I just built a seasonal building in the catskills. The heating is radiant. The cooling is forced air. It is a very well insulated tight building.

The problem now in the winter is excessive humidity, double pane windows are dripping water on the inside. It's up around 70%. I was not sure if this was just construction moisture from painting and cleaning. But now it seems like it is not going away and I need to find a solution.

Since this building will be rarely used in the winter and the heat will be kept at 45 or 50 degrees I'm thinking a whole house dehumidifier tied into the the cooling ducts might be the best solution. Or would an ERV be the way to go? I know the dehumidifier does what I want it to do but I'm not sure about a ERV. Plus a dehumidifier would be a lot easier to retrofit.

My other reason to tend towards a dehumidifier is that since the use will probably be only a few hours during the winter the humidifier will turn off once it has reached the desired levels. I don't know how an ERV could be tied to humidity levels.

Any advice would be appreciated.

No need for an ERV while you aren't there. But a dehumidifier makes sense.

Your problem is all about the dewpoint.How cold is the outside air and what temperature is the indoor being heated to. You say the house is extremely tight ,just how tight of construction is the building.Any bathrooms ,kitchen,etc.How about indoor plants or fountains.Raising the temperature may be all that is needed or adding infiltration . A dehumidifier will work but first the grains of moisture that need to be removed must be calculated to size the dehumidifier. There is more to the picture then what you are describing.

Either ERV or HRV under dehumidistat control would be cheaper to operate than running a compressor and air handler just to purge moisture during winter weather, and unlike some dehumidifiers, it even works at 40F interior temps. Many/most ERV vendors have dehumidistat control options.

A dehumidifier would convert that latent heat into sensible heat, lowering the space heating load somewhat, but probably not enough to matter. Even running a kilowatt of air handler (let alone the compressor) to handle a wintertime latent load on an unoccupied house is like using a howitzer to kill flies. The mean midwinter outdoor dew points in the Catskills is about +15F, which is plenty dry enough to dilute 50F/70% RH (dew-point=41F) to below the window's interior temp. If you set the dehumidistat to 50% RH, with a 45-50F interior you'd never get liquid water on the windows, only a light frost when it was wicked-cold out and the window surface temps dropped to the freezing point.

ANY house that's tight enough to meet IRC 2102 (and may that aren't that tight) needs a ventilation system anyway, and spending the money on the ventilation system first makes more sense than yet another compressor.

I didn't know there were dehumidistat control options. Outside 24F, inside 50F.

ICF basement, 8"SIP walls, 12" Sip roof. Windows are OK but not triple pane. It's pretty tight.

If you can have a dehumidistat control then since ERVs and dehumidifiers are about the same price I guess I'll go with that. THough venting it will be a lot harder.

Is there any manufacturer you recommend? I've put in a systems by Honeywell and ReNewaire. (Appoloonaire?) They seemed about the same.

Sorry to keep adding things. One other aspect of this is that there are some valuable paintings in the space. I'm wondering if the ERV option will be as good overall as a dehumidifier if humidity levels are very important.

And thanks Dana1 !

If you have a very tight home what is your mechanical ventilation. A tight building needs ventilation so if you have none start there. If the home is very cool and unoccupied then I would assume you dont have plants, aquariums etc that would add moisture. So there should be a low moisture load mostly from the last time it was occupied. I would start with venting and go from there

if the ERV option will be as good overall as a dehumidifier if humidity levels are very important.

> Since this building will be rarely used in the winter and the heat will be kept at 45 or 50 degrees

When it's 70% RH and 50F outside and 50F inside, running an ERV (or HRV) will only serve to increase your power bill. If you have a problem (other than post construction startup) at typical (ie colder) winter temps while unoccupied, then you need to find where the moisture is coming from and stop it - don't waste money running an ERV or a dehumidifier.

Your comment about valuable paintings is an interesting comment. Paintings need a controlled enviroment to properly preserve their integrity.Not only is humidity of concern but indoor temperature as well.Have you ever been to a art gallery where it's 50 degrees inside.

I have not been to a clothing retailer where it is 50 degrees inside either. Even the vegetable section at the grocery store is warmer.

I did ask a registrar at a major art institution and he said 50 degree temps were not a problem as long as the humidity was controlled and the temperature swings were not too fast.

1. How old is the house? It takes months & months for all the construction moisture to dry out.
2. you haven't said if you have ventilation. Do you have an HRV? It is necessary in a tight house & helps tremendously with mosture levels. Get the HRV before you go to dehumidifiers.
3. Did you put insulation and poly under the slab? You'll get tons of moisture through the slab if it is poured directly on dirt or gravel.

Posted By Dana1 on 07 Feb 2013 03:30 PM
Either ERV or HRV under dehumidistat control would be cheaper to operate than running a compressor and air handler just to purge moisture during winter weather, and unlike some dehumidifiers, it even works at 40F interior temps. Many/most ERV vendors have dehumidistat control options.

...snip...

ANY house that's tight enough to meet IRC 2102 (and may that aren't that tight) needs a ventilation system anyway, and spending the money on the ventilation system first makes more sense than yet another compressor.

My understanding is that an ERV transfers both sensible heat and humidity from the outgoing stale air to the incoming fresh air, while an HRV only transfers sensible heat.  If the objective is to get rid of excess humidity from the house, wouldn't the HRV be the desired choice, or is there a way to run an ERV where it is transferring sensible heat but not moisture? 

