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Community and Q&A

Fresh Air Supply in a Tight House

paula_builds | Posted in General Questions on

Dear GBA friends,

-I’m hoping to make my home pretty air tight, as I have high ceilings throughout.
-Air tightness complicates other things too, though.  A brief overview of my situation.
-Western Oregon, climate zone 4, mild & rainy winters, hot dry summers.
-Woodstove (Morso 7110) has the option to duct air into the back, which I didn’t do yet.  Stove is installed.
-Midea Heat Pump (ducted) will be installed with short duct runs to two wings of the house from a central closet.  The current plan is to have the air supply via a louvered closet door.
-Shower fan is 100 cfm, set to run via humidistat, with a manual switch override.
-Powder room fan is 80 cfm, runs on a timer.
-Stove vent hood is 250 cfm max.
-I do not have plans to install a dryer.
-Crawlspace is conditioned, and the plan is to circulate air from crawl into house via a fan, through a filter.  Something like this (
-Air return to crawlspace can be through a grate, with filter again if necessary.

-If anyone recalls my previous post, I have now installed plastic on the crawlspace floor, which I hope will reduce further moisture/mildew in the crawlspace.  I have also mixed up a bucket of white wash with which to coat the underside of the subfloor, and floor joists, as some mildew had formed prior to my putting plastic down.  All this to say that I hope I will have a somewhat pleasant crawlspace, and I will monitor the humidity down there.  I know the idea of sharing air between home & crawlspace is controversial.

But the main point of this post is to try to learn more about air balancing.

I hope to finish air sealing around my windows and doors with backer rod & caulk.  Other penetrations have been detailed carefully.  Then I can get a blower door test done to determine how tight my build is.  I do not have insulation or drywall up so presumably I can fix things if the envelope isn’t very tight yet.

1) I could install an outdoor air supply kit to my stove.  Should I wait to see how the wood stove performs first, before doing this?  I understand that installing this kit would prevent the danger of backdrafting.

2) I am curious how the air supply kit wouldn’t be a big air leak.  Is there a way to open/close it, as one does with the damper in the front of the stove?  Does it close when there is no fire?

3) When I think about the big picture of my home envelope, I have 3 places where there will be intermittent exhausting.  Aside from the possible addition of an air supply to the wood stove, I do not have any other fresh air being added into the home (yet).

4) I had considered a Heat Recovery Ventilator (HRV) but backed away from that, as it seemed excessively complicated and expensive.  Also, I have been told that these make more sense in places like Vermont where winters are very cold.

5) Is there a way to supply air into the home on an as needed basis?  Like if I were to be running a bath fan AND the stove vent hood simultaneously, then some mechanism would open and allow fresh air in through (perhaps a mini HRV)?

6) I do not intend to ALWAYS keep my windows closed.  My home is located in the country, and from time to time I do hope to experience bird song, or the sound of rain.  It has been pointed out to me that life in a perfectly sealed envelope isn’t the greatest.  But when it’s damp & clammy out, I am sure I’m better off stepping outside rather than opening a window and letting the warm air rush up to the ceilings.

I would like to take it slow and add complexity on an as-needed basis, but at the same time, now is when my walls are open, so if there are systems that must be added, this might be a good time.

I look forward to hearing more.  Please let me know if you need more details in order to respond, and thanks in advance!


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  1. charlie_sullivan | | #1

    First, I want to say that your house looks terrific, in a beautiful setting. It looks very inviting.

    Part of the idea on air supply kit of the stove is that you then keep the vents into the stove from the room closed, so the only air flow path is into the stove and up the chimney, not into the room. But that isn't perfectly air tight, and even with no fire, you can get cold air flowing into the stove and up the chimney, driven by stack effect from the heat in the room, and thus cooling the room. So some people install dampers in the intake--I've heard of people even using large plumbing valves to get a really tight seal.

    Sorry not to answer more of your questions but at least that's a start...

  2. GBA Editor
    Kiley Jacques | | #2

    Hi Paula,

    I second Charlie's compliment. I have enjoyed reading your updates--you ask such thoughtful questions. In fact, if you are interested, I'd love to hop on the phone to talk about your build for possible editorial coverage on GBA (maybe FHB too). If this is of interest, email me at kjacques@taunton.

    Regarding your questions, here are a few resources that should prove helpful:
    How to Provide Makeup Air for a Wood Stove, this Q&A conversation, and this episode of the BS* + Beer Show: Playing With Fire: Indoor Combustion.

