Moisture on underside of roof deck
jtlloyd
| Posted in General Questions on
So a client of mine removed existing batt insulation from a rafter bay that happen to be touching the underside of the roof deck (no air space) and there was condensation on the rood deck. However when he removed the insulation from the remaining rafter bays there was no condensation or discoloration of the wood deck. And now that the insulation has been removed, the spot that was wet has dried out.
So was the condensation just a cause of the natural vapor transmission through the materials and because there was no air movement, the moisture had a chance to condense on the surface?
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Replies
Joshua,
Condensation occurs when warm, moist air — usually originating from the interior of a house during the winter — encounters a cold surface (in this case, the underside of the roof sheathing). The usual cause is an imperfect air barrier at the ceiling.
Something about the rafter bay with the condensation was different. Possible sources of leaking air include a can light, a partition top plate, a plumbing vent, or a defect in the ceiling.
Fiberglass batts are the worst possible type of insulation to use for cathedral ceilings. Because they are permeable to air, they often lead to the type of problem you describe.
Joshua,
I just re-read your questions. To clarify:
1. While you speculate that the condensation occurred because "there was no air movement," I contend that the condensation occurred because there WAS air movement.
2. It is highly unlikely that this condensation had anything to do with vapor diffusion.
Well it is my thought that the batt insulation trapped the warm moist air against the underside of the roof deck and that is why it condensed since they were in contact. Because everywhere else in the attic where the insulation was not touching the roof deck and allowed the air movement there was no condensation or discoloration of the wood.
Joshua,
Your original question made it sound like you were describing insulated rafter bays, so I assumed this was a cathedral ceiling. Now you mention that this is in an attic. Can you describe it better?
Are these batts on the attic floor which happen to block the air movement near the perimeter of the attic?
Are there soffit vents?
Is there a ridge vent?
I heard this from a meteorologist::
Condensation & Evaporation are happening at the same time...all the time.
Condensation becomes visible when Evaporation can not keep up.
Joshua - Fiberglass insulation cannot stop or trap air - it is air permeable. Figure out where the air is getting through your air barrier. If it is not obvious - an experienced Building Analyst can perform some zonal pressure diagnostics to help pinpoint the air leaks.
It doesn't take a weatherman to tell which way the wind blows.
All dynamic processes in nature occur all the time in both directions. As conditions vary, one direction dominates.
In winter, water vapor diffuses outward at the same time that liquid water diffuses inward. And the wind can blow in two directions at once, creating shear zones which are deadly to aircraft and hot air balloons (but you wouldn't know that listening to the weatherman).