If the excess humidity were a problem during the summer time, neither the ERV nor the HRV would be effective in removing the moisture, but the A/C should be. 

I suspect that the reference to IRC 2102 should be to IRC 2012? 

I do have foam and poly under the slab and gravel under the foam. It was built in the last year (slowly). It was just painted before winter so probably a lot of moisture is coming from that.

Now I am getting more confused. We have suggestions for ERV, HRV and for a dehumidifier. I should also mention that it is not a house. It's a painting studio. Acrylic paints.

So you have water evaporating from the concrete, the framing, the drywall and the paint = hundreds of gallons of water, so the moisture should be expected. Its a very tight building so the moisture has nowhere to go. In a poorly built building it would escape through all the air leakage points. So you need to ventilate so the moisture can escape. Raising the temperture of the building above 45 will also help since it probably isn't drying out much at that temperature.

HRVs and ERVs are similar devices with different cores. ERVs are typically recommended in humid, cooling dominated climates, HRVs in heating dominated climates like the northeast. Both will bring in fresh, tempered air. Airtight houses need ventilation to moderate the humidity as well as to modify offgassing from everything in the building. I've found that typically a building with an HRV will maintain a good humidity level, which they do by the simple method of ventilating the building. The humidity control (which you can set on the main control panel IF you buy the main control panel) will raise the fan speed from normal low speed to a higher speed when the humidity levels get higher than what you want.

The opposite is also true; in a normal winter with little human activity your humidity levels could be very dry - probably drier than would be good for the artwork. An HRV will keep the humidity levels higher. (I've observed this in a 20 yr old seldom used vacation house where the strip oak floor and the painted woodwork has never shrunk from when it was installed)

A dehumidifier wrings the moisture out of the air. It does nothing for ventilation. If you go this route, you may need a humidifier for winters.

This changes everything. Need a lot more information.Your building requires mechanical ventilation. HRV possibly would work , ERV probably not. Both would be in defrost mode often because of your high internal humidity

My ERV (Renewaire) has no defrost cycle. I think some models preheat air in frost situations. I've seen studies saying that the enthalpy wheels fail in a year or two in far north climates because of excess moisture. Renewaire's manual said to call if the only intake/exhaust option is through the roof. Turns out they want backflow preventers so cold air doesn't settle through the unit and cause condensation.

OP, it took more than a year for my house to dry out from construction. Last winter, RH > 70 percent. This year, 40-50.

I do agree that more than likely the building needs to dry out from the construction. In the mean time,RH of 70% is not beneficial for the artwork,building materials, and human occupancy.Oil paintings indoor winter design should be 65-70 degrees F and 40-45% RH. Any conditions outside these parameters will cause damage to the artwork.Rent a portable dehumidifier and reduce RH to design conditions.There is still more pieces of the puzzle missing here.

Rsipgeo - as AC says, before doing anything else, rent a few dehumidifiers for a week and bring the moisture under control. Than see what happens over the next couple of weeks.

Posted By Lee Dodge on 08 Feb 2013 09:56 AM

Posted By Dana1 on 07 Feb 2013 03:30 PM
Either ERV or HRV under dehumidistat control would be cheaper to operate than running a compressor and air handler just to purge moisture during winter weather, and unlike some dehumidifiers, it even works at 40F interior temps. Many/most ERV vendors have dehumidistat control options.

...snip...

ANY house that's tight enough to meet IRC 2102 (and may that aren't that tight) needs a ventilation system anyway, and spending the money on the ventilation system first makes more sense than yet another compressor.

My understanding is that an ERV transfers both sensible heat and humidity from the outgoing stale air to the incoming fresh air, while an HRV only transfers sensible heat.  If the objective is to get rid of excess humidity from the house, wouldn't the HRV be the desired choice, or is there a way to run an ERV where it is transferring sensible heat but not moisture? 

If the excess humidity were a problem during the summer time, neither the ERV nor the HRV would be effective in removing the moisture, but the A/C should be. 

I suspect that the reference to IRC 2102 should be to IRC 2012? 

In a Catskills location ERV is still the right choice, to lower the summertime latent cooling loads. Yes, it isn't as efficient at wintertime dehumidification as HRV would be, but since the moisture transfer is nowhere near as effective as the sensible-heat transfer it still dehumidifies when the outdoor dew point is lower than the indoor dew point.  It would still would use far less power than a whole house dehumidifier to achieve that function, even if there were periods like  jonr's worst-case senario when the outdoor temp was equal to the indoor temp and the duty cycle went to 100% for some periods. During those periods it wouldn't be dehumidifying, but there also won't be puddles of condensation on the window sills to worry about under that scenario.

And yes, you have to duty-cycle the ERV during Catskill summers rather than running it under dehumidistat control when the outdoor dew points are high, and the AC would be providing the dehumidification function.  It's spelled out in the manual when you buy an ERV control with a dehumidistat function- SFAIK there are no "dumb" dehumidistat controls for ERVs, but creative idiots can defeat even the smartest control systems when given a chance.  I'm sure there are people who "set and forget", but then there's the rest of us. The ERV will lower the latent loads that come in on summertime ventilation air, but comes nowhere near to eliminating it, and over-ventilating in summer adds to the latent load (but HRV adds considerably more latent load at a given ventilation rate.)

Since the IRC 2102 has yet to be written, so maybe we SHOULD stick with IRC 2012.