  3. arnoldk | | #3

    I just purchased the SBI Fresh Air Intake Register with airtight damper for my wood stove setup I will have in my new build that's based on Passive House standard. I believe they sell this in the 3, 4 and 5 inches.


  4. paula_builds | | #4

    Thank you for the responses! And for the compliments, that is nice to hear! :) I think I'll have plenty to read following up with the links you have provided so far, so thank you!

    It will be interesting to find out how tight my house is, and then I can compare my situation to those I am reading out.

    I will follow up with more comments after I read & study the resources linked.

    1. chicagofarbs | | #7

      Hi Paula,

      Make up air for combustion could be in the hundreds of cfm. If you plan on having a tight home, you’re going to want to plan for a dedicated make up air solution for the wood burner. Something like the mechanical airtight damper Arnold poster. I didn’t watch the BS link Kiley posted, but I imagine they suggest something similar.

      For me, exhaust only ventilation in a tight home is not something I’d like to roll the dice on, especially if you have a wood burning and/or other gas equipment/appliances (?). If you insist on this approach due to cost, I highly recommend tracking your indoor air quality, to make sure your CO2 levels and other indoor air pollutants don’t get to high. If they do you can open windows to force airflow in while your exhaust fans run.

      There are H/ERV’s that run only when ventilation is needed, like the Build Equinox CERV. This system will track CO2, VOCs, and small particulates and bring in fresh air when those pollutant thresholds are met. When fresh air isn’t needed, the system circulates indoor air around house to keep things balanced.

      Heat and Energy recovery ventilators are about bringing in fresh air in, in an effective manner, to keep your home’s indoor air quality high. I don’t understand the comment about them being better in Vermont. Looks like western Oregon can still see 30F in the winter. You will still want to recover heat from your exhaust air to precondition the incoming cold fresh air during these months.

      I would revisit an H or ERV again, as indoor air quality to me is just as important as energy efficiency.

      1. paula_builds | | #9

        Hi Chicago,
        Thanks for responding, and providing specific products I can look into.

        Just a clarification about the Vermont comment - our winters overall are much milder than Vermont, so while we occasionally have temperatures below freezing, much of our winter nights have lows in the 40s. So I guess I was thinking that while there is still an advantage to an HRV/ERV as opposed to the alternative (cracking a window at worst, passive air inlet as perhaps a compromise), it is less of an advantage.

        But certainly all the reasons to install a fresh air supply and retain the energy in the warm air are valid.

        The woodstove is the only combustion appliance I have planned at this point. Range will be electric.

        1. chicagofarbs | | #10


          In my opinion, balanced ventilation (with energy/heat recovery) is an advantage in every single way except for first cost...

          In your scenario, even if you crack a window or have a passive inlet, you are still letting 40F dry air into your home that will need to be heated ~30F (or whatever your desired set point is). In this same scenario, if you are using an E/HRV with 80% recovery effectiveness, you preheat the incoming air from 40F to 64F, which would then require very little additional energy to reach your indoor set point. With an energy model, you could calculate the operating costs for both scenarios.

          For me, the reason is to pursue an E/HRV is to provide high levels of IAQ in the most energy efficient way possible. With more people working from home, COVID has really shined a light on IAQ and how inadequately we have been approaching it. I’ve been tracking our IAQ at home with an Awair Element and we frequently break 1,000 ppm CO2 in a large space with only 2 humans and 2 dogs. We have a leaky home with exhaust only ventilation.

  5. Expert Member
    Akos | | #5

    I've never regretted installing an air exchanger in a new build. The BOM cost is pretty much a wash when you can eliminate 2 or 3 bath fans, plus you eliminate the holes through your walls for each bath fan. In some ways it is a simpler install.

    It does require some up front design and extra bit of ducting.

    With a ducted mini split, the simplest install tends to be a hybrid ducted setup similar to the one bellow here:

    You run stale air pickups to each bathroom, kitchen and laundry room and the fresh air supply to the mini split return. Since the fan on the mini split runs 24/7, it can be used to distribute the fresh air to the rest of the house.

    A couple of things you do have to watch for this type of setup. Make sure to get an auto-balance ERV/HRV and the pressure drop in the mini split return ducting should be pretty low (less than 0.1" wg).

