when exterior walls dry to the inside does that affect the interior humidity substancially? I've noticed my interior humidity up at least 10 percent.

I'd be concerned with the question of why they are getting so wet.

What temperature and RHs are you referring to as being up? What type of construction is the home? Where is the home? How old is the home? Drying by diffusion boundary to the inside?

In very dry environments this is often a useful function. Wall insulation normally should not have an RH of greater then 18% or you risk mold and mildew formation.

Hot and humid outside with cooler inside gives you higher humidity inside until air is conditioned.

Brian

I ran some sample numbers and got 1 pound/pint of water per hour of diffusion at 1 perm. Summer air infiltration can greatly exceed that.

Posted By jonr on 11 Jul 2015 09:50 AM
I ran some sample numbers and got 1 pound/pint of water per hour of diffusion at 1 perm. 

Show your calculations. Something isn't right, as movement of water vapor by pure molecular diffusion through a 1 perm resistance is exceedingly low. It's why 1 perm was chosen as code for a reasonably closed interior vapor retarder, and why air leakage is far more important as a water vapor transport mode. I once did a calculation on this, assuming a 2x6 @16"oc framed wall, with plywood on the exterior, and a 1 perm retarder layer on the warm side. With inside air at 40% RH and 70 F, and very dry outside air with a dew point of zero F, and ignoring any water buffering capacity of insulation, it would take something like 10 months for water vapor diffusing outward to raise the moisture content of the framing and plywood from a very safe 10% to the 19% considered borderline for "dry" wood. Zone 6 winters aren't that severe for that long.

If you have a million square feet of wall you'd get even more than a pound per hour, eh? :-)

Without the actual area, the absolute mass of the water migration per unit of time cannot be calculated.

We have no idea what actual numbers for the OP are, so these are random, extreme case values. On the other hand, it's not hard to exceed 1 perm with some wall/ceiling designs.

Outside (wetted or moist exterior)
100F, 85F dewpoint = 63% RH = 1.21"Hg

Inside
75F, 60F dewpoint = 60% RH = .52"Hg

10,000 sq ft of surface area
1 perm (total)
7000 grains per pound
US perm = 1 grain of water vapor per hour, per square foot, per inch of mercury.


10000*(1.21-.52)/7000 = 1 pound/hr

Framing/plywood moisture content and the length of Winter have nothing to do with what I'm calculating.

Posted By jonr on 11 Jul 2015 09:50 AM
I ran some sample numbers and got 1 pound/pint of water per hour of diffusion at 1 perm. Summer air infiltration can greatly exceed that.

This is the first order of business. A blower door to determine if there is any migration via permeation.

Posted By BadgerBoilerMN on 14 Jul 2015 07:00 AM

Posted By jonr on 11 Jul 2015 09:50 AM
I ran some sample numbers and got 1 pound/pint of water per hour of diffusion at 1 perm. Summer air infiltration can greatly exceed that.

This is the first order of business. A blower door to determine if there is any migration via permeation.

Naw, a blower door can only tell you if there is any migration via air infiltration.

A steel can could be hermetically sealed or have  holes big enough to drive a hummer through, but there is no migration via permeation.

That's what I meant of course referencing Jonr's enlightened reference to the significance of infiltration on indoor rH.

Thanks for keeping it real...

Absolute mass, seriously, where have all the scientists gone?

Lets say you had a 5,000 sq ft house with .35 ACH natural under the same conditions.

(5000*9)*.35*6.51/7000 = 15 pounds/hr.

10% of the entering moisture being due to diffusion is possible.

That's a pretty leaky house- it would be more than 2x the code max 3ACH/50 air leakage, use any of the models for converting blower door numbers to the approximate ACH natural.

True, but it would also happen if someone used a HRV (vs ERV). Hmm, that's 1.4 tons of AC running continuously just to remove moisture.

Do you have a chart of the actual Water vapor transport comparison of different ERVs and the enthalpy gains? I see lots of claims but not usually hard numbers and calculations. Manufacturers makes lots of claims. My experience with most ERVs is that they do not do a great job of controlling humidity on their own and require separate dehumidification especially in places like the SE US.

Brian

I don't know of any ERV manufacturer claiming to "control" humidity. They simply transfer sensible and latent heat recovery energy otherwise lost to uncontrolled infiltration. This reduces the heating and cooling loads with "total effectiveness" of up to 75%. Different technologies produce varied results.

Latent heat is the heat energy due to phase change which is the change in relative humidity in the air stream. ERVs have a permeable membrane HE or an enthalpy wheel which can condense and or move moisture between the air streams. This provides partial humidity control of in coming air on a hot humid day and retains humidity in the building when cold and dry outside air is coming in the building.

Claims of Latent or enthalpy recovery is humidity control of the air streams. Additional humidity control is typically needed to maintain RH in the 40 to 60% range ideal for humans. Calculating and optimizing the incremental humidity control equipment sizing is the reason for the question. Since efficiency of ERVs are all over the map based on many variables air flow, balance, air stream temperatures, RH etc. It would be nice to have a table or set of standard equations to model performance of actual available models in stead of the peak ideal numbers published by manufacturers.

Most people just take a SWAG at it or over size the equipment. I would rather actually calculate the best solution for the specific application, building and micro climate.

Brian

So which brands are worth looking to evaluate?

It looks like the general math may be here. But I'll just guess and use 50%.

So under conditions 1/2 as extreme as I picked earlier, a 50% moisture (not total) efficient ERV might save $1/day over a HRV in reduced AC for moisture removal. Or 1/2 that since all the air doesn't flow through the ERV (some comes via leaks).