About to start my basement - vapor barrier and insulation Q's.

13 years ago

More than just that really - a quick sanity check on what I'm doing is appreciated, as I start I want to be sure I've covered enough with my contractors and haven't missed anything.

Background - my currently unfinished basement in a circa 1920's NYC two family brick home. Have agreed with the contractors to start next week, with the following task list:

- Move some plumbing around to accommodate new floor plan

- Level existing concrete floor

- Install porcelain tiles

- Wire electrical outlets and ceiling fixtures

- Insulate and frame foundation walls

- Vent and enclose boiler room (this area was hideously out of code until now)

- Add kitchen and bathroom

I'm stuck on two items in particular - whether I'd like to add a recirculating pump and baseboard heating, and what type of insulation should be used on the surrounding walls.

I still have several bats of foiled fiberglass insulation left over from our renovation on the first floor a few years ago. I haven't had any moisture issues upstairs, but a basement is a new animal. From a few searches, I've seen that some people don't recommend the use of fiberglass bats in the basement, because they don't abate the moisture. In others, I've seen it recommended, but only with the use of a vapor barrier.

I've decided that I want to seal the foundation walls prior to beginning, so I'm off to Home Depot for some Drylock. I had applied Thoroseal to half the basement walls sometime back, and haven't had any issues with it, so I'm not sure how much of a concern moisture will be anyhow.

Is there a reason to skip the fiberglass bats and go with extruded polystyrene (foam board insulation) instead? I see that Dow makes a product called Wallmate, not sure whether that makes sense instead (and I'm sure the cost will be considerably different). If I go with the bats, would they need a vapor barrier, or is the foil side sufficient?

Finally - anything else I should consider asking these guys to do as they go on? I've already asked them to add a drain to the boiler room (from what I can tell, there was one prior, but it hasn't worked since we bought the house). I haven't spent much time in finished basements, and throughout our home ownership there hasn't been any issues down there, so I'm in the dark, any advice is greatly appreciated.

Thanks!

-John

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Comments (5)

worthy
13 years ago
All you need to know about basements from the leading building scientists is available at the link below.

The appropriate choices for the wall are extruded polystyrene (XPS), expanded polystyrene (EPS) or spray closed-cell foam (ccf). Fibrous insulation is only appropriate with a hard to find Membrain vapour barrier. Fibrous insulation with a conventional poly vapour barrier is the absolute worst system. Wallmate, which is XPS, is an excellent system if you don't mind surface mount wiring.

Tiling on a basement concrete floor will tend to crack at pressure points. I use an uncoupling membrane, the most well-know of which is DITRA. You might want to consider floor systems using XPS and ply, if you have the headroom and the budget.

Here is a link that might be useful: Building Science Corp. on Basement Insulation
giantcrazy
Original Author
13 years ago
Thanks worthy!

Just to recap - looks like I should go with XPS directly applied to the foundation walls, nailed/screwed on, and then put the framing over it (metal studs/tracks)? There isn't a vapor barrier required afterward?

Also - would you recommend a surface coating for the cement walls before installing the XPS? Something along the lines of a Drylock/Thoroseal?

I'm sure this will cost me more as far as materials go, but would you say it is more or less labor intensive than installing fiberglass bats onto the framing? I don't mind paying the materials, but my labor costs are already high (I'm in NYC, where everything seems to be extra), and I'd hate to hear some crazy estimate for what seems to be a straightforward install, might even be something I'd do myself first.

Regarding the DITRA - given that I have no cracks on my cement slab at the moment, would you say it's imperative or just a good idea to use the DITRA? Less concerned about cost here, and more about the space. There's not a lot of headroom in the basement, and even if it's only a matter of a couple of inches, I hate to give up any more.