  6. brp_nh | | #6

    For some background reference, I'm not a builder or building professional, but did GC our small house build in climate zone 6. We paid attention to air sealing, along with our shell contractor, and tested well under 1 ACH50. There's basically no make up air when building this tight (everything is taped, caulked, foamed, and sealed up) so you have to account for all the exhaust systems you're installing. Better to add some complexity now versus having to retrofit later.

    I don't have experience with wood stoves, but from reading GBA, I believe you'll have to add a dedicated air supply.

    We went with an exhaust ventilation system based on bath fans and passive air inlets. It has worked out ok, but in retrospect, I would have skipped the bath fans and installed a quality HRV. If you're not totally committed to your two bath fans, I'd seriously consider pulling them and installing a HRV as your ventilation system for the house.

    Is your stove gas or electric? If electric, I'd consider a re-circulating range hood so you don't have to factor in the 250 cfm max vent. But if you cook a lot, the planned vent could be good and you'll have consider some make up air for it.

    The house looks great.

    1. paula_builds | | #8

      Thanks Brian.

      I'm open to installing a dedicated air supply for the woodstove, and I'd be open to pulling out one of the bath fans & installing an HRV, or maybe just installing an HRV in addition to what I have installed now.

      My range is electric. Also I believe that re-circulating hoods aren't allowed here. I cook elaborate meals infrequently, so perhaps just cracking a window while vent hood is on could work for me as far as addressing the 250 cfm there.

      I am interested in the passive air inlets you mention - could you link me to a product? Something like this?

      What is it specifically about this solution that you do not like/love? I like the idea of having it supply air when the fans are on, and then do nothing when the fans aren't on. Do you like an HRV better because it would save more energy (exchange heat)?

      I had bookmarked this ERV as well:

      I hate the idea of having humming machinery running - someone had mentioned an HRV that was squeaking until it was tuned/fixed. The more complexity that is introduced, the more likely I'll come across a glitch that I'm unable to fix. I'm a woodworker, not an engineer or technophile. I am finding my way but reluctant to introduce excess complexity if not necessary.

      I know there are people here who are going for 99% efficiency - I am not going for perfect Passive House or some other standard, but just for good quality efficiency, comfort. I'll be heating with wood that is basically free, a lot of the time, but I also would like the envelope to be relatively easy to heat and keep comfortable.

      Still following up on links provided - thanks to all who've responded.

      1. brp_nh | | #11

        I think that one of the overall benefits of a properly sized HRV is that it meets all of your ventilation needs, so you don't have to mix/match bath fans with something like that small Panasonic ERV. I hear you about not wanting to introduce complexity, but installing one HRV now could be simpler (and make more sense) than dealing with multiple bath fans, passive air inlets, etc.

        "I like the idea of having it supply air when the fans are on, and then do nothing when the fans aren't on." If your house is tight, you're going to need continuous ventilation when your windows are closed, or it may feel stuffy with elevated CO2.

        Yes, we have those Panasonic air inlets. They are rated 12-18 cfm, but we've found that's optimistic and they probably only allow 10 cfm. As I mentioned, our house is extremely well air sealed...even though we only run one of our bath fans at 30 cfm continuous, we've had to install 4 passive air inlets for enough makeup air. We can't boost the fans to 80 cfm (unless windows are open) because it will significantly de-pressurize the house. The downsides to this system: 6 holes (2 bath fans, 4 inlets) in the exterior to deal with (flashing, trim, cleaning), too much cold dry air in the winter, can't boost fans when house is closed up, not balanced, slightly elevated CO2 at times, contributes to slightly elevated radon levels. This system certainly hasn't been a disaster or anything, but if I could go back, I would definitely ditch all this for a HRV. We may retrofit the house with a HRV after this winter (and rip out the bath fans and air inlets).

        "I know there are people here who are going for 99% efficiency - I am not going for perfect Passive House or some other standard, but just for good quality efficiency, comfort." I completely understand. All I can say is that if I could go back to when our house was under construction, one of the big do-overs would be installing a HRV instead of the bath fan and air inlet combo.

        1. chicagofarbs | | #12

          Great testimony from Brian!

          Paula, one thing to note, if you elect to go with a HRV decoupled from your bathroom exhaust fans, you will be exhausting out a good amount of air that will not be available for heat recovery. My suggestion would be to go all-in, one way or the other, and to not mix-and-match approaches.

          1. paula_builds | | #13

            Thanks Brian and Chicago. I will be discussing all this with my HVAC person - as well as a neighbor who specializes in energy efficiency. This input is really valuable so I thank you.

