Tightened up House and Now Getting Condensation and Mold :(

keyserjose

I live in southern NH and recently had a company come in and blow cellulose insulation in the walls and attic and air seal my 2,200 sf, two-story, two-family house. There was a pot of energy efficiency money from the state that made the cost very reasonable. The problem is that my previously "breathable" 1900 Victorian house now seems to be collecting moisture that's causing mold around the windows of the 2nd floor unit that we rent out.

I've tried to identify sources of moisture and can't seem to find an obvious culprit. Two people live upstairs with a cat and my wife and I live downstairs with a dog and cat. There are probably a dozen small to medium sized plants on each floor. The upstairs has steam heat, which I thought was the culprit initially, but I changed out all of the air vents just in case and no change in humidity. Bath vent upstairs works well and windows are vinyl replacement although not very good. I ran two 90-pint dehumidifiers over an entire weekend in the upstairs unit, while the tenants were away, continuously (only stopping to empty reservoir) and wasn't able to get the humidity below 45%. While they are home we run a single dehumidifier continuously and the humidity ranges from 50-65%.

I've basically come to the conclusion that I need to provide some type of mechanical ventilation upstairs to combat the humidity. I got a few quotes for HRVs and whole house dehumidifiers and the cost is ranging from $$$$ to $$$$ just for that 1,100 sf space. Now I'm thinking that before we dive into such an expensive installation I could try to add a 600 cfm ventilation fan near the location that seems to have the most humidity (I have three wifi hygrometers up there). There is an unfinished attic space above the second floor unit that I think will be a convenient location for the fan, duct work, etc. The rough plan is my head is to cut an opening in the ceiling, install a grill and duct to an in-line fan and then exhaust the fan out the gable wall. I'm hoping that this will bring enough fresh air in through cracks in the house - similar to what the bathroom fan is currently doing. Is this a reasonable solution to try before going big with the HRV? Any advice, including "that's a dumb idea," is much appreciated. If it's not such a dumb idea I'd love to know where there might be a resource to help me to size the grill, ducts, etc.

Jose

Robert_25

The moisture is coming from somewhere.  Does the house have a basement or crawlspace  If so, what is the humidity level down there?

Jamie Hall

Not a dumb idea, @keyserjose . Worth a try -- but whether it will be adequate or not is an open question. It might be, provided enough outside air can still get into the house to actually reach its rated flow. To control indoor air quality in a house your size I would want to see a minimum of 600 cfm exhaust and intake; preferably 1200 cfm A draughty old Victorian will not have a moisture problem, in general, as there is enough air exchange. Sealing it up without providing for air exchange is just about 100% guaranteed to create a moisture problem, as you have just discovered.

Do you have any fuel burning appliances in the house, such as a gas stove, gas or oil furnace or boiler, gas water heater, etc.? If so you may need to provide additional outside air for them.

Larry Weingarten

Hi @keyserjose , If you have a crawlspace, and even if the ground feels dry, that can be a big source of water getting into the house. I'd start by putting heavy plastic down on it and sealing around the edges. See what effect that has on the problem. Water is coming from someplace and that's where I'd focus my attention. Have a look at your water meter also, when you know everything is off. See if it moves!

Yours, Larry

keyserjose

@Robert_25
The house has a basement that doesn't seem to be very damp. I will run a dehumidifier down there for a few days and see what effect that has on the humidity upstairs. Or, maybe I should be running the dehumidifier in my (1st floor) unit where we are likely producing humidity...

@Jamie Hall
I have two oil burning boilers and two natural gas water heaters in the basement. Both are vented to the outdoors. We have a gas range (oven is electric) on the first floor - it has a downdraft vent that we use in the warmer weather, but not so much now. There is a gas range/oven on the 2nd floor that is unvented. You may recall me from this recent post that you were helping with:
https://forum.heatinghelp.com/discussion/186036/could-steam-heat-cause-mold-mildew-around-windows-and-mouldings#latest

I think I'd like to try a 600 cfm fan first. If it does the trick then great, if not I'll go from there. I'm fairly handy, but don't know much about HVAC work. Do you know how I could size the intake grill that would get installed in, I presume, the ceiling? Then it seems I'll need some type of transition duct (plenum?) to go from the intake grill to the (round? rectangular?) duct work that I would also need to size? I'm willing to try to gather these materials, cut into the ceiling, assemble duct/fan and cut into the gable wall with a bit of guidance if you or one of the other good Samaritans on this site is willing to give it.

