Finishing Basement - Doubling the wall insulation

[theparkway]

I am finishing my basement, and have a question on insulation. The home builders installed a fiberglass "blanket" drape over the entire cement foundation walls of the basement. This drape has a plastic layer that faces the living area, I am assuming acting as a vapor barrier.

When the basement walls were framed out, the drape was left in place and the walls were spaced out from the foundation walls. My question is this. If I want to add additional insulation between the wall framing, to up the basement wall R-value, what do I need to do, as it pertains to the vapor barrier?

Removing the plastic from the wall drape isn't really an option, because that is how it is attached to the wall. Can I simply add new fiberglass insulation between the studs, then add another vapor barrier to finished wall? Or is it a no-no to have a vapor barrier sandwich like I would then have?

 

[Dana]

The plastic-bag fiberglass has a pretty crummy history in terms of mold issues over time. The "right" thing to do is to pull it off and install a continuous layer of the requisite amount of rigid foam-R for dew point control for the intended amount of fiberglass, and re-install the old stuff. When you have it stripped to the foundation take the time to caulk the foundation sill to the top of the foundation, caulk all of the seams of the band joist to floor joists, foundation sill, subfloor etc, and cut'n'cobble rigid foam against the band joist sufficient for dew point control on any fiber insulation you are installing the interior side. Cut the band joist foam for a loose enough fit that you can seal it in place with can foam. Be sure to put foam on any ledge on top of the foundation too. Tape the seams of the foam with housewrap tape, and seal the top & bottom of the wall foam to the foundation with can-foam to prevent convection, which can transfer quite a bit of moisture.

Bluffdale UT is in US climate zone 5B:

In zone 5B you need a minimum of R5 on the exterior side of R13 for wintertime dew point control, at which point you can use latex paint on wallboard as the interior side vapor retarder. If the bag o' fluff is 3.5" thick, you only need 1" of polyisocyanurate or 1.5" of EPS. You can get a labeled R5 at only 1" if you used XPS (pink, blue, green, whatever), but that stuff eventually becomes R4.2/inch as it loses it's HFC blowing agents, so really it would take 1.5". Cheap 1lb density box store EPS is R5.8 at 1.5", which is fine.

IRC code minimum for zone 5B is R15 continuous, or R19 if thermally broken by studs. But putting studs next to a cold foundation has wintertime moisture accumulation issues as well as ground moisture issues, and is risky.

Using continuous 3" polyiso would get you there too. This guy seems to have a stack of favorably prices factory seconds or overruns, which is a cheap option if you have a truck and can go get it. There is also often reclaimed 2" and 3" roofing foam available at 20-35% of retail, which makes it pretty cheap too. (If found the above link by searching your local craigslist for rigid insulation, but commercial roofers will sometimes have a supply they're willing to sell for cheap rather than scrap.)

If you use polyiso, keep the bottom edge off the slab, since polyiso can wick ground moisture. A 1/4" of air is plenty, or cutting in an inch of EPS or XPS as a capillary break.

Are you intending to finish the basement with a 2x4 studwall? If yes, put an inch of EPS under the bottom plate of the studs as a thermal break too.

If you're planning to use rugs or hardwood flooring an inch or two of EPS (not polyiso) keeps the bottom of the rugs or subflooring to prevent accumulating summertime moisture. Even though outdoor dew points in your area are below the deep subsoil temperatures, the indoor air dew points can be higher (especially if you air condition with a swamp cooler), which leads to mold on things in contact with the cold slab.

 

[Sponsor]

 

[theparkway]

I should have been more specific, the reason that pulling the drape off the wall is also not an option is because all of the interior 2x4 walls are already installed, as well as the electrical in those walls is complete.

Although I appreciate the response, this house was just built 5 years ago, so it's not like the way it is right now isn't exactly how the vast majority of homes in this area are being built now.

I am really just looking to know if I can install un-batted insulation in the 2x4 wall cavity and then put a thick plastic lining over that before the drywall, or if I am better off to just leave it how it is and put drywall up.