        2. paula_builds | | #15

          So if I were to get a HRV or ERV, would it be intelligent enough to sense the need for air when an exhaust fan is running, and to supply air accordingly? Brian you said that the passive air inlet is only 10 cfm - is there a sensor for the HRV that can tell what air is needed to be added in order to balance things? Does an HRV come with a cfm range that it can supply?

          My bath fans are already installed (I have two because I put the shower in it's own room, and the toilet/sink has a separate room).

          Also the siding is done, and the house is painted, but I can figure out a way to make another hole if necessary.

          1. brp_nh | | #16

            Looking over your previous posts, it seems you decided on a HRV at one point: "He came up with a plan to use the HRV to ventilate the bathrooms, move air in & out of the crawlspace as well as provide fresh air to the dwelling."

            The HRV or ERV would be a closed system with roughly equal amounts supply and exhaust for itself, and not meant to supply make up air for other systems (like a bath fan).

            HRVs are available with different specs for their range of CFM. They are often configured to run continuous at a lower volume (ex: 50 cfm) and then have a boost feature when higher volume is needed (ex: boost to 100 cfm when showering).

            Keep in mind I'm just a homeowner that was pretty involved in my own house build, but it seems like you have a couple main options:

            1. Go with your current setup. Assuming your house is really well air sealed and there will be significant periods of the year with windows closed, you'll need to install enough passive air inlets (or another make up air system) to roughly meet the CFM specs of your bath fan. Is your main bath fan a model that can run continuous at a lower volume and then be boosted during showers?

            2. Go back to the HRV plan and take out the bath fans. Size and setup the HRV properly. If setup properly, the HRV can exhaust air from the bathroom(s) and maybe another location, along with supplying fresh air in chosen locations.

            The 250cfm range hood is another story, you'll probably have to open a window when running that.

          2. chicagofarbs | | #17

            Paula -

            I agree with Brian's main takeaways above.

            I would not try to combine passive inlets, bathroom exhausts, and an HRV all together. You could probably make an HRV work just fine and keep your bathroom exhausts in, it just won't be as efficient on its heat recovery (since you are decupling).

            You know both of our preferences are for option 2, get the HRV installed properly, and route your bathrooms to it.

            Agreed that the kitchen exhaust will likely need to be supplemented with a near by window when you run it.

            There are HRVs that auto balance so it can ramp up/down depending on pressure. The Zehnder unit comes to mind.

            Hope this helps!


  7. Deleted | | #14


  8. brian_wiley | | #18

    Hi Paula,

    First, like the others said, you have a beautiful home there! I grew up in Southern Oregon and that picture made me miss it.

    I totally get where you're coming from in terms of the added complexity of an HRV or ERV. Like others have said, it may be the best way with your walls still open and accessible. However, there's an older article on this site (link below) that addresses the use of all these different ventilation systems. It'd definitely be worth a read, but there were two things that might be applicable to you situation. One was that passive inlets don't always work well—and sometimes don't work at all—due to pressure differentials, which I hadn't considered before.

    The other was the idea of a central-fan-integrated supply ventilation system, and that these systems can be balanced ventilation by tying together your air handler, a fresh air intake, and your bath fans. I became interested in this approach due to the ability to selectively apply positive pressure to the envelope during wildfire smoke events in an economical way, which may have some applicability given your location.


  9. paula_builds | | #19

    Hi Brian,
    I did read a large part of that thread, thank you. I am letting all these thoughts percolate.

    In the mean time I installed a vapor barrier in my crawlspace and I see it is doing its job already, so that is a good feeling. I am hoping once gutters are on there will be less visible moisture under the 10 mil plastic layer.

    So far I have decided that I'm likely going to install the air supply to the woodstove.

    Then after I get the air sealing done, I'll have a blower door test done, and decide based on that information whether to proceed with installing a HRV such as the lunos e2.
    My understanding is that this is on the cheaper end of possible HRV installations?

    I have vaulted ceilings so no attic space within which to house a larger unit (although I do have that central utility closet).

    The house has been sided and painted, so I'm wondering about punching a hole through the siding, but assuming there is a way, should I choose to proceed with the HRV.

    One more question for today - how does one locate one's previous posts? I have often wished I could go back and re-read some of them - after I deleted the emails that alerted me to replies.

    Thank you all,

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