Thanks for the help!
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Yes, I believe this is a perimeter drain. We also have a sump pump, but I've never seen any water in it. We are in a mixed climate (central NJ). The basement walls only extend about 18" above grade. I have another question. What about insulating only half of the wall? My foundation was constructed with poured concrete half way up, then the top half is cement blocks. The framed wall is about an inch away from the foundation at the bottom, but 1/2 way up, the wall is closer to 4" away from the blocks. I could easily put insulation on the top half of the wall without worrying about it touching the foundation. Would this be worth while? Also, I was thinking about using Drylock to perhaps help with the humidity. Without removing the wall framing this is going to be a real hassle, so I don't know if it is worth while. Would I do only the concrete block part of the wall, or is this supposed to be applied to the solid concrete lower half of the wall also? Thanks guys for your replies.
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A minimum thickness of 1"-1.5" XPS would have been more effective in your climate. It is very possible that the moisture you are seeing would not be condensing if a tight air-barrier, such as taped sealed drywall, were in place over the fg. To be more certain of forestalling moisture problems, you can either remove the stud wall to increase the thickness of the XPS or use a moisture tolerant insulation in the stud bays--either more XPS or less costly expanded polystyrene (EPS) which, depending on density, has an R Value of up to 3.92 per inch. (See Table 2, example 4 in the linked document below.) Adding a poly vapour barrier all but guarantees problems. As Yost and Lstiburek point out in the linked publication from Building Science Corp., researchers have realized that "a vapor barrier (usually polyethylene) on the interior side of the basement wall assembly inhibits drying of the wall more than it prevents wetting of the wall." Here is a link that might be useful: Basement Insulation Systems
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First, the tuck tape. Look for "sheathing tape". It's usually sold in the box stores, sometimes it's red colored. You could use Tyvek tape as well. The vapor barrier is a bit tougher to diagnose. Your exterior foam, "expanded polystyrene" is usually quite leaky with regards to moisture transmission and is not a strong vapor retarder. "Extruded polystyrene" is a much tighter product, and it could indeed be a vapor barrier, probably a Class II barrier. If the exterior foam is indeed continuous on the walls and well detailed, then it might be best to omit the interior poly and just use the Roxul. Your walls may take in some vapor over time, but the vapor should also transmiss back into the bathroom during drying cycles. The biggest problems occur when you have two barriers and one or both are poorly detailed. Over time moisture gets in but has difficulty getting out. Thus the "sandwich" that you referenced. The "imperfect" in your system is the exterior foam. I'm a proponent of foam, but only if it is the right type and the proper thickness. With your exterior foam probably being a Class II on the exterior side of your wall, your best bet is probably to omit the interior poly and simply use other methods on the interior walls. You can use something like Hydroban in your shower (if the shower is on an exterior wall), it's a waterproof barrier but does allow vapor to transmiss through it. For the remainder of your walls, simply use two coats of latex paint over your interior drywall. Latex paint is classified as a Class III vapor retarder. As such, it'll slow moisture vapor transmission that wants to get into your wall, but if any does, it'll allow your wall cavities to dry out during a drying cycle. It's an imperfect solution for an imperfect situation. I'd be completely confident in my reply were your exterior foam thicker than it is and if I knew it was well-detailed. My concern is the interior side of the foam reaching dew point temperatures in winter and any moisture vapor within the framing cavity condensing on the interior surface of the foam, between the foam and the sheathing. That may not be a problem. But it is a consideration, especially since I tend to go towards the conservative side with an "over the internet diagnosis" such as this one. I'd certainly do more research if you can, you might get better information than what I'm offering. You are addressing moisture generation within the bathroom. That's good. Other stuff: 1) I do sometimes insulate interior walls, Roxul is good for that. 2) A simple yet effective sound deadener behind your washer and dryer is nothing more than a second layer of drywall over the first. 5/8" is better than 1/2" due to the added mass. Beyond that you can go fancier: 2a) sandwich a 1/2" sheet of homasote between the two layers of drywall. 2b) Use RC clips and metal channel between the studs and the drywall. 2c) Use green glue between the two layers of drywall. 2d) If you do research you'll probably see references to mass loaded vinyl, but I usually use that only when doing fully integrated sound deadening room "systems". I think you'd do better (and save money) with just using drywall.
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At this point, the vapor barrier against the exterior foundation/basement wall is probably a good detail for you, moisture wicking into a basement through the exterior walls can be a big source of moisture in basements. Many current basements would use a low permeability foam insulation at this location against the outside wall. The insulation has other benefits in regards to changing the temperature profile through the wall section, but minus this knowledge, the vapor barrier here is an overall positive in my view. The interior vapor barrier behind the drywall is unwanted, but let's hope that the exterior vapor barrier does its job and keeps a major source of moisture out of the wall assembly. At this point you want the wall assembly to be able to dry to towards the interior. The general leakiness of the house is a good thing in terms of lowering indoor winter humidity. Other suggestions would be to keep the interior basement walls leaky - don't gasket switch plates or outlets, use vapor permeable paints on wall, no semi-gloss paints, etc. and no wallpaper. That's probably about all you can do. And do keep the humidifier running. I wouldn't get into a fight with your spouse at this point. At the moment you don't have mold. Keep a close watch and at some later date, maybe make an exploratory cut in the drywall to check on things. Also educate yourself, the Buildingscience.com web site has a lot of information about wall assemblies and proper vapor control.
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worthy
13 years ago
The most expensive but best insulating process is closed cell spray foam. For a large space, with ten foot ceilings it might be no more costly than board insulation. XPS is more labour intensive. For NYC, I'd use t&g 4'x8' x1" boards; if you can't find those, then use 2'x8'x1" t&g. Seal all the joints with building tape--Tuck and Tyvek and two popular brands.

I've not used sealers, but a building writer whose information I've always found to be sound, swears by Drylok. (I assume this is not a rubble or brick foundation.)

The advantage of steel framing is it's neater to put up (no sawdust) and there are no subsequent nail or screw pops. However, as it's sensitive to moisture, I'd have the baseplate resting on strips of XPS not only to create a thermal break but to keep it away from any minor floods that may occur. Since one inch of XPS is R 5, I always add high density fg batts, adding another R 15.
No vapour barrier is necessary, though if this is a permit job, the inspector may insist on one. It wont' hurt anything. (See Building Science links in my first post.)

After two years, a number of tiles cracked at pressure points on a brand new home I built. And that's with the tile on top of at least 10" of reinforced concrete and power tamped gravel. Since then, I figure it's a lot cheaper in the long run to use an uncoupling substrate such as Ditra. (If you dont', be sure to keep a lot of extra tiles just in case.) Ditra and cement add maybe 3/8" extra to the flooring height.

NYC labour. My condolences!
worthy
13 years ago
Here's why you don't want to use fg behind a vapour barrier. Moisture gets trapped from after being driven inward in the summer. It then nurtures the mould caught in in the fiberglass.

Mould Behind Basement Vapour Barrier Source: Building Science Corp.
giantcrazy
Original Author
13 years ago
Thanks worthy - I saw those images from the links you had provided, and that was enough to convince me I needed the XPS. The contractor and I are meeting later, I'm hoping that he doesn't go overboard on the labor costs (and thanks for the condolences - apparently, contractors here haven't heard anything of a financial crisis or recession).

I have one other question - what of the areas that are near plumbing that is too close to the wall to fit the XPS behind them? Is it sufficient to use spray foam insulation? If that's the case, where would I be able to buy it in quantity? I've seen smaller cans of expanding spray foam, but I'm guessing that there must be something available in building material size that allows for closing up larger areas?

About to start my basement - vapor barrier and insulation Q's.

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About to start my basement - vapor barrier and insulation Q's.

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