Replies

1. 
   Andrew808 | Nov 05, 2020 02:09am | #1

   On a related note - I have a friend that had his roof replaced. It was coated in a white acrylic or silicone membrane. When the roofers removed the roof, they found a lot of trapped water. The roofers stated this was because the roof was not able to breathe, so moisture from the home was trapped under the air and water tight coating.

   Do cap sheets like bitumen and torch-down membranes really breathe air?

   I have heard of concerns because the roofer did not allow the roof to dry before applying the membrane, but that is an easy fix.

1. GBA Editor
   Kiley Jacques | Nov 05, 2020 08:06am | #2

   Hi Andrew,

   Part of your question is addressed in this article by Martin Holladay, where he talks about ways roof sheathing can dry, which conversely explains why sheathing may stay wet. In terms of cooling your roof, the answer is yes, that can make the sheathing more susceptible to moisture accumulation. Martin explains how that can happen in this article: Night Sky Radiation.

1. 
   Andrew808 | Nov 05, 2020 12:53pm | #5

   Thank you Kiley,
   I just read. We have a "vented unconditioned attic"

   I note that "Vented unconditioned attics are naturally dry. They tend to get baked by the sun, which beats down on roof slopes facing east, south, and west. This type of solar heating just about guarantees dry conditions."

   So, it sounds like we are ok as long as the attic gets warm enough.
   With a reflective top layer and foam... is it possible to cool it down too much?

   My guess -- we will still have relative warm in the attic space ~ at least 80deg +

   Andrew

2. 
   Andrew808 | Nov 06, 2020 01:55am | #11

   I just got through reading the "Night Sky Radiation" article you mentioned.
   It does state,
   "warm sheathing is dry sheathing — and that the easiest way to keep sheathing warm is with a continuous layer of exterior insulation."
   or, paint the roof black. We don't want that since we are trying to prevent heat from gettin into the home.

   However, how does exterior insulation help to prevent this heat loss? In this case is the insulation holding onto heat from the building envelope and preventing it from loss to the clear sky (worst case)?

   This would sound like an unvented Polyiso + capsheet and nothing against the underside of the sheathing may be ok?

2. 
   creativedestruction | Nov 05, 2020 08:19am | #3

   "What causes moisture under the sheathing in this environment?"

   Showering, cooking, and exhaling. Sealing the ceiling airtight is your first defense to reduce the amount of moisture that reaches the roof sheathing.

   "Even if we “cooled our roof” with Polyiso + reflective coatings (high SRI), could we ever make our sheathing too cool and cause condensation?"

   What is your climate exactly? Open-air sounds like California.. The answer is kinda-sorta. You're taking about bringing the attic into the conditioned envelope which requires mechanical airchange within the attic. This doesn't seem prudent if you don't mechanically heat or cool currently.

   "Does the average higher temperature and ventilation in the attic appropriately handle the moisture?"

   Measure the moisture content of the wood; that will tell you if it's appropriately managed. Or look for signs of mold or rot. Adding a reflective "cool roof" could make matters worse as the attic temperature spikes are what currently dries your roof -- so says your graph.

3. 
   Andrew808 | Nov 05, 2020 12:52pm | #4

   Jason,
   We are in Hawaii - no heating or cooling, windows always open.
   I will do some reading. I am hoping we can adequately ventilate the attic to mitigate any moisture concern. We just rebuild the home, so air-sealing the ceilings at this point would be a pricey re-do.
   "attic into the conditioned envelope" -- do you mean because I am putting insulation on the outside? We do not have any air conditioning, so wondering if this helps.

   Thank you,
   Andrew

1. 
   creativedestruction | Nov 05, 2020 04:26pm | #9

   A good read if you haven't found this one yet: https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-042-blue-hawaii

   A "vapor diffusion ridge vent" described therein would mitigate the risk added by a reflective roof.