 

[Dana]

As long as the thickness of the plastic-clad insulation up against the foundation is more than R5 (probably is, if it's at least a couple inches thick), you can install unfaced batts (or kraft faced batts), in the 2x4 cavity, and use only standard latex paint on wallboard as the interior side vapor retarder. You could also use blown fiberglass or blown rock wool in the wall cavities for a more perfect fit & fill (or even blown cellulose, if there is zero chance of flooding.)

Do NOT install sheet polyethylene vapor barriers under the wallboard, and don't use vinyl or foil wallpapers, or you will create a moisture trap. The pre-existing insulation will keep the cold side of studwall cavity fill above the average wintertime indoor air dew point, and there will be no significant moisture accumulation. By keeping the interior side comparatively vapor open (about 3-5 perms, compared to ~0.05 perms for 6 mil polyethylene), in the event of minor flooding or wicking of ground moisture through the slab, the wall cavities can dry at reasonably fast rates to the conditioned interior space. It's the amount of insulation exterior to the studwall that makes this possible.

If insulating the studwall with batts, take the time to install them well, tucking the edges and corners all the way to the plastic facer on the old stuff, then tugging them back gently so that they're just proud of the stud edges for a compression fit. Use a batt knife (or a 8-10" bread knife) to split the batts around wiring & plumbing, and around the electrical boxes. Air seal the electrical boxes with can foam or caulk on the back side and insulte behind them with cut pieces of batt. Compressing the cut piece that goes behind the boxes is fine (it's more R/inch at the higher compressed density), but don't just stuff an un-cut batt behind the box leaving a thin spot (=lower R spot) and a void that would allow convection to occur. Air leaks can move more than an order of magnitude more air into the stud bays than vapor diffusion through 5-perm paint (standard interior latex).

 

[theparkway]

Thank you very much for the reply. I will move ahead with your recommendations. Just as an FYI, there is about 3.5 inches of insulation in that drape.

 

[Dana]

Go ahead and tape over the slit vapor barrier (and any other openings you find) with something aggressively sticky but flexible- housewrap tape would be a good choice. Any air getting into the fiber in winter will condense, but it'll have a heluva time getting back out, since vapor diffusion thorough the 10 mil vinyl or whatever is impossible.

If the drape insulation is 3.5" thick and the studs are 4.75" from the concrete it seems you have about 1.25" of space between the framing and the vapor barrier on the old insulation(?). That's not great- ideally the studs would be in contact with the vapor barrier on the drape everywhere. The cavity fill needs an air barrier on both sides to perform at it's rated R, and if you leave a cavity/voids between the studwall/new insulation and the older stuff it will be a convective path robbing the batts of some performance, and a potential thermal bypass between the new and the old. If there's a gap it would be better to blow fiberglass in there to guarantee a full cavity fill no voids.

To do it with batts, you'll still have to deal with voids along the exterior edges of the framing which is less than ideal. If it's really 1.25" pretty much more or less everwhere, cut up some 1.5" foam board (any type, though foil-faced polyiso is easiest to cut) into 1.5" wide strips, put some dabs of foam board construction adhesive on them and and slip them to the back side edges of both the vertical & horizonal framing. That will leave a ~5" deep nominal void, which is easy to compress a low-density R19 into. That will pretty much eliminate the convection and thermal bypass channels.

R19s are really "fluffed up" R13s it's the same amount of fiberglass per square foot, and it will perform at R13 when compressed to 3.5", and R18 when compressed into a 5.5" deep cavity. At 5" deep it'll deliver about R16. (R19s are only R19 at it's full 6.25" manufactured loft.) Even if the compression fit squeezes the drape insulation to ~3", the R-ratio still has plenty of dew-point margin with R16 on the interior.

Cutting foam into narrow strips can be pretty easy using a 4" taping knive that has been sharpened on it's edge using a straight edge to guide it. A short demonstration video can be found here. Low density box store 1.5" EPS with facers is pretty cheap, but it'll still leave a lot of crumbs using this method, and the strips are easy to break. Foil faced polyiso doesn't have that issue.