Finishing Basement-Insulation

[jjmjones]

New to this forum so forgive me if I'm asking about something that's been covered many times in the past (I did search,have read the Building Science Corp suggestions on insulating/finishing basement but still have a question).

Background- trying to finish a walk-out basement. The slab is 4" on 6" of stone, no vapor barrier and no insulation. I have external footing drains and have no obvious signs of water infiltration. My plan is to finish the floor using XPS rigid foam and two layers of 1/2" ply-floating on the slab.

Question-how much foam should I be using? The lit. I read suggests anywhere from 3/4" to 2". Is more counter productive? I gather that considerations when I insulate a wall are different from a floor.

Thanks- jj

[frenchie]

Re: Finishing Basement-Insulation

There's a thread over in finish carpentry that mentions this technique... here it is:

http://forums.jlconline.com/forums/s...ad.php?t=37987

All I know is that ResCheck likes a lot of floor insulation; I was surprised at how much of a difference it made. And I can't think of a real disadvantage to laying down more... I'd put as much as I could, within reasonable limits (headroom clearance).

[jjmjones]

Re: Finishing Basement-Insulation

Thanks for the heads-up- there appears to be much variation re how to handle finishing on top of a concrete floor.

I too thought more insulation was better but had read where it should not be over 3/4" to allow for moisture to migrate through the insulation and be dispersed through the living space.

jj

[frenchie]

Re: Finishing Basement-Insulation

Where did you read that? It sounds completely back-assward. Why would you want to let ground moisture seep into your living space?

Maybe if you had a vb under the slab, then it would (kind of) make sense, to avoid a moisture trap between two vapor barriers... but even then I wouldn't be worried, the slab would act as a buffer, I'd expect.

Anyhow, academic: you don't have a vb under the slab. I would want at least 1 1/2" of foam, then, so it WILL be a vapor barrier, and keep any gound moisture out of the living space.

You shouldn't run into mold issues unless you create a moisture trap, for example laying a poly vb and then foam on top of it.

[jjmjones]

Re: Finishing Basement-Insulation

Here's the document I've been reading

http://www.buildingscience.com/docum...term=basements

It's report #0202 at buildingscience.com

Not sure how 1"1/2 of foam becomes a vapor barrier

[WarriorWithWood]

Re: Finishing Basement-Insulation

JJ,
That's a great site and one I haven't read to boot, Thanks

[WarriorWithWood]

Re: Finishing Basement-Insulation

Not sure how 1"1/2 of foam becomes a vapor barrier

If you use foil faced Polyisocyanurate it is a form of vapor barrier AFAIK.

[frenchie]

Re: Finishing Basement-Insulation

Even unfaced: less than one inch is semi-permeable, more than one inch is semi-impermeable.

Look at the roof assembly on Lstiburek's Aspen house, the only vb is an inch of XPS behind the sheetrock.

http://www.buildingscienceconsulting...iles/aspen.htm

1-1/2 was off the top of my head, and reflects my over-cautious nature.

Jim, it's late enough for me that I'm cross-eyed, and I haven't read it closely yet, only skimmed... but I'm not seeing the part where that document recommends using less than 3/4" to allow vapor diffusion inside...

In fact, they say their best performing wall was using 3.5" of foam... and one of the details shows a full inch on the floor with 2" on the walls... the only reference to drying towards the inside that I'm seeing is where they explain FG's a bad idea... ?

[jjmjones]

Re: Finishing Basement-Insulation

Page 11 first bullet generally talks about needing to be able to dry to the interior on below grade applications.
My concern is having enough insulation to keep floor warmer but not too much to prevent any drying to the interior that might be needed. I do see a figure with 1" on floor.

Thanks for your help- Skippy

[frenchie]

Re: Finishing Basement-Insulation

Ah - see it - that was a little confusing... (page 11 according to the pdf reader is page 10 by the numbers on the page).

Okay... you need to take a look at Lstiburek's discussion on vapor-retarders... and then forgive me for using "vapor barrier" and "vb" when I should properly have been saying "vapor-retarder".

http://www.buildingscience.com/docum...nce/main_topic

So, all they're saying, on page 10/11, is to avoid any completely impermeable barriers. No poly sheeting, no vinyl wallpaper, no foil-faced foam.

Not sure how much foam it would take to become a true vapor-barrier (less than 0.1 perms)... but it would have to be a lot more than 2" or 3".

If it helps any... Figures 13 & 14 have 2" of foam under the floor, as well as on the walls; and on page 9, they talk about 3.5" as their best-performing wall.

Go ahead an lay down as much foam as you can afford - just make sure none of it is foil-faced.

[WarriorWithWood]

Re: Finishing Basement-Insulation

Hey JJ,
Can you fill in your profile please.

[greentree]

Re: Finishing Basement-Insulation

If your basement is a walkout how are planning on ignoring the existing sill height of your doors?

[WarriorWithWood]

Re: Finishing Basement-Insulation

If your basement is a walkout how are planning on ignoring the existing sill height of your doors?

And on that line of thinking, what about the first tread on the stairs?

[jjmjones]

Re: Finishing Basement-Insulation

What I had planned for was to use a floor system of 2x4 PT 16"OC shot into the slab with 1-1/2" foam between the PT with 3/4" T&G ply.
So I set me door sills and stairs to accommodate this and I added 3/4" for finish flooring.

I was later convinced to go with rigid insulation with two layers of 1/2" ply. So I think it will all still work.

jj

[frenchie]

Re: Finishing Basement-Insulation

Warrior - re jj's profile...

Mike's made it real clear that this particular forum is open to anyone. Rob & I got reamed for profile-policing... just thought you should know.

FWIW, the remnants of that thread:

http://forums.jlconline.com/forums/s...ad.php?t=35596