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Puzzling mold issue on kraft paper batt facing in hot/dry climate…

sponger | Posted in General Questions on

Hi GBA Community – I’ve got an interesting BS puzzle from my in-laws’ Zone 3B ~3000ft elevation Southern California desert home. The annual rainfall here is 5″ and it’s very hot (~105F) in the summer and dry all year round (summer outdoor RH during the day is ~15%). They completed a remodel of their laundry room and garage about 18 months ago. The laundry room is separated from the house by a well sealed exterior door and then the garage is accessed from the laundry room. Both share an attic above the ceiling plane that is separate from the rest of the house’s attic by a firewall (the laundry room used to be part of the very generous garage). To create more storage in the garage the ceiling joists were redesigned, a plywood subfloor was installed on top of them to create storage, a nice set of drop down “attic” stairs were installed, both the rafter bays and ceiling joists were insulated with kraft-faced fiberglass batts, and the entire garage was drywalled and taped (and textured of course it’s Cali)! There’s plenty of open soffit vents around the area in question with a large gable vent for extraction (with Martin’s favorite a PAV installed baby – it’s only turned on during the summer I am told). Only other detail of note is there’s an atmospherically vented water heater in the garage that draws combustion air from the garage and then vents it out above the roof. 

Finally to the problem. I was just up in the “storage attic” and noticed a good amount of mold remnants on the kraft paper batt facing between the rafters. It’s currently summer and it was at least 110F up there at 15% RH so the mold is obviously not active but the question is how did it get there? There’s zero way in my mind that it could be happening during the summer, something would need to be getting the kraft paper facing to 80+% RH for a continued period for mold to grow and it’s just wayyyy too hot up there (which would require a TON of moisture) and we’ve used a good ol’ sling psychrometer to verify the RH was 15% (same as outdoors, inside the house it’s 30%). So I don’t think it’s a summer issue…the fact that the RH is low and matches the outdoors also makes me believe it’s not caused by the PAV depressurizing the space and sucking moisture in from the laundry room (no doubt it is somewhat but it’s not the source of the problem).  

In winter I’m wondering if the kraft paper facing is really cold as it’s being well washed by the soffit -> gable venting as well as the fact that the shingles are really cold from the low (~30F) temperatures and massive night sky radiation. Then all we need is a source of warm moist air leaking into the attic and then condensing on the kraft paper. Only potential idea there is a leak from the water heater flue pipe that’s dumping super moist flue gases into the attic. 

We just bought a wireless temp/humidity sensor that we’ll install up there and monitor things over the seasons. I also suggested putting a CO detector up there as that would be a further clue that it’s flue gasses causing the moisture issue. Any ideas, thoughts, questions are welcomed! I love these puzzles!   


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  1. user-2310254 | | #1

    Hi Ryan,

    Maybe Martin's article on night sky radiation offers some clues (

    Does the house have a cool roof (likely in California)? I wonder if radiation to space is causing the attic to get really cold during the winter. Cold surfaces combined with exhaust leaks from the water heater vent might provide the right conditions for mold growth on the kraft paper. What do you think? Just WAGing here.

  2. sponger | | #2

    I did see Martin's article as well as the recent cases from the Pacific NorthWest about mold developing on the sheathing near the soffit vents because of the voracity of night sky radiation (which is crazy, the BS field is never boring!!). The roof is indeed a cold roof, but it's somewhat of a mixed bag. The ceiling plane is insulated with batts but the rafters are as well (kraft paper faced fiberglass in both with the kraft paper on the inside. The ones in the ceiling plane are "relatively" well air sealed by the drywall but the batts in between the rafters are not so air is washing all through them. I'm doubt they are doing much but I guess they might raise the roof sheathing temp a tad in summer and keep it a little cooler in winter. This roof area does develop frost on the shingles in the winter more than the other parts of the house so I do believe the insulation is keeping the sheathing a bit colder.

  3. sponger | | #3

    Now that the attic has been instrumented for a while it’s clearly not a summertime issue. Can’t wait to see the data when it cools down.

  4. tommay | | #4

    I think you are on the right track with the HW heater flue. Perhaps it's not leaking but just creating enough heat in the winter to create some condensation with the colder air coming into the soffits and keeping the attic space just warm enough to grow the mold. How much of the flue is exposed in the attic? I'd say either staple some poly over the insulation and observe if any moisture accumulates and if it does, the poly may be enough or else box in the flue.

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