@Larry Weingarten
No crawlspace, just the basement. Good thought on the water meter - I'll have a look at that.

Hot_water_fan

600 cfm is gigantic - I think 1/10th of that is what’s usually prescribed. Fix the source of the humidity! 

Robert_25

Much of my 110 year old house is insulated with blown in cellulose and we have more of a challenge keeping it from getting too dry than dealing any any excess humidity.  The only time I have ever seen condensation on the windows is when my wife was making turkey stock on the gas stove and forgot to turn the hood on. 

Do your tenants cook a lot?

I also think it is worthwhile to monitor and try to reduce the humidity in the basement

Jamie Hall

Hot_water_fan said:

600 cfm is gigantic - I think 1/10th of that is what’s usually prescribed. Fix the source of the humidity! 

Standards vary, and the local building code may have specifications. I have seen specifications from about 100 cfm on the low end for that size building to 1,500 cfm.

If there are no local standards, the desirable rate depends in part on humidity control, as we notice here, but also on control of other contaminants, notably volatile organic compounds and virus and bacterial problems. Paradoxically, these are much less of a problem -- particularly VOCs -- in older buildings, which aren't likely to have significant amounts of building materials or furnishings which contribute VOXs. Modern buildings can be much more problematic.

MikeAmann

Let me throw this out there:
My basement is unheated, and I while I am working there if it is cold I use a tank-top propane heater to take the chill out of the air. I know, I know........ not supposed to do this. Anyways, the dehumidifier was failing, but I didn't know it because the fan was running, but the compressor wasn't. My north basement wall was sweating so much that it looked like I washed the wall down with a garden hose. Come to find out that a 20# propane tank puts 4 gallons of water into the air!!!
You are using gas appliances. Do you think there might be a connection now that the house is tighter?

Hot_water_fan

There’s something else happening here: air sealing doesn’t necessitate 600 CFM for 1100 sqft. That level of ventilation might double or triple the heat loss. If they ever stopped the ventilation, they’d be at risk of whatever is actually causing the mold. Time to hire a local pro. 

Jamie Hall

Hot_water_fan said:

There’s something else happening here: air sealing doesn’t necessitate 600 CFM for 1100 sqft. That level of ventilation might double or triple the heat loss. If they ever stopped the ventilation, they’d be at risk of whatever is actually causing the mold. Time to hire a local pro. 

First, it's 2200. Second, you should be very happy you don't live somewhere -- such as Canada -- with good indoor air quality codes. In Canada that area would require the 600 cfm in commercial properties, and no less than 200 cfm in residential. By code.

pecmsg

Congratulations on sealing up that home now you see an issue, not enough outside air to remove indoor pollutants and moisture. It is recommending a complete air change every 6 - 8 hours. Installing Whole house dehumidifiers connected to outside air is one solution. Air to Air ERV's HRV's are another. With a house that size a combination of all may be required.

April Air as well as several other companies deal with this more and more.

Hot_water_fan

@Jamie Hall  It’s the second floor having the issues, which is why Jose was considering the ventilation up there. We know it’s residential not commercial. Any way, 600 is too much, by either 3x or more depending on requirements. 

EdTheHeaterMan

If you are handy enough to install a 600 CFM exhaust fan, then maybe you could try this item.

This will provide you with some energy recovery. Loading the outdoor air with some of the exhaust heat. Your exhaust fan will only draw in cold air and exhaust heated air that you paid $$ to heat.

The phenomenon that causes the winter indoor air to be so dry for most homeowners is the fact that the relative humidity of 40 or 30 degree outside air may be near 100% but when it enters the home and warms up, it increases in volume and has the capacity to hold more water vapor. This increase in the ability to hold more moisture is directly related to the measurement of "Relative Humidity". It is relative to the temperature of the air. As warmer air expands and has more space to hold humidity. If no humidity is added, then the air is Relatively dryer.

This simple air exchange devise will take the cold air and heat it with the exhaust warmer air and is doing so, will decrease the colder outdoor air's relative humidity. If you are looking at the more expensive energy recovery units that also retain a percentage of the humidity that is leaving, then you will need a larger capacity unit. Try the lower cost option first, and keep the heat but not the humidity.

This is a better "try this first" than the 600 cfm energy vacuum you are proposing.

Just an opinion

Mr. ED

Jamie Hall

And those things are easy to install, @EdTheHeaterMan and would probably do the trick very nicely, actually. I highly recommend them.

