Rigid insulation overtop of fibreglass?

[rmarie5]

This is in a basement. Is it a good idea to put these rigid fibreglass boards overtop of the fibreglass insulation, and then staple plastic around it? (Pictured on left side)

 

[woodgeek]

In older houses, it is important to seal the rigid board around the edges to stop drafts. If you do that, the fiberglass doesn't add much benefit over the (sealed) rigid foam by itself. IN newer homes, the sill plate sometimes has a gasket and is well sealed, and the insulation is all you need, in this case the rigid foam is not adding much.

 

[woodgeek]

And for the record, stapled plastic sheeting is often a problem. It would not provide airsealing, and would keep the area from drying out if it ever got wet. Canned sprayfoam would be conventional, some airsealing tapes might be ok too.

 

[midwestcoast]

Poly vapour barriers (plastic sheeting) are not recommended in basements.
XPS foam boards (the kind shown) can provide a good air barrier and vapour retarder if all edges are sealed with caulking or canned foam. But you want the foam on the other side (cold side) of the fiberglass to give air sealing right at the rim joist & prevent condensation inside the fiberglass.
So if you put the foam in first, seal all edges with caulking or canned foam, then replace the fiberglass you have a well insulated rim joist.
In Ontario you want that foam to be at least 2" thick (maybe more depending where in ON you are). Multiple layers are OK to build up 2".
Also you probably have to cover the foam with a fire barrier. Check you local codes on that to see what's required.

 

[semipro]

"Stapling" plastic sheeting up won't provide much of an an air barrier.
As has been said, you probably don't want a vapor barrier here. A vapor retarder is more appropriate.
You mention fiberglass board but what's shown doesn't appear to be fiberglass but its hard to tell from the photo.
BTW, I've recently found that Roxul batts work very well in this application. Once you seal the rim joist with foam board and/or spray foam you can insert the Roxul. The stuff is cut like bread, even using a serrated edge knife, and then fit snugly in place.

 

[moey]

Id pull out the fiberglass and put two layers of foam board seal the edges with spray foam and put up a piece of drywall anchored on both sides on top. Theres a decent article here on it http://www.homeconstructionimprovement.com/rim-joist-insulation-insulating-options/

 

[Laurent Cyr]

What Midwestcoast said. I am currently finishing my basement. I have installed rigid 2" inch foam (pink) insulation, used cans of spray foam to seal the edges, then added the pink fibreglass over top. DO NOT add plastic over that, or anywhere else in the basement. In the present case, the foam acts as the vapour barrier, and you don`t want to add plastic over that as it will create an area where the vapour will accumulate. In your search on internet, you will see a lot of different methods to insulate basement walls and sills in Canada. Probably the best method is spray foam. Look at the costs, and then decide what you want to do.

 

[TradEddie]

I put the xps against the rim joist, sealed the edges with caulk or "great stuff" and I put Roxul over top as a fire barrier. It has worked great, and I've occasionally removed the Roxul to check for any unforeseen problems, and none so far.

TE

 

[semipro]

I put the xps against the rim joist, sealed the edges with caulk or "great stuff" and I put Roxul over top as a fire barrier. It has worked great, and I've occasionally removed the Roxul to check for any unforeseen problems, and none so far.

TE

No matter how much personal protective gear I put on, working with fiberglass made me miserable.
I recently started using Roxul in our basement and love working with it compared to fiberglass.
You can cut it with a serrated knife as if it were bread. Once cut it can be placed tightly in openings like those between joists. I've even been able to cut it to fit well in the triangular spaces in floor trusses.