the average water table (sometimes below the water table during the spring rainy season), so I stopped the iso & gypsum a couple inches from the slab, and used flood-tolerant PVC casing board (yes, an evil vinyl product :-) ) as kick board to finish out the bottom.

Dana-

Great information. Thanks for the help. We need to solve some basement drainage problems first, and then I hope that we can insulate the basement walls.

Thank you also for your earlier comments on adding foam to the outside of the regular house walls.