High moisture in Attic — is it reverse stack effect?

**Scott C Simpson** · 02-23-2018, 03:52 PM

The attic has a high humidity level. While patching some holes in sheetrock on exterior walls and the ceiling, I noticed the patching compound pushing outward (it is winter and the heat is running).

Does this suggest the stack effect is reverse of what one would expect it to be?
Would this draw moist air into the attic?
If this is the case, how would the situation be addressed?

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**Jerry Peck** · 02-23-2018, 06:28 PM

Originally Posted by Scott C Simpson

I noticed the patching compound pushing outward ...

Pushing outward (inward into the interior, I presume), or sagging?

Drywall compound has a big tendency to sag.

If air is being pushed into the walls (from outside or from inside) that would increase the pressure within the walls and "push" away from the inside of the walls ... but gravity could cause the same effect.

To find out if it is gravity (gravity always wins) and sagging or if it is pressure, tape a piece of plastic wrap over the hole. If the wall is negatively pressurized compared to the interior, the plastic wrap will be "sucked inward) toward the wall cavity, if the plastic wrap is "pushed outward" into the room, then the wall space is positively pressurized, probably from the outside as any fan pressurizing the interior space and getting into the walls space would similarly pressurize the walls cavity. You could also feel or measure the temperature of the air "coming out of" the wall - if it is cold, it is likely from outside, if it is warmer, it is likely from inside.

Just some thinking off the top of my head - others will likely have other ideas, and maybe even say that the above is illogical. (Where is Spock when you need him?)

**John Kogel** · 02-23-2018, 06:59 PM

Need more info - age of the house, East Oregon or West?
Is there drywall with a vapor barrier behind it?
Is the vapor barrier old and full of holes?
Is the attic ventilated at the soffits and ridge?

**Scott C Simpson** · 02-23-2018, 07:19 PM

I don't think it is sag — it happens instantly. Sort of like a bubble. Also, I have seen it happen on exterior walls as well.
I will try the plastic on other areas though.

Originally Posted by Jerry Peck

Pushing outward (inward into the interior, I presume), or sagging?

Drywall compound has a big tendency to sag.

If air is being pushed into the walls (from outside or from inside) that would increase the pressure within the walls and "push" away from the inside of the walls ... but gravity could cause the same effect.

To find out if it is gravity (gravity always wins) and sagging or if it is pressure, tape a piece of plastic wrap over the hole. If the wall is negatively pressurized compared to the interior, the plastic wrap will be "sucked inward) toward the wall cavity, if the plastic wrap is "pushed outward" into the room, then the wall space is positively pressurized, probably from the outside as any fan pressurizing the interior space and getting into the walls space would similarly pressurize the walls cavity. You could also feel or measure the temperature of the air "coming out of" the wall - if it is cold, it is likely from outside, if it is warmer, it is likely from inside.

Just some thinking off the top of my head - others will likely have other ideas, and maybe even say that the above is illogical. (Where is Spock when you need him?)

**Scott C Simpson** · 02-23-2018, 07:37 PM

The house was built in 2004 in Western Oregon — Willamette Valley.
There is no vapor barrier in the attic — only thick insulation batts.
It is a flat roof with vents on all sides.

The other question — what should the relative humidity be in the attic to prevent condensation.
There has been at least 1 condensation 'event'. I assume due to extreme cold weather.

Currently it is:
Outside: 38 degrees/64% humidity
Inside: 72 degrees/27% humidity
Attic: 44 degrees/ 71% humidity

Originally Posted by John Kogel

Need more info - age of the house, East Oregon or West?
Is there drywall with a vapor barrier behind it?
Is the vapor barrier old and full of holes?
Is the attic ventilated at the soffits and ridge?

**John Kogel** · 02-23-2018, 08:56 PM

2004, there is definitely a vapor barrier under the insulation, right on top of the drywall. Unless this house was built without a building permit and municipal inspections.

Where moisture can leak from the interior is through light fixtures and the juncture of interior wall and the ceiling. The standards now call for vapor barrier to be tightly sealed with tape and gaskets around all openings in the drywall. Maybe not so strict in 2004 where you are.
Warm interior air is the most prevalent source of moist air in the attic. When that warm air hits the cold surface of the roof deck, you get condensation, beads of water.

Two things to do are, 1) repair any holes in the vapor barrier with construction tape (Tuk Tape). Look for dark patches on the surface of the insulation batts. That is where air is leaking up from below. 2) Make sure all exterior vents are open to allow that moist air to escape.

As long as you have heated air inside, the stack effect is working, warm air is rising.

**Scott C Simpson** · 02-23-2018, 09:14 PM

Unless the vapor barrier looks like the backside of sheet rock — it's not there.
Also — this doesn't explain why the wall patch compound is being pushed out.
I will get an insulation company to check it out, hopefully it will be part of a free quote.

Originally Posted by John Kogel

2004, there is definitely a vapor barrier under the insulation, right on top of the drywall. Unless this house was built without a building permit and municipal inspections.

Where moisture can leak from the interior is through light fixtures and the juncture of interior wall and the ceiling. The standards now call for vapor barrier to be tightly sealed with tape and gaskets around all openings in the drywall. Maybe not so strict in 2004 where you are.
Warm interior air is the most prevalent source of moist air in the attic. When that warm air hits the cold surface of the roof deck, you get condensation, beads of water.

Two things to do are, 1) repair any holes in the vapor barrier with construction tape (Tuk Tape). Look for dark patches on the surface of the insulation batts. That is where air is leaking up from below. 2) Make sure all exterior vents are open to allow that moist air to escape.

As long as you have heated air inside, the stack effect is working, warm air is rising.

**John Kogel** · 02-24-2018, 09:32 AM

Sounds like you are using the drywall compound that comes premixed? Just a guess.
It is a bit runny and gravity pulls it out of the hole, normal. If you left a blob of compound to air dry for a bit, it will be stiffer.

Absolutely, get your attic checked by a professional.
There is another major source of attic moisture and that is the attic hatch. It should be fairly heavy with weatherstrip seal all around.
Humidity in the attic should be similar to outdoors. But your flat or low slope roof reduces the volume of air in the attic, and there is some warming from below that affects temp and humidity.