Walkout Basement Insulation/vapor Barrier Question - Pic Included

[Boozle]

Hi all,

I am remodeling my walkout basement right now. The basement was previously finished, but I tore out all the drywall to make a lot of electrical and plumbing modifications, and also to alter some of the framing.

The entire basement has a concrete wall 4 feet high with a 4 foot 2x6 wall built on top of the concrete (8 feet total ceiling height). The 2x4 framing to finish the basement was built about 1 inch in front of the exterior concrete/2x6 wall combination.

There is existing blown in insulation with a vapor barrier on top filling the 2x6 cavity. Further, within the interior 2x4 cavity there was faced fiberglass insulation only up to the top off the concrete. I am in Colorado, but this basement was never cold. Further, there was no evidence or moisture or mold when I tore out the walls, so this system seemed to have worked OK.

What I would like to do now, however, is add Roxul rock wool insulation inside the 2x4 cavity for the full 8 feet of height. I am putting a home theater in the basement, so I want the additional sound damping characteristics of the rock wool insulation.

Question: Is it ok to leave the existing blown in insulation and vapor barrier in the 2x6 section, put the Roxul the full height of the 2x4 cavity, and put drywall on top? Or should I put a vapor barrier on top of the 2x4 cavity after installing the Roxul? Any insights are appreciated!

In the picture you can see the 2x4 wall sitting in front of the 2x6/concrete wall, and you can see the insulation and vapor barrier covering the 2x6 cavity.

 

[Dana]

I don't see the vapor barrier details in that picture. Is it air tight? It looks like a loose flap where it goes over the concrete, which means leakage into the wall can convect cold air to the sheathing, which isn't great.

What type of sheathing and siding do you have on the exterior?

Are you in the zone 4A part of NJ, or are you in the zone 5A part (the NW corner of the state)?

If it's too hard to tell just looking at the map, what county are you in?

In zone 4A it's generally better to not have ANY vapor barriers in the stackup, but you do need air barriers. In zone 5 it only needs a vapor barriers with certain types of siding, not with others.

If the existing vapor barrier is (or can be) detailed as an air barrier with all penetrations seams and edges sealed will be OK to leave it, and it's fine to leave the existing blown fiberglass in place.

Putting fiber insulation in direct contact with the concrete is risky, even if you've gotten away with it so far. It looks like you have something like 1/4" of space between the stud edges and the concrete. If you can slip some 1/4" or 3/8" fan-fold XPS (the UNperforated versions only) in there from floor to ceiling, and tape the seams with housewrap tape. That would be a ~1 perm vapor retarder and air barrier. If you go that route, take the 6 mil polyethylene out, since having at least some drying capacity toward the interior gives it more resilience, and the vapor retardency of the XPS limits the rate & amount of moisture accumulation in the sheathing over the winter to very tolerable low levels. Air tightness is key.

Without the intermediate air barrier & vapor retarder, in zone 4 you don't really need it. In zone 5, depending on siding type you may need some amount of vapor retardency, but not 0.05 perm 6 mil polyethylene. You can buy quite a bit of resilience installing 2-mil nylon (eg Certainteed MemBrain) detailed as an air barrier on the interior side of your studs, just under the wallboard. The 2-mil nylon is variable permeance. When the air in the cavites is dry (as it will be in winter, as the cavity air dew point tracks with the temperture of the sheathing), it will be under 1 perm, limiting moisture diffusion from the indoors into the cavity. But should the moisture ever build up to mold-supporting level it becomes vapor open, letting that moisture dry into the room. It's no super expensive, but more expensive than 6 mil poly. Some box stores are now carrying it in shorter roll lengths. If it's not on the shelf near you, it can be ordered online for local store pickup.

 

[Sponsor]

 

[Boozle]

Thanks for the response! I am actually in Denver, so zone 5 (just updated my profile).

The existing "vapor barrier" isn't really sealed tight at all. There are a lot of flaps and overlaps that aren't airtight, and it is overall very loosely stapled.

The exterior of the house is partially brick and partially Hardi board siding. It appears there is 3/4" or 1" foam board on the exterior sandwiched between the brick/siding and the 2x6 wall.

I'm thinking I can leave the existing 2x6 insulation and "vapor barrier" as is, install 1/4" XPS behind the 2x4 framing up to the top of the concrete section of the wall, and then install Roxul from floor to ceiling inside the 2x4 cavity?

Thoughts on that approach?

 

[Dana]

So you're Denver, NJ, eh? What exit is that? That's zone 5B.

The vent cavity behind the brick would be a drying path for the structural sheathing, were it not for the exterior foam. The type of foam and it's R-value matters. For 2x6/fiberglass you would ideally have sufficient dew point control at the sheathing with at least R7.5 of rigid foam, but you have less than that, but not dangerously less. The ratio of the foam-R to the total R with ~R20 fiberglass in the wall isn't so low that the wood sheathing temp is TOO much below the dew point of the conditioned space air. But when you add another R15 on the interior ratio changes, and the sheathing runs considerably colder, potentially too cold. If the foam is sufficiently vapor permeable that the sheathing can dry toward the exterior nothing bad happens, but if the foam is much below 1 perm there's risk.

If the exterior foam has a foil facer the poly has to come out, otherwise it becomes a moisture trap with the sheathing that dwells at average wintert temp low enough to get wet and stay wet. With ~1-perm fan-fold XPS as the air barrier & vapor retarder it will still be able to dry toward the interior over the spring into summer, and it won't get wet enough fast enough from diffusion alone to become a mold problem in spring. With poly it might take all year to dry, and during the summer the temps will be warm enough for mold to grow.

