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-   -   foam board between joists (http://www.diychatroom.com/f2/foam-board-between-joists-83029/)

fubar79 10-04-2010 12:22 PM

foam board between joists
 
Hey

I used this site a lot last winter when I started finishing a bedroom in my basement. It's been a great help!

But now I'm second guessing something I did. I used Roxul in between the stud walls and used a 6mil Vapour Barrier. In between the rim joists I used 1 or 1.5" foam board against the concrete, and used expanding foam to fill the gaps around it. THEN I used Roxul to increase its R value, and sealed that with the 6mil VB and accoustic seal. Leaving me with: concrete then foam board, then roxul, then 6mil VB.

I guess I created a double vapour barrier didn't I? Is the solution to just cut out the VB?

The VB for the walls does have a layer of accoustic seal for itself, so it shouldn't be affected.


Cheers!

1910NE 10-04-2010 09:41 PM

well, neither Roxul of ridged foam is going to absorb moisture, so thats not a problem.

fubar79 10-05-2010 09:32 AM

True, but the joists are wood.

steveel 10-05-2010 10:13 AM

"I guess I created a double vapour barrier didn't I?"

if I understand right, your studs are up against the concrete ready to grab all the moisture they can and wick it into the wall cavity where the poly will trap it. To me that sounds like a single vapor barrier, and on the wrong side of the assembly.

If you're concerned, you could open the wall down in a couple places down near its base to check for mold on the studs. A moisture meter probe would be useful too, perhaps.

fubar79 10-05-2010 10:26 AM

My issue is not with the stud walls. I built them out an inch from the concrete, then used Roxul in between studs then a 6mil VB, then drywall.

My concern is in between the rim joists where I used the pink foam board ( extruded foam?) It's tight to the concrete and the perimiter sealed with expanding foam. Since it has a low R value, I then added Roxul to fill the rest of the void, and then a VB. My concern is that using the foam board and then a 6 mill VB I have created a double VB in between the joists.

So I think I could just eliminate the 6mil VB in between the rim joists leaving the pink foam board tight to the foundation, with a bit of Roxul for added R value.

steveel 10-05-2010 11:18 AM

sorry, for some reason I thought you first said the foam was between studs. Probably just me being a knucklehead.

So.... you're talking about the floor of this bedroom, right?

fubar79 10-05-2010 11:31 AM

No, i'm talking about the area between the rim joists. Where the joists lay on the foundation wall.

From what I've read, for Ontario climate, using rigid foam board then Roxul is ok, just not sure if a 6 mil vapour barrier is needed?

steveel 10-05-2010 12:17 PM

ach so! sorry I didn't get it from the start. I really don't know and hope you get a real answer. My best guess is (a) you didn't need the poly (b) but don't bother tearing apart your work unless there is a problem and (c) if there are no smells or water stains a moisture meter may help you monitor the situation and head off any problems when they are still small (assuming its a problem at all and maybe it isn't)

But like I said, that's just a best guess. I'll be interested in others replies.

Gary in WA 10-05-2010 01:12 PM

Air spaces are not good; http://joneakes.com/jons-fixit-datab...basement-walls

The poly in a cold climate: http://www.buildingscience.com/docum...ent-insulation

Gary

fubar79 10-05-2010 04:26 PM

:eek:

The more I read in regards to finishing my basement, the more confused I get. I was not able to get my answer from either of those links. :icon_sad:

I'm now worried since the room that I did finish, i tried to leave a gap between the concrete wall and the Roxul... according to your first link, that is not a good idea. :furious:

steveel 10-05-2010 05:21 PM

Quote:

In between the rim joists I used 1 or 1.5" foam board against the concrete, and used expanding foam to fill the gaps around it. THEN I used Roxul to increase its R value, and sealed that with the 6mil VB and accoustic seal. Leaving me with: concrete then foam board, then roxul, then 6mil VB.
Rim joist: runs along top of sill plate on top of concrete wall
Ceiling joist: crosses the room

If i understand right you laid the foam flat on top of the concrete wall, sealed it in place with blowfoam, then set some Roxul on top of the foam and pushed up against the exposed part of the rim joist in between a pair of ceiling joists. Then you blocked that area with poly.

Am I getting it now?

But then, maybe you have a foundation wall type I am unfamiliar with.

Tom Struble 10-05-2010 05:30 PM

you should be ok if you do have a double vapor barrier in the area of the rim joist its just a small area that can diffuse thru the bottom and top of the rim

fubar79 10-05-2010 06:19 PM

steveel, you are close but I guess I used the wrong terminology.

I put the rigid foam board vertical up against the rim joists, in between the ceiling joists.

tomstruble; for the rest of the basement should I avoid the 6mil vapour barrier in this area? Would the rigid foam board be considered a vapour barrior?

steveel 10-05-2010 06:59 PM

gotcha. so you're air sealed very well, and the worry is condensation in the rockwool behind the poly? If you have enough foundation exposed on the outside, you can keep an eye on splashing and wicking from outside (including winter) and keep an eye on sources of moisture on the inside, and I bet you'll be OK

Tom Struble 10-05-2010 07:05 PM

1'' foam without a foil faceing is not impermiable,you should be ok


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