[zander123]

Note image. - it appears to be 3/4" foam insulation with foil on either side.
Anyone know what it is? Specifically- is it likely to be a vapor barrier?

You can see where I got it from in the image. Behind the 22 year old builders fiberglass I pulled out of the exterior stud wall which sits on top of the concrete wall in my walk-out basement (this part above ground). When I cut it out- brick can be seen on the other side. Home is in Ontario.

Builder did have a vapor barrier inside (also as seen in image) so presumably the foil should not be an external vapor but I don't discount the possibility of something being done wrong the first time around. Some of the fiberglass bats do show signs of blackening..

Thoughts?

 

[Windows on Wash]

Foil is a Class 1 Vapor retarder in this case.

http://buildingscience.com/documents/information-sheets/building-materials-property-table

http://c.ymcdn.com/sites/www.polyiso.org/resource/resmgr/Technical_Bulletins/PB_PIMA_PerformanceBulletin_.pdf

 

[Bud9051]

Hi zander,
Yes it is a vapor retarder or as WOW states, a class I vapor retarder. And your concern about two vapor barriers is justified. Discoloration on fiberglass insulation can be mold or it can be an accumulation of dirt where air is leaking in or out. But I don't see an easy solution. 3/4" is not thick enough to permit leaving the Vb on the outside while eliminating the VB on the inside and it doesn't sound like removing the exterior Vb is an option.

If you remove the Vb on the inside and maintain as low of a relative humidity during the winter you may be able to get by. Check local codes to see what they require.

Bud

 

[zander123]

Thanks for the information - sure glad I took a closer look at that. If understand correctly:

What I have is foil-faced Polyiso. It is a vapor barrier being a Class 1 vapor retarder so technically everything to the interior *should* be allowed to dry to the interior. The builder was wrong to install a second vapor barrier on the stud wall effectively trapping moisture in the middle - likely why there are a few blackish spots in the fiberglass.

Moving forward, sounds like my best option is to remove the fiberglass and replace it completely with either roxul or 2.5" XPS rigid foam (in either case no additional vapor barrier needed.

Much appreciated!

 

[Bud9051]

I mentioned that 3/4" is not thick enough and I should explain why. With an air permeable insulation, most batt styles, on the inside the rigid on the outside needs to be thick enough to prevent the inside surface from cooling below the dew point. Related link below.

Now, this involves the ration between the batt insulation and the rigid insulation so adding too much batt insulation can be as bad as not having enough rigid. You mentioned adding more rigid and that would indeed resolve the problem.

http://www.greenbuildingadvisor.com...rmal-mass&utm_campaign=green-building-advisor

Bud

 

[zander123]

Scratch the above - I wrote it before reading Bud's reply. Thanks Bud and Windows for the information - sure glad I took a closer look at this.

Sounds like the best alternative is to replace the fiberglass with 2.5" rigid foam within the studs, seal them using foam can and then leave open. ie like this:

http://www.homeconstructionimprovement.com/walk-basement-wall-insulation/

Not a perfect solution (thermal transfer would still occure via the studs) but likely better than nothing..

 

[Bud9051]

Wood studs are about r-1.2 per inch so better than metal studs. My preference with the remaining gap would be to custom fit one of the batt insulations in there to reduce air circulation. But the added rigid by itself will be a big improvement.

Bud

 

[Gary in WA]

Welcome to the forums!

Could you remove the cavity ff foam or is it on the outside of the studs? Is there sheathing outside the foam? Is there a WRB or is the foil acting as that as well? Describe the wall make-up, please.

Gary

 

[zander123]

No, the ff foam is behind the studs (no gap between).

From the inside working out: plastic vapour barrier, Studs+fiberglass, ff foam, what seems to be a black paper like material (WRB?), exterior brick

 

[Gary in WA]

Then as said earlier, no plastic inside with the ff foam= 2 vapor barriers. Too bad as cold climates can use the solar radiant heat in winter, rather than reflect some away with the foil-faced..

Gary

 

[Gary in WA]

Further thoughts; a vapor barrier is required per code, on the inside for your location (as the exterior foam doesn't warm the cavity above dew-point). IMO, and a lot cheaper than adding rigid foam board to the cavities is using a "smart vapor retarder" on the stud edges after filling cavity with Roxul. DO NOT install fibrous insulation without an interior vapor barrier.

Patch the foil section back in and tape all cracks with foil tape or you could experience a lot of moisture through the gaps from "solar drive" in the brick to inside. The black builders paper is very permeable to moisture.

What do you have planned for the basement concrete walls, if finishing now?

Gary