Basement floor

Thanks in advance for any advise.

I'm planning on finishing my basement. Although I have never seen any liquid water coming into my basement, it is still quite humid so I've been looking into best practices for sealing things up. I've come across Superseal Carpet Subfloor Membrane which looks like a smaller version of the type of dimpled membrane used on exterior foundation waterproofing. It seems like an economic solution to provide a vapor/moisture barrier and also some degree of thermal break. If anyone has any experience with this kind of floor, I would love to hear about it.

I also plan on adhering 2" rigid polystyrene to the concrete walls before building standard studwalls in front, covered in sheetrock. Living on Long Island in NY, we do get cold winters but not too harsh (a typical cold night will drop into the 20s). Is this enough insulation for the occasional use of the basement? It won't be used on a daily basis, perhaps once a week or when our niece and nephews come over to play so running the heat a bit extra during these times isn't too bad. I understand that adding fiberglass insulation below grade can lead to all kinds of mold problems and as the basement is kind of humid now, I don't really want to risk it.

Again, I want to thank the experts here. I feel I'm pretty handy, but when it comes to things like airflow and moisture I really want to get as much information as possible. I would hate to have to tear out my work in a few years because I didn't fully understand how moisture might be trapped.

 

Have you measured the humidity or done a simple moisture test? Take a plastic garbage bag and some duct tape. Lay the garbage bag flat on the cement and tape it to the ground. Wait a couple of days and see if moisture puddles or condenses underneath the plastic.

First step is to get a handle on the humidity or you will be making adjustments to your work in a few years.

 

Good advice... if the slab shows no sign of liquid moisture you could use any of the membrane products out there. Course the basement moving air dries out (evaporates) any moisture as it is formed) Do the instructions require air-sealing the edges to complete the 'pressure equalization" system as per science; Fig. 3:BSI-003: Concrete Floor Problems — Building Science Information


Notice 2b, 3, 4; http://www.cosella-dorken.com/bvf-ca...stall_inst.pdf

For Zone 4; 2009 IECC Climate Zone Map - New York

R-10 continuous (foamboard) OR R-13 (cavity insulation) clicking on your county. You could also combine the two with R-2.5 fb and R-13 cavity fiber. IRC FAQ: Insulating Sheathing Vapor Retarder Requirements — Building Science Information This would stop condensation at cavity insulation/f.b. interface. With R-2.5 fb, safe to 35% Relative Humidity in 68*F room air, may need a dehumidifier or thicker f.b. With R-5 (1"XPS) and R-13 cavity fill, safe to 40%RH and below. With R-10/R-13, safe to 49%RH. Are you aware of install- drywall/foam plates, fire-stopping?

Gary

 

I have turned off the dehumidifier for the past few days to see what it would be without it. It is currently at just over 70% humidity around 66 degrees. I've taped some plastic wrap to the walls in a few places and on the floor 24 hours ago. So far no moisture either on the surface of the plastic or under the plastic, although it hasn't rained for a few days either. I'll give it a few more days and see what happens.

 

Thanks for all the great links.

My plan was to install the non-load bearing stud walls on top of the dimpled membrane. Would I still need the foam sill plate gasket? I will probably splurge for the mold resistant (green) drywall just to be on the safe side. I have 5/8" drywall over the oil burner, but do I need any other fire stopping?

 

The membrane would shield the wall plate from the moisture below slab but not insulate or air seal as well as using foam board. The other concern is running the air-barrier membrane past the wall to air seal the perimeter edges at the concrete wall instead of in front of the frame wall. You would have only that one line of defense against air rather than the fb under the plate with two beads of caulking (seals concrete discrepancies) along the length to give much more air sealing. Check locally on fire codes, different as per locations. Your RH is high.... have you air-sealed everywhere down there, are the window frames leaking, pipe to exterior, holes in floor above- wiring/plumbing/etc.?

Gary

 

The windows are probably 40 years old, single pane and not very well sealed. Part of the basement is also open to a small dirt floor crawlspace which I am in the process of encapsulating. All other holes have been either caulked or spray foamed.

If I am understanding correctly, you suggest that I insulate the concrete walls with 2" foam board, build the stud walls in front of that with a foam sill plate gasket under the bottom plate, then run the membrane up to the bottom plate of the stud wall and caulk the perimeter between the membrane and the bottom plate. I was thinking that with the membrane run up to the the concrete walls and caulk applied between the membrane and concrete wall, with 2" foam board on the concrete wall and the stud walls built on top of the membrane would provide better air sealing than the thin foam gasket.

 

The sill sealer poly gasket is not nearly as effective as rigid foam board under the plate for an air/capillary/thermal barrier. IMO, stop the membrane 1/4" short of the frame wall assembly and air seal it there, as per diagrams already given. You would have two (1-seal at membrane perimeter, 2- seal under wall plate- caulking, two continuous beads the whole length, one above the f.b., one under the fb/concrete) Use foam compatible adhesive in 1' square grid pattern on the wall f.b., with a continuous perimeter bead to air seal completely; MYTH: LEAVE AN AIR SPACE BEHIND THE INSULATION IN THE BASEMENT TO AVOID CONDENSATION.

Canned foam the top/bottom/joints of sheets or use furnace mastic/mesh tape as f.b. sometimes shrinks to break joint tape bond over time; http://www.buildingscience.com/docum...a-foam-shrinks

ADA the drywall; Info-401: Air Barriers

There should be a weather-stripped door between conditioned basement/unconditioned crawlspace- no air exchange. If conditioned crawl- important for air exchange to meet minimum code; http://www.buildingscience.com/docum...ance-and-codes

Air sealing, new windows or insider storms are an option, you need to get the RH down...

Gary