I want to make it warmer, but how?

May 17, 2009 7:03 AM

I'm in the midst of a bathroom remodel and about to insulate and close up the walls. I'm a little confused on the best way to insulate the exterior walls of my 80-year old house. A diagram of the existing wall can be seen here: http://www.flickr.com/photos/ctracey/3538183459/ I've checked a lot of online resources and books, but few of them deal with a wall construction such as this.

From the outside, there is: a layer of brick, a 1/2" gap, a layer of tar paper placed over, some 1x10 pine boards, and finally 2x4"(ish) studs. I am using hardibacker as the wall material (tile and plaster on top) and denim insulation. Everything I have read says to put a layer of tarpaper or other moisture barrier behind the hardibacker, but that may only apply to tub installations where water may seep through. The tub is all on inside walls.

So, I guess the question is whether or not I should place the hardibacker on top of another layer of tarpaper or not? If I did, would that create a trapped layer of moisture in the walls.

posted by buttercup to home & garden (7 comments total)

You need to put your insulation into all the spaces between the the studs (you may need to custom fit it if the distances between them aren't equal). Then you need to cover all the walls with a layer of heavy polyethylene plastic. Check what thickness is code for your area. You staple it into place as you go. Then cover al the seams, staples and areas around protruding pipes and electrical outlets with red plastic tape called "tuck" tape (not "duct" tape). Then your drywall or concrete backer goes over that.

The idea is that you have a barrier on the warm side of the insulation to keep the moist bathroom air from getting into the wall, cooling off and condensing within the insulation.
posted by bonobothegreat at 8:03 AM on May 17 [4 favorites]

..and if the ceiling is below an unheated attic, it should be treated as an exterior wall as well.
posted by bonobothegreat at 3:58 PM on May 17

I haven't worked with denim insulation, but from what I see online, it does need to be protected from moisture. Bonobo is correct that the primary moisture concern is vapor pressure from the interior which will be warmer than the cavity.

You also have a legitimate concern about locking moisture in the wall. If you are set on using the denim insulation then you might consider the product MemBrain. If you are not set on the denim insulation you might look at damp applied or densepack cellulose (similar to the denim but with recycled newsprint), which has the benefit of offering greater Hygroscopicity - defined in this pdf as capacity of a material to absorb and release water as a gas (water vapour) from and to the air as the relative humidity of the air changes. Cellulose insulation installers may warrant your condition without the addition of another moisture barrier.
posted by meinvt at 3:59 PM on May 17

Am having the kitchen redone on our 1850s house. I have great confidence in the carpenter. He put in fiberglass battens, then covered the walls with plastic sheeting, which was sealed with tape (joints and around outlets). In a few places, he used some foam-in-place material, then the wallboard went up.
posted by RichardS at 4:47 PM on May 17

Condensation inside the insulation/cavity is your greatest concern. Installing a vapor retarder on the warm side of insulation prevents water vapor diffusion into the cavity. The warm side is going to change from season to season - so , what to do? Consider that in a bathroom, the major problem is going to be warm moist air created when showers are taken. Without a vapor barrier to prevent moisture from migrating through the wall, you're going to have condensation inside the insulation.

Faced insulation can meet both R-value and vapor retarder requirements. When unfaced batts or blown-in batts are used, a vapor retarder is usually provided. The denim insulation does not have a vapor barrier included, so you need to consider that when you select insulation.

For vapor barrier, The closer the perm rating is to zero, the better the vapor barrier. A material with a perm rating of 1 or less is considered a vapor barrier. Tar paper or the Tyvek type barriers are pretty good answers. Even some paints provide a good perm rating.

You bring up a situation where it's a bit of a conundrum. If you make the assumption that the moisture-laden air is almost always on the inside of the wall then that's where you'd like the vapor barrier. However, it's not always the case in the summer when it's hot and humid outside. The biggest concern would be holding moisture inside the cavity and batts and allowing them to become saturated. Fortunately, you're not looking at creating the vapor barrier throughtout the house, so I think providing the extra layer of vapor barrier in this one location isn't going to create a problem and will help your insulation to stay dry.

Wet batt insulation doesn't insulate and allows mold and rot to happen. That's one reason we're seeing a big change in the approach to insulating commercial construction and some forward-thinking residential construction in the humid warm climates. The newer approach is to use rigid sheets of polyiso insulation on the outside of the studs. The dewpoint/condensation point occurs in the middle of the rigid insulation. But, wait, isn't that bad? No - since the rigid insulation doesn't allow moisture instrusion.

If you're hooked on learning more about housewraps, here's a good article.
posted by mightshould at 6:57 AM on May 18

You definitely have a non-standard wall design. And you're right about not putting in another moisture barrier and trapping moisture inside the wall. The current setup is probably pretty good, in that it allows moisture to migrate through the brick but has a vapor barrier that prevents it from coming through to the inside. Are there weep holes or something similar at the bottom of the walls to allow moisture to escape?

There's no need for the tarpaper behind the hardibacker if it's not on the tub/shower surround. If you're just doing decorative tiling on a wall you should be fine. Just make sure your bathroom is properly ventilated to the outside and use the fan religiously.
posted by electroboy at 7:23 AM on May 18

Forgot to add, don't use the denim insulation. You're asking for trouble in this context. Unfaced fiberglass or foam board is preferable.
posted by electroboy at 7:26 AM on May 18