Replies

1. 
   GCourter | Jul 09, 2005 11:06pm | #1

   Ever consider an inch of spray foam for the vapor barrier?

2. 
   csnow | Jul 11, 2005 04:58pm | #2

   Masonry buildings hold a lot of moisture.  Bricks are like sponges. Vapor perm is not the primary source of moisture in wall cavities in most structures.  Poly can do more harm than good in many cases, and a masonry building is likely to load a lot of water into the wall cavities in the summer.  Look into alternatives such as "airtight drywall approach".

1. 
   Junkheap | Jul 12, 2005 03:43am | #3

   Many thanks for your reply. The brick (vs. wood) construction element isn't something I really thought about. I appreciate your insights! Mark/Portland, ME

1. 
   csnow | Jul 12, 2005 04:54pm | #4

   Brick is tricky.

   If the interior wood walls are being reframed, a good design would be a contiguous and sealed layer of rigid foam (set away from brick just slightly to allow drainage plane) then place your framing inside of that.  The remaining wall cavities could them be filled with any sort of insulation, since the foam is doing the heavy lifting.  If the foam were a full 2 inches, you could skip any further insulation (ignore R values as not relevant in this case, unless you must comply with code).  Essentially, you would be building a 'weatherization wall' within the structure, leaving the brick to do what it does best, which is keep out the rain, and hold up the roof.

   For existing framing, you can strive for something similar using rigid foam by sealing the edges of the foam to the framing with gun foam.  This is obviously more time-consuming than more typical methods, but brick is a tough moisture situation.

   If you must use other insulation materials, you would not want them to contact the brick, which may be a challenge.  One big advantage for rigid foam is that it is not absorbant.

1. 
   Junkheap | Jul 14, 2005 05:50am | #5

   Again, thanks! I'm seeking advice a little late because one of the two attached townhouses I've already framed up.I left the exterior-wall plaster/lath/strapping (after five dumpsters of other stuff). I didn't mind losing space because the place is huge (and fiberglass is cheaper than foam board), so I framed up with 2x4s three inches out from the old plaster. One unfaced layer of R-13 batts runs horizontally behind the studs, and a second layer runs vertically between the studs. (The floor joist areas are only R-13.)
   In this case, would you recommend plenty of air-infiltration measures but no poly barrier?Your foam board idea sounds good for the other unit (still without ripping out the existing walls). Rather than strapping the foam, I think for ease of wiring, and to avoid all the cutouts for boxes, I would stud up 2x4 or 2x3 walls against the foam. Question: if I add fiberglass, won't wintertime interior water vapor migrate though the fiberglass, hit the foam, and condense?I greatly appreciate your assistance. Mark.

1. 
   csnow | Jul 14, 2005 05:16pm | #6

   "so I framed up with 2x4s three inches out from the old plaster. One unfaced layer of R-13 batts runs horizontally behind the studs, and a second layer runs vertically between the studs. (The floor joist areas are only R-13.) In this case, would you recommend plenty of air-infiltration measures but no poly barrier?"

   The idea is that the poly barrier may trap moisture in the cavities half of the year (or more).  Imagine how much consensation occurs inside a brick wall on a hot humid day.  So the (admittedly still heretical) thinking is that poly VBs may do more harm on net than good.  Controlling air migration into the cavities is the key, but from a code perspective, it is easier for an inspector to check for the existence of a VB.  Now, this VB will be riddled with holes, but hey, it's present.  Air control is achieved through carefully attending to hundreds of tiny details- where's the checkbox for that?  Controlling air is very important with your design, since my guess is that the original plaster wall will be where moisture will choose to condense on a cold day, though this will vary depending upon ambient.

   "Question: if I add fiberglass, won't wintertime interior water vapor migrate though the fiberglass, hit the foam, and condense?"

   The theory is that if bulk air movement into the cavities is truly controlled, the net moisture load is very low.  Also, with 2 inches of foam, the dew point should not be achieved within the cavity.  Anything inside the 2 inches of foam is essentially a conditioned inside space.  The R value may sound low, but R ratings are not all they are propped up to be.  That's the (heretical) theory anyways.

2. 
   Junkheap | Jul 16, 2005 01:04am | #7

   Again, thanks. I appreciate your heretical thinking, and pointing me toward airtight-drywall, and the related caulk-and-seal approach to reduce air leaks. Having the dew point potentially at the existing plaster wall (in the townhouse I've already insulated) seems more benign then vapor condensing on the sheathing of a wood-frame building. And things will get better on the 2nd half with the foam board. It'll make a nice little laboratory: 10 years from now I can do a dissection to see how the two compare after summer & winter cycles! Best wishes, Mark/Portland