Warmer air usually has higher humidity.

Duplicate post.

>"In the winter we will run a humidifier on the second floor. We don't run too crazy with the humidity though, keeping the house around 40-45%."

That actually IS crazy in a New England climate! The dew point of 70F 45% air is about 48F, whereas the average wintertime temperature of the wall sheating or roof deck is 15-20F (or more, in northern New England) colder than that. Without a Class-II or tighter interior side vapor retarder and VERY good air sealing the interior moisture drive will create excessive moisture content in the exterior sheathing.

Holding the line at 35% RH @ 70F would yield a much less crazy 41F dew point temperature, and significantly less moisture risk for the building, while still solidly in the "healthy & comfortable" range for humans.

>" Generally the temp rise is only a few degrees (3-4 degree rise), but my perception is that the humidity is very, very different. At times the humidity is in line with the outside, especially when it is lower (say 40%). But the bulk of the time the humidity readings in the attic is 15-20% higher than outside. This morning it is 67% outside and 87% in the attic."

Without the temperature at which the humidity is relative those numbers are meaningless- that's why it's only the RELATIVE humidity.

The absolute humidity can be expressed as dew point or wet bulb temperature. Building science types prefer to use dew point, because it's simpler to explain where & why moisture can accumulate. HVAC types usually prefer wet bulb temperature since it can be measured directly with simple equipment. But either is more meaningful than RH % when the temperature to which it is relative isn't given.

Soffit to ridge venting is ideally split 60/40, with 60% of the free vent area at the soffits, 40% at the ridge. If there is more free area at the ridge than at the soffit, stack effect drive depressurizes the attic relative to the indoor space below, drawing air in from the indoors into the attic, which in winter will raise the absolute humidity levels in the attic.

With insulation is at the attic floor the attic space is counting on venting to the outdoor to keep the moisture content of the wood from reaching mold/rot potential. The proper location to air seal is at the upper floor ceiling plane, to prevent humid indoor air from getting into the attic to be taken up by the exposed wood in the attic. Every flue & plumbing stack chase, every electrical penetration needs to be sealed. But even if the attic floor/upper floor sealing is air tight, the attic STILL needs to be vented to the outdoor to purge the (much lower quantity of ) moisture that is getting in via vapor diffusion. So if that's what you're doing, you're doing the right thing, and expressing the humidity in absolute rather than relative terms would probably reflect that.

A blower door, or even a large window fan can be useful in chasing down the leaks, once you've fixed the biggest and most obvious. The $200 FLIR One IR imager makes it even easier to spot those leaks when depressurizing a room with a fan, but a $50 pistol-grip IR thermometer can still be useful, if a bit more tedious, and prone to missing smaller leaks.

There is a Vapor Barrier paint available now. I get it from Sherwin Williams. It is not a cure-all and some on here don't believe it, but I have used it. Could be a easy help for you.

I do agree with the thermal camera Dana mentioned and love mine...I use in from the inside, I tried outside and was no help, never tried inside the attic. When I find a spot I drill a small hole in the drywall and spray foam carefully through it and then touchup the paint.

Use a psychometrics calculator to associate and better understand elevation, dry bulb temp, wet bulb temp, dew point temp and relative humidity:

https://www.borstengineeringconstruction.com/Psychrometrics_Calculator.html

Do a Glaser moisture analysis to better understand how vapor diffusion can result in water condensation formation within building assemblies:

https://www.borstengineeringconstruction.com/Building_Assembly_Moisture_Analysis_Calculator.html

Posted By newbostonconst on 16 Nov 2019 12:20 PM
There is a Vapor Barrier paint available now. I get it from Sherwin Williams. It is not a cure-all and some on here don't believe it, but I have used it. Could be a easy help for you.

I do agree with the thermal camera Dana mentioned and love mine...I use in from the inside, I tried outside and was no help, never tried inside the attic. When I find a spot I drill a small hole in the drywall and spray foam carefully through it and then touchup the paint.

Vapor barrier paint (about 0.5 perms, a Class-II vapor retarder, not a true vapor barrier) is only rarely needed for a vented attic in New England, even if they're over-humidfying the air to 45% RH @ 70F.

But if it's kept that humid indoors it's likely to be needed to keep the WALL assemblies sufficiently dry, unless there is a class-II or tighter vapor retarder under the wallboard.

But whether walls or ceiling, air tightness is far more important than vapor retardency.