There is a very wide range of opinions regarding required air changes per hour -- as the debate between myself and @Hot_water_fan shows. I don't think it is quite fair to try to come with one specific number; it has much more to do with the indoor contaminant loading and occupancy type than anything else. It also has to do with the tolerance level of the occupants for contaminants which can be sensed -- humidity and odours, particularly. Admittedly my experience along these lines is much more closely related to two remarkably different occupancies: schools and passive solar houses -- and derives from experiences in the Carter years, when there was a tremendous emphasis on sealing up everything very tightly for energy conservation. I was closely involved at that time with some public health aspects of schools, and, frankly, the results of cutting off almost all the outdoor air exchange in schools -- which was done remarkably successfully -- was simply catastrophic in terms of public health. Nobody died, true, but... respiratory illnesses of all sorts went through the roof, and there was a remarkable spike (which endures in that age group) in allergic sensitivity. Which, perhaps, has made me somewhat over-sensitive.

On passive solar houses, work which my father-in-law and I did in the 1980s found that unless adequate air exchange -- through heat recovery ventilators -- was provided that sensible air quality became problematic (odours and humidity and allergens) and testing showed that VOC concentrations were unhealthy.

keyserjose

@EdTheHeaterMan
Great tip Ed. Is that 80cfm unit sized for the 2nd floor 1100 sf unit or the 2200 sf house? I’ve had it in the back of my mind that the first floor is contributing moisture to the second floor. Would the below product be recommended for a 2200sf house with 4 brs?
https://www.supplyhouse.com/Fantech-SHR150R-SHR-Series-Heat-Recovery-Ventilator-w-Recirculation-Defrost-6-Side-Ports-up-to-3600-Sq-Ft

Or do you think I should just go with the 80cfm unit?

pecmsg

You should get a blower door test performed to find out exactly what size is needed.

keyserjose

pecmsg said:

You should get a blower door test performed to find out exactly what size is needed.

Can you elaborate on that? I may be able to get the people that tightened up the house to do a blower door test, but I’m not sure they’ll know what to do with the information. Will I just need a [flow] @ [pressure] measurement? Once I have that info, can you refer me to a resource that I can use to help with flow sizing recommendations?

pecmsg

It will tell you the rate of leakage. With that you can decide how much outside air is required and the best way to get it. 

EdTheHeaterMan

keyserjose said:

@EdTheHeaterMan
Great tip Ed. Is that 80cfm unit sized for the 2nd floor 1100 sf unit or the 2200 sf house? I’ve had it in the back of my mind that the first floor is contributing moisture to the second floor. Would the below product be recommended for a 2200sf house with 4 brs?
https://www.supplyhouse.com/Fantech-SHR150R-SHR-Series-Heat-Recovery-Ventilator-w-Recirculation-Defrost-6-Side-Ports-up-to-3600-Sq-Ft

Or do you think I should just go with the 80cfm unit?

Look at this table in from the instructions
this is the instruction booklet
https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Fantech-AEV80-User-Guide.pdf
I think the 80 is enough for the 1100 sq ft apartment. You can try one and if it works you are done. If not or you like the additional fresh air, then add one for yourself.

I always liked to offer the least costly solution with an option to go more if needed. The cost of the larger one does not save that much, and the individual control for each unit may be to your advantage in the event that you heat and cool differently than your tenant. You can use the fresh air differently also.

Also there is a video in the SupplyHouse.com listing of SHR150R you posted. You are looking for a HRV and want to stay away from the ERV. The problem you are trying to solve is humidity related and the HRV will reduce the indoor humidity at a greater rate than an ERV.

keyserjose

This has all been super helpful, thanks! One thing that the internet can't seem to help me with is the difference between an AEV (the 80 cfm unit you you initially suggested) and an HRV. The reason that I ask is mostly because I'll need to install this thing in my cold attic and my understanding of HRVs is that they produce condensation that needs to be drained or pumped somewhere (they also seem to not be recommended for attic installation). The AEV instruction manual that I went through did not mention anything about connecting to a drain, but otherwise everything seemed similar to the HRV installation. Does the fresh outdoor air mix with the stale indoor air within the unit? I know that this is not the case with an HRV. I have also rejected ERVs as a solution for the same reason you mentioned.

Have you (or has anyone) had any success installing an HRV in an attic in cold climates? Do they just need to be insulated or maybe heat trace tape added to the unit and the drain line?

