Water content of closed cell foam

[LindaWhaley]

I ran a scenario of a double stud wall that ended up having a very high water content in the closed cell foam layer. The layers modeled included the hardi plank, furring strip, felt paper, plywood, foam, densepack fiberglass, gwb. Perms for paint was added to both sides.

I saw on another thread regarding fiberglass that "Cellular glass is considered to be hygrically inert. It has completely closed gas cells (open porosity=0), it does not take up or transport any water (neither liquid nor vapor). It also has no defined moisture storage function, so it is quite irrelevant what the w_max is, but WUFI needs some number in that place, and that number was probably just as good as any other.

Since the moisture storage function for the cellular glass is left undefined in the database, WUFI uses an internally defined default moisture storage function which depends on w_max and thus on the porosity. But WUFI only uses this default function because it needs a well-defined moisture content for all materials, even for materials which cannot take up any water at all. Any water contents reported by WUFI for those materials are thus spurious and should be ignored. So again, the precise number for porosity used in this case does not matter."

Does this apply to closed cell spray foam as well? Here are the film results of the wall section I ran, the component assy info, a WUFI BIO of the foam/plywood intersection and a picture of the wall after the foam was applied but before the densepack fiberglass was sprayed in:

http://existingresources.com/?page_id=1142

Would there be a greater benefit to modeling this wall in WUFI 2D instead of Pro since the spray foam layer is used as an air barrier and it completely isolates the interior of the structure from the external 2x4 and plywood wall (the plates are also coated with foam from the inside)?

Thank-you,

Linda Whaley

[Thomas]

I saw on another thread regarding fiberglass that "Cellular glass is considered to be hygrically inert. It has completely closed gas cells (open porosity=0), it does not take up or transport any water (neither liquid nor vapor). It also has no defined moisture storage function, so it is quite irrelevant what the w_max is [...]
Does this apply to closed cell spray foam as well?

Hi Linda,
this does not apply to spray foam. The spray foam (I suppose you are referring to 'Sprayed Polyurethane Foam, closed-cell') may have closed cells which greatly reduces vapor diffusion through the foam, but the polyurethane material making up the walls between the cells is not completely impermeable to vapor diffusion. In fact, the material database lists a mu-value of 89 for this foam, so you will have some moisture intruding into and through the foam layer by means of vapor diffusion.

In the cellular glass, by contrast, the cells are closed and the glass walls between the cells are practically completely vapor-tight. That's why no water content is to be expected in the cellular glass under any circumstances.

Here are the film results of the wall section I ran, the component assy info, a WUFI BIO of the foam/plywood intersection

Apparently the plywood becomes very wet. Either because of rain water which can not dry out quickly enough, or because of vapor coming through the foam from the indoor side, condensing at that cooler location.

Would there be a greater benefit to modeling this wall in WUFI 2D instead of Pro since the spray foam layer is used as an air barrier and it completely isolates the interior of the structure from the external 2x4 and plywood wall (the plates are also coated with foam from the inside)?

The foam will not completely isolate the two parts of the construction from each other because it has a low but non-negligible vapor permeability.

Even if it did, I do not see why this would mean that a 2D calculation had any advantage.

Regards,
Thomas