Subscribe or Renew Membership Subscribe Renew

Insulating basement (fieldstone walls)

EllenK's picture


I am a recent addition to this forum and think it is fantastic.  That being said, here are a couple of (hopefully good) questions, relating to our efforts to finish our basement.  Background: 1913 colonial, outside Boston.  Fieldstone foundation walls.  House is on a hill so we don't have problems with water infiltration.  Two years ago did a gut rehab - as part of that we had the existing basement slab removed, excavated down about 8" (to gain ceiling height) and had a new slab poured.  We unfortunately ran short of cash so other than the mechanical room being framed out, the basement was left unfinished.  Fast forward to the present - still no cash, but undeterred and resourceful homeowners that we are, equipped with Paslode cordless framing nailer and Ramset, we forge ahead to finish the space.   An important detail about the new slab needs to be explained (pictures would probably help) - since the excavation lowered the slab below the bottom of the existing fieldstone walls, our structural engineer designed a concrete "shelf" that runs the perimeter of the slab, approximately 8" high by a foot deep.  This shelf is tied into the slab with rebar so an elevation (section) would look something like this:

|                                                      |  <-- WALL

|_____                                            _____|  <-- SHELF

      |                                          |  

      |__________________________________________|  <-- SLAB

So we faced two choices in framing the perimeter walls - either frame as tight to the fieldstone as possible and then have this funny "lip" running around the basement (which is essentially unusable as floor space), or frame out to the front edge of the shelf and have the drywall hide the shelf.  We chose the latter, figuring that we could use the extra wall cavity space for insulation.  So we built 2x4 stud walls on top of 2x6 PT plates ramset into the shelf, where the plate overhangs 1 1/2" over the edge of the shelf.  The vertical surface of the shelf where it meets the slab has 2x4 PT on the flats as a nailing surface for the drywall (this also leaves some space for piping for baseboard slant-fin radiators).  Behind this stud wall and against the fieldstone walls I have adhered 1" T&G EPS rigid insulation (actually maintaining an irregular 1/2" to 1" air gap between the EPS and the stone).  I chose 1" EPS in order to still have some vapor permeability, and read somewhere that you should also use fiberglass (unfaced) batting behind the drywall, but leave an air space in between the EPS and the fiberglass so the batting doesn't get wet in the event that there is condensation on the inside surface of the EPS.

So now the questions: (1) The original plan to use fiberglass batting remains an option, but a friend felt this was overkill and the rigid by itself would be enough.  Do I need the additional insulation?  I have enough room where I could even put in R-19, but considering that I have about 120 linear feet of perimeter walls, it would be a considerable additional expense, not to mention a pain to work with, transport and store.

(2) For finishing the floor, we plan to use cheapo Armstrong commercial vinyl composition tiles, at $0.53/sqft.  The slab is not incredibly level, and floor leveler would be expensive, so I am hoping that some minor height variations (no more than 1/2" across the basement) would not be a problem.  I would need underlayment under the tile, and originally settled on using Dricore or Subflor, but after perusing this forum I am considering the Delta-Fl product instead, with some type of subflooring/underlayment on top of that, since it seems to be more economical (Dricore is $1.30/sqft at the Home Despot).  I have read good things about both Advantech and Halex - are there any opinions about which of these would work best?  It seems that Advantech panels might be a better subfloor, being more structurally rigid, T&G, and the thickness would be an advantage in evening out the height variations, however might not be smooth enough to set resilient flooring directly to it.  The Halex might be a better tile-setting substrate since it is specifically made for this purpose (smooth faces, void free, etc.), but since it is thinner than the Advantech and not T&G I worry that the finished floor will reveal the seams of the Halex.  Maybe I should use both? In other words, Delta-Fl on the concrete, then 3/4" Advantech (maybe with a few scattered Tapcons), then 6mm Halex (stapled?) and finally 1/8" vinyl (in thinset).  This would still be less than 1 1/2" total height, but again (as in question #1) is this overkill?  Can I get away with just the Advantech?  Or should I go back to my original idea of the 2x2 Dricore and put Halex on top of that?

Sorry to be so wordy.  TIA for any input/suggestions.


(post #66941, reply #1 of 2)

Hey Ellen

Concrete shelf is to hold the old foundation in place since the slab was below foundations.

The first ? 1" EPS  R-Value is 5 if budget allows I would use 3" EPS R-Value 15 that will be fine no need for fiberglass insul.

The second ? Did the slab guy use poly vapor before he pour the slab? If he didn't I would put poly on the concrete and then 1x4 ACQ @ 16" O.C and ramset in the floor use shims where ever you need it and then 3/4" Avantec over the 1x4" make sure you use constuction adhesive and nail or screw it in, just like a "sleeper deck". Thats all! The walls I would put up same way using 2x4 K.D (typ.) I would put it before the shelf not on top it. Hope my info was right.  Have Fun!



Jack of all trades.................Master of none!

(post #66941, reply #2 of 2)

Thanks for the response. The reason I used 1" EPS was because I was under the impression that you want to maintain some vapor diffusion, and EPS any thicker than 1" functions as a vapor barrier (which we are (these days) supposed to be avoiding). I get the impression that I probably should use some fiberglass batting in front of the EPS, provided that it is unfaced.

Also, in case it matters, the basement is below grade but not by much - because the lot slopes on one side the ground level is about 5 feet up the basement wall, on the other side it is about 3 feet. So a substantial amount of the foundation wall is above the frost line.

The slab was poured on a 6mil poly. I'm pretty sure the order of things (bottom to top) was ground, sand (to level), poly, gravel, rebar and finally concrete.

Using 1x4 sleepers and 3/4" decking, that already brings the floor up 1 1/2", and on top of that I might need to use additional underlayment (like the 6mm Halex I mentioned in my first post). Using the Delta-Fl, I might be able to gain 1/2" of ceiling height, but the downside might be a more uneven floor. Also, aren't 1-by sleepers not supposed to be as good as 2-bys? I'm not entirely sure I understand the reason for this, sure the 2-bys flex less but if they are lying directly on a slab what difference does it make? If I go with the sleeper approach, should I put them 16" or 24" O.C.? Again, thanks for the help!