Vinyl Siding With Foam Insulation

[Brian A. MacNeish]

Originally Posted by prussell

I would be too, vinyl siding is not a water tight product and neither is the foam that is installed under it.

Every building needs a vapor/moisture barrier under the siding, no matter what type of siding is used. In some cases, especially in coastal area's a secondary barrier is wise to use such as felt paper.

Expanded foam insulation itself holds water and essentially becomes a resevoir for water bypassing the siding. On sunny sides of the house, this water heats, turns to vapour and can move though the permeable sheathing membrane into the wall system due to higher vapour pressures. I have seen the sheathing and sometimes structural elements rotted. In our area, the rain screen principle has been added to the provincial building codes in 2004 to allow water to drain and not be held in and behind porous sidings.