If the exterior foam is unfaced EPS (white, with distinct bead structures) or XPS (pink, blue, green, gray, but smooth consistent color) it's sufficiently vapor permeable that the sheathing can dry toward the exterior, and the polyethylene can stay. At 1" unfaced EPS will be over 2 perms, unfaced 1" XPS would be 1-1.5 perms.

^^EPS^^

^^ XPS^^

Whichever stackup it ends up being, take care that every seam is taped and every edge is sealed on the fan-fold and you'll be in pretty good shape.

The dry wintertime air in Denver helps your building-science conditions, but if you actively humidify the interior air it's a problem. Keeping the wintertime interior air below 35% RH @ 70F in winter would be prudent.

 

[Boozle]

Thanks for the help - I'm learning a lot.

I have a few new facts.

My exterior walls are only 2x4, not 2x6.

The exterior foam board is 1/2" and white, but doesn't really have a bead structure to it, and it appears to be foil faced. See picture.

Also, the gap between the interior stud wall and concrete wall varies. On two of my walls, the gap is about 1.5," and on the other two walls the gap is basically 0," making it impossible to slip anything behind it without relocating the framing.

With that said, how would you recommend I proceed knowing I want to incorporate the Roxul into the interior 2x4 cavity? This basement has always stayed pretty warm, so I'm not concerned with achieving optimal insulating characteristics, but I do want something that will insulate against noise, and most importantly have little to no potential for moisture and mold issues.

 

[Dana]

It's really sort of an off-white, looks very much like polyisocyanurate. Derated for Denver winter temps on an exterior side stackup it's good for about R5-R5.5 per inch, so at 1/2" you're looking at about R2.5-R3 dew point control at the sheathing which is almost good for about for the first R11-R13 of cavity fill for skipping the vapor retarders, but not for any more than that. Code min says R5 for zone 5 for 2x4 framing, which presumes a maximum of R15 cavity fill.

When you install an air tight class-II vapor retarder (such as the fan-fold XPS) at least 25% of the total R needs to be on the exterior of the vapor retarder, which means you'll be fine. The polyiso is good for at least R2, the fluff in the exterior studwall about R13, and the 1/4" XPS about R1, which adds up to R16. With R15 rock wool you'd be at R31-R32 total, so you'd have more than half of the total-R on the exterior side of the vapor retarder, which is great! If the fan-fold XPS is fully air tight, and no air can convect around it into the exterior 2x4 cavity you won't need an interior-side vapor retarder layer other than latex paint on wallboard to keep mold out of the R15 rock wool.

If you're obsessive about air sealing the XPS it will be a very robust stack up. Beyond taping the seams and foaming the edges, caulk or can-foam the XPS to the framing along the perimeter of EACH stud bay before installing the R15s. Caulk the bottom plate of the studwall to the slab with polyurethane caulk too.

 

[Boozle]

So, a new development. Two of my walls have virtually zero space between the concrete and the interior 2x4 wall, so I can't get 1/4" fanfold behind them without bumping out the existing framing.

On the other two walls that have a sufficient gap, it would be difficult to install airtight fanfold due to the way the wiring has been done (run though the gap behind the framing, but pulled inside the framing and stapled to every other 2x4).

What if I skipped the fanfold, removed the poly covering the exterior 2x4 cavity, installed the Roxul in the interior 2x4 cavity floor to ceiling, and then install Certainteed Membrain over the top of that, just under the drywall?

 

[Dana]

MemBrain does work, so where there is no better option you should install it. Use polyurethane caulk to seal the framing to the concrete in each bay and to limit convection behind/around the vapor retarder layer.

It would be preferable to install the fan-fold XPS where you can though. R15 rock wool's manufactured depth is 3.5", not 3.75". When installing it in a 2x4 wall with exterior sheathing you tuck it tight to the sheathing on the edges and corners, then tug it out gently to where it's just proud of the interior stud edges for a no-voids compression fit. That's fine for 3.5" nominal depth, but stretching it another 1/4" or more doesn't always work out so well. Unimpeded air channels that allow air to move freely is the enemy of both thermal and moisture performance.

 

[Boozle]

MemBrain does work, so where there is no better option you should install it. Use polyurethane caulk to seal the framing to the concrete in each bay and to limit convection behind/around the vapor retarder layer.

It would be preferable to install the fan-fold XPS where you can though. R15 rock wool's manufactured depth is 3.5", not 3.75". When installing it in a 2x4 wall with exterior sheathing you tuck it tight to the sheathing on the edges and corners, then tug it out gently to where it's just proud of the interior stud edges for a no-voids compression fit. That's fine for 3.5" nominal depth, but stretching it another 1/4" or more doesn't always work out so well. Unimpeded air channels that allow air to move freely is the enemy of both thermal and moisture performance.

So with either the fanfold xps or membrain route, I would want to remove the 6 mil poly that I currently have covering the exterior 2x4 framing, correct? Thanks for all the information!

 

[Dana]

Yes, get rid of the polyethylene. In the revised stackup the foil faced polyiso and the polyethylene forms a moisture trap. With woefully insufficient R-value to the polyiso on the exterior for dew point control at the sheathing the polyethylene is more likely to cause a problem than fix to one.