For guidance on this I use buildingscience.com research report 0006 by Armin Rudd which goes over the math for pressure vs airflow vs return air path dimensions. I just corrected a situation that had 76 pa difference in room pressure because of a forced air return. I got it down to 6 pa which is still bad, but a lot better then before. Homeowner had trouble opening the door. I needed 800square inches of return airflow to do it. 10 cfm airflow with a 3 pa max difference requires 5.4 square inches of opening so for a 32" door this would be 0.16" of undercut~3/16". Hell be generous and use 1/4 inch undercut and you pressure difference will be under 2 pa. Cheers, Eric

Posted By joe.ami on 09 Oct 2013 09:35 AM
"Data is available for supply only at 10 CFM. As I recall, it was something like 1/4" under the door and you will still get some pressurization of the room. In a closed door room it is best to use a supply and a return, both with dampers."

There is code and then there is opinion and data. Unfortunately they are not always on the same page.

And that's a fact!

There is stunningly parsimonious data behind the ASHRAE 62.2 ventilation rates, which were determined by average the WAGs of a bunch of pros, but there is some serious push-back going on from the building science crowd, who prefer measurements to opinions.

Ventilation should be closer to 15 CFM per person based on CO2 levels alone. Put two people in a closed door bedroom and the necessary underdoor cut becomes a real noise and light problem.

Trying to bring this thread up as it has my information - I've lived in the house for 2 years now achieving outstanding results in energy efficiency. My bills in the summer were $ 500/ month and now are $120 / month at summers peak keeping the house at 69-71 F

. I've got no fuel bill and my heat costs are around $ 200 at the peak for winter, my Geothermal units run about 90% of the time during the peak winter season and I've had a few times where it ran 22-23 hours chugging along keeping the house at 70 - 72 in the winter.

My sealed house only has exhaust fans (3) in the bathrooms. My RH is steady at 50% in the winter and 40-45 % in the summer.


I could not be happier, however I never installed a HRV /ERV and am looking into that now so my questions are :

1 - Since I have a 2 story house and two separate HVAC units ( one in basement and 1 in attic ) whre should I place the HRV / ERV ? up or down? Sleeping area is upstairs so thats my first guess.

2 - ERV? I really dont have an excessive humidity problem, but at 50% - if I make the wrong choice with a ventilator I could introduce more RH and that would be an issue - Can anyone recommend one over the other? I'm fine in the summer, but winter i dont want any more RH.

3 - Should I get something thats capable of ventilating the entire house CFM wise? If its placed upstairs it could be too much airflow ( based on whole house ) and not have even air distrubution.


Thanks in advance for your help!

An indoor humidity of 50% @ 70F is an excessive humidity issue (for the building) in winter in cooler climates, and is the threshold at which dust mite populations can thrive. Where are you located? Climate matters! (Based on your high air conditioning bills I suspect you're in US climate zone 2A or 2B.)

Ideally in a cold climate you'd want to keep it 30-35% RH in winter to limit the moisture in the wall assemblies, while still keeping it in the health & comfort range for humans (30-50% RH) . While 2" of closed cell polyurethane is a Class-II vapor retarder, if the wall stack up does not include back-ventilated siding a 50% indoor humidity could still be pushing it in a zone 6 or colder climate.

The benefits of an ERV is that it tends to preserve the interior RH at higher ventilation rates. It doesn't add moisture, only exchanges moisture between the incoming and outgoing air streams. In winter the outdoor air is dryer than in summer, so an ERV would help keep the house from becoming uncomfortably dry as compared to running an HRV at the same ventilation rate.

ERV cores are more readily damaged by ice than HRV cores, if you're in a cold climate. Generally speaking US climate zone 4A and lower A-zones may be candidates for ERVs, but north of there it doesn't much matter unless you plan on really high ventilation rates in summer, when the outdoor air carries a lot of humidity. If you're in the gulf-coast region the summertime outdoor humidity is pretty high, so a higher venilation rates adds a latent load in summer, but that latent load is less if using an ERV. From a total air conditioning power point of view it's not an excessive load, but if you have reduced the total AC load with all of your building upgrades the fraction of the load that's latent is larger. With an HRV you may have to back off the ventilation rates on really sticky days to keep it under 50%, and would be able to run a higher rate with an ERV.

I am in southern New Jersey - sorry for the lack of info.

I was always under the impression that 30-50% RH. Was acceptable? I misrepresented my utility bills as that's the total bill - with lighting etc included. Lastly - if you had to choose where would you introduce fresh air - upstairs or down?

Both - and in every occupied room where the door is kept closed.

In a 2 story home with 2 zones that would require 2 units. That's not in the cards at the moment so where would the most benefit occur? Up or down? And would you size it for the entire house or just the unit feeding that floor?

At NJ's wintertime temps ideally you's want the interior humidity to be below 40%RH @ 70F. The 2" of closed cell foam will likely save you from the mold threat as long as it's below 50% though.

30-50% is acceptable for human health, but over 40% during the coldest weeks becomes a moisture accumulation issue for the building materials unless you have at least a class-II vapor retarder on the interior side. The 2" of closed cell is a class-II vapor retarder, but it's located closer to the exterior side of the stackup. The lower vapor permeance sufficient to protect the sheathing, but in a higher-R house it may not be sufficient foam-R for dew-point control for the fiber layers. But you're in a warm enough climate that it's not a large risk- it's in fact quite small.

When you add ventilation the wintertime indoor humidity WILL be lower. The dew point of 50%RH/70F air is about 50F, which is warmer than the average mid-winter outdoor air temp. Any outdoor air being brought into the house (even on foggy days) will be drier then the indoor air, lowering the interior RH. You may have to back off the ventilation rates during the coldest days to keep it above 30%.

Thanks - would you add it up or down? I'm thinking up because the bedrooms are upstairs - but wanted your thoughts.


Thanks for all the input - it's appreciated!

Which floor has the largest source of moisture? Bedrooms with humans in them 8+ hours a day, and bathroms upstairs?

Both need ventilation, but not at the same rate or in the same way. It's common to set them up for supply-only in the bedrooms, and exhaust only at the kitchen & bathrooms, since the latter are greater sources of indoor air pollution. In that sort of approach, if the partition walls & doors are super-tight you may need a door cut or tiny jump ducts to keep the opening/closting bedroom doors from unbalancing the system.

DJV.... what is your goal?
are you wanting more fresh air in the house...
trying to keep moisture down, ?

Are your existing bath fans on timers ?

Just throwing it out there... You could always put your 3 fans on timers and see what it does for you. It's not a perfect solution but it is certainly cheap.

Another option would be to put the HRV upstairs and do a spot HRV or ERV downstairs or vice versa.
If you put the HRV upstairs, would it hit every bedroom?