2. 
   qofmiwok | Dec 06, 2020 12:46pm | #13

   I have a similar house in Hawaii. No insulation, heat, ac, etc. The house is 2x4's with exterior plywood sheathing, and drywall on interiors. I am extremely mold sensitive and do fine there.
   Every time I check my attic it is dry as a bone as there is nowhere for water to collect. I can't imagine wanting to air seal...

4. Expert Member
   BILL WICHERS | Nov 05, 2020 02:45pm | #6

   Are you actually seeing water in the form of condensation on the underside of the roof sheathing? If you are, that means your sheathing is dropping below the dew point of the air in the attic, probably sometime at night. With enough ventilation, this shouldn't happen to any significant degree UNLESS excess moisture is getting into your attic from your living space.

   A full air seal job might sound costly, but you might be able to fix things "enough" by just sealing up the worst offenders. If you have can lights, they're probably of the "AT" type if it's a recently built home, which are "airtight" in manufacturer fantasy land. In reality they are more like "LL" lights, for "Leak Less". You can either box them over on the attic side, or replace the light with an airtight LED conversion assembly. There are MANY LED conversion kits available for recessed can lights, and some cap the can itself so they can seal against the ceiling drywall on the interior side. Use these with gaskets (Halo makes some), or some rope caulk to get a good seal. Done correctly, the seal can be hidden under the flange of the light so that it can't be seen. Just be sure you seal the light to the drywall and not the metal edge of the can assembly since you want to make sure there are no sneaky air leaks around the edge of the can. I've never found a can with a gasket that actually seals well to the hole in the drywall so I don't trust any of them to work very well.

   Get a few smoke sticks and see if there are other leaks. You can rig a box fan into a window as a sort of DIY blower door to help with this. Seal anywhere the smoke stick shows leaks. Classic places are electrical fixtures and any ceiling penetrations (since you're mostly concerned with leaks into the attic here). I'd also check to make sure you don't have anything like a dryer or kitchen vent that is exiting a wall right under a soffit vent, and make sure your bathroom exhaust fans vent directly to the outdoors and not into your attic.

   Hopefully a few relatively easy air sealing steps will help enough to solve your problem but you'll have to try it and see.

   Bill

1. 
   Andrew808 | Nov 05, 2020 03:21pm | #7

   Thank you Bill,
   I will do some inspection, but do not think we have any condensation due to the heat levels. Maybe at night? I'll have to check.
   The concern is with a cool roof and insulation on the exterior -- will the attic then be cool enough to be a problem.
   We do not have any venting into the attic -- 3 ceiling vent vans vent to the outside.
   I will need to consider the lighting solution -- thank you for the ideas on that.
   We could try smoke flow, but the attic tapers to no space at all, so very hard to see where anything flows.

1. Expert Member
   BILL WICHERS | Nov 05, 2020 04:03pm | #8

   Use the smoke sticks inside your living space. No need to go into the attic. Just assume air leaks near the ceiling are leaking into the attic and seal whatever you find.

   Bill

5. 
   Andrew808 | Nov 05, 2020 06:06pm | #10

   Ok - I assume most windows in the home are closed, and just open them in the room you are testing?

   I am also wondering about putting a moisture detector on the sheathing to monitor the moisture level.
   I found the one below.
   Additionally, I have a temp/humidity bluetooth reader in the attic now. It is just sitting in the opening I have access to, but I could mount it near the sheathing.
   To have condensation, wouldn't RH have to be near 100 (saturation)?

   Andrew

   -------- Moisture Datalogger
   https://www.testequipmentdepot.com/protimeter/moisture-meters/thermo-hygrometers/environmental-datalogger-0-100-humidity-bluetooth-low-energy-1-unit-bld2025.htm?ref=gbase&gclid=CjwKCAiA4o79BRBvEiwAjteoYN63fOx6ZNnW0ADSnR1SFu2w0M02gBipApxZxF55xmDTTGqAuvzDoRoCGu4QAvD_Bee

6. 
   Andrew808 | Nov 11, 2020 02:08am | #12

   Any ideas -- is moisture underneath the sheathing really a thing in Hawaii?
   Our homes are old and leaky.

   Thank you