MikeAmann

Maybe this will help, maybe not.
When I bought my house, a friend of mine told me that instead of installing the usual bathroom exhaust fan, to install a sauna fan in the attic and run the ducting to the grille in the bathroom. All you hear is the airflow - no noisy fan motor running. He was right and the unit is made of plastic, so it can handle the humidity and does not rust. And it does have a small drain hose. BTW, the unit is a Fantech and well built.

pecmsg

keyserjose said:

This has all been super helpful, thanks! One thing that the internet can't seem to help me with is the difference between an AEV (the 80 cfm unit you you initially suggested) and an HRV. The reason that I ask is mostly because I'll need to install this thing in my cold attic and my understanding of HRVs is that they produce condensation that needs to be drained or pumped somewhere (they also seem to not be recommended for attic installation). The AEV instruction manual that I went through did not mention anything about connecting to a drain, but otherwise everything seemed similar to the HRV installation. Does the fresh outdoor air mix with the stale indoor air within the unit? I know that this is not the case with an HRV. I have also rejected ERVs as a solution for the same reason you mentioned.

Have you (or has anyone) had any success installing an HRV in an attic in cold climates? Do they just need to be insulated or maybe heat trace tape added to the unit and the drain line?

https://www.linquip.com/blog/hrv-vs-erv/#:~:text=The main difference between HRV and ERV systems,more humid airstream to the least moist stream.

keyserjose

pecmsg said:

https://www.linquip.com/blog/hrv-vs-erv/#:~:text=The main difference between HRV and ERV systems,more humid airstream to the least moist stream.

Thanks @pecmsg, but the question was about differentiating AEVs and HRVs. ERV vs HRV is all over the internet, but not AEV vs HRV. The product that @EdTheHeaterMan recommended was an AEV.

rick in Alaska

Whatever way you go with, you will need to make sure you are letting in as much air as you are taking out. They need to balance out. With an hrv, the ductwork to the outside takes care of that. If you went with a basic large fan, you will need to put in some kind of damper to allow air back in.
Most of the places where I am at use an HRV, but people who are on a budget will just put in a good bathroom fan and install what are referred to as fresh 80's, which is basically just a tube that is installed high in the wall in different room, that has a damper on it which opens when air is pulled in, and also has a way to manually shut them off. The bath fan is controlled by a timer in the bathroom that is typically set to have it run once an hour for 20 minutes. Both scenarios have there good and bad points.
Use the chart for the hrv and it will tell you what you will need for the proper size. I can't remember, but my 2200 square foot house was about 110 cfm. This will depend on what your blower door test tells you though.
Rick

rick in Alaska

Whatever way you go with, you will need to make sure you are letting in as much air as you are taking out. They need to balance out. With an hrv, the ductwork to the outside takes care of that. If you went with a basic large fan, you will need to put in some kind of damper to allow air back in.
Most of the places where I am at use an HRV, but people who are on a budget will just put in a good bathroom fan and install what are referred to as fresh 80's, which is basically just a tube that is installed high in the wall in different room, that has a damper on it which opens when air is pulled in, and also has a way to manually shut them off. The bath fan is controlled by a timer in the bathroom that is typically set to have it run once an hour for 20 minutes. Both scenarios have there good and bad points.
Use the chart for the hrv and it will tell you what you will need for the proper size. I can't remember, but my 2200 square foot house was about 110 cfm. This will depend on what your blower door test tells you though.
Rick

Jamie Hall

The description of operation which I can find in the literature for an air exchange ventilator is not clear. In one place, it clearly states that it mixes recycled air with incoming air. In another place it states that the incoming air is warmed by using the heat from the exhaust air, but says nothing about mixing.

If mixing is indeed permitted, I'd be very dubious.

That said, you mention condensation. Whether you are specifying a heat recovery ventilator -- which completely separates the inlet and exhaust air streams in a heat exchanger -- or an air exchange ventilator, which may not, there may be a problem with condensation on the exterior of the inlet ducting if it is a space with a dewpoint above the inlet air temperature, and possibly on the interior of the exhaust duct, if that is located in a space where the air temperature is below the exhausted air dewpoint.

Either problem can be alleviated by providing adequate insulation which is impermeable (closed cell foam or equivalent) to the ducting to the exterior or interior, as the case may be.

I cannot recommend energy recovery ventilators for air quality control, as they also transfer water vapour -- and potential contaminants -- as well as sensible heat.