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

giantcrazyJuly 25, 2010

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.



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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

    Bookmark   July 25, 2010 at 3:31PM
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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!

    Bookmark   July 26, 2010 at 12:09PM
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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!

    Bookmark   July 26, 2010 at 4:56PM
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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.

    Bookmark   July 26, 2010 at 5:04PM
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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?

    Bookmark   July 28, 2010 at 11:15AM
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