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Community and Q&A

Tight Homes/Good Ventilation

GBA Editor | Posted in Energy Efficiency and Durability on

I am huge advocate of building tight homes. Air leaks, to me, represent poor workmanship that wastes a huge amount of energy. But, just for a minute, I want to be the devil’s advocate, primarily because I hear this argument all the time. Tight homes need good, dependable ventilation systems or the occupants will suffer and the durability of the building will suffer……whole house ventilation systems, exhaust fans in bathrooms, hoods, sealed combustion gas appliances, etc. My concern for these energy efficient buildings is that occupants must be knowledgeable and conscientious enough to use all the ventilation methods, and they must maintain these systems.

Bathroom exhaust fans must be used and cleaned periodically to maintain optimum air flow. And we want very quiet fans that won’t disturb people….which is good, except when it malfunctions and needs to be fixed or replaced. Of course, fogged up windows and mirrors will give occupants good indications, but then they must respond. I’m just saying this because I have a hard time getting my life’s partner to even turn on the fan. I think occupancy sensors with quiet fans are the answer as long as they stay on for 20-30 minutes after occupants leave. Yes, I see gunk matted up in the fan shield, but it takes me forever to go get a ladder and clean the damn thing.

And what about whole house ventilation systems. They are quiet and supposedly work for a long time. But what about 5, 10, 20, 30+ years from now, after the house has been sold 5 times. Homeowners, from my experience, are not very knowledgeable, nor are they very conscientious, especially about quiet systems like ventilation systems. You know it when the heating or cooling system is not performing….not many people, even myself, would know it if air flow is decreased over time in the quiet ventilation system. What happens? People start feeling uncomfortable, sick….mold starts to grow…will they even think to suspect the ventilation system? How do we require or expect homeowners to have good HVAC inspections/maintenance every year…when we are only now beginning to train them to know anything about ventilation systems? Obviously good homeowner education is the key, but there will be many cases of neglect and sometimes very unpleasant consequences.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Ed,
    Excellent questions! Ones with no easy answers.

    When I performed inspections of multi-family housing projects, I often found HRVs with filters that had never been changed, or HRVs with exterior air inlets clogged with spider webs and dried leaves.

    I also saw LOTS of disabled bath exhaust fans.

    In addition to worrying about IAQ, I also worry about wall assemblies that don't have condensation problems as long as the indoor humidity is under control. Once the ventilation fan is disabled, however, the indoor humidity can rise to the point that the wall assembly is tipped into failure. That's a good argument for robust walls with forgiving materials. And enough to keep many builders tossing and turning all night.

  2. Riversong | | #2

    Ed,

    You're quite right that the more we try to "solve" the technical problems our "better" building practices create with yet more complex technologies, the more potential failure modes we are creating.

    While I disagree with the tendency to make our homes like spaceships that are hermetically sealed against a hostile environment, I do think that - like all well-engineered technologies - our homes should not be made "fool proof" (since that's impossible) but rather "fail-safe".

    All artifacts fail. Those of us who've been around a while with our eyes open understand many of the potential failure modes. Houses can not, and hence should not, be built to be perfect (like Joe L's "perfect wall", or PassivHaus uber-tight, or REMOTE/PERSIST hermetically-sealed boxes) - because even perfection will eventually suffer imperfection.

    Instead, houses should be designed and built to fail gracefully. Walls and ceilings will someday leak, so they should be breatheable and moisture-tolerant and show evidence of leaks before they become problems. Occupants will ignore maintenance or forget to use fans, so houses must be made of hygroscopic materials that can buffer relative humidity, with inside and outside "skins" that are vapor permeable to allow drying in either direction.

    The more we rely on plastic sheeting and foam and impermeable membranes, the less tolerant our homes become to the inevitable failures down the road. If a house is almost completely air-tight, then it cannot support life without artificial respiration (mechanical ventilation). A little natural air exchange is a good thing from an occupant health perspective and a house health perspective as well. Exhaust-only fans with passive make-up air inlets allow for some non-powered ventilation when the fans aren't working or aren't used.

    We are building our homes like space ships, but the terrestrial environment is not yet inhospitable to life. A shelter should be a semi-permeable membrane that does not completely isolate its occupants from the natural world. Flexibility and resilience are the keys to sustainability - in nature and in human artifacts.

  3. User avatar
    Danny Kelly | | #3

    Hi Robert - sounds like you are against rigid foam sheathing on the exterior of walls. I have read a lot (on this site) of the benefits of this - added R-Value, ruduced thermal bridging, etc. I understand all of that and have finally accepted this is a better way of constructing a wall although I still cannot get over the fact that most of the rigid foam boards out there are less than 1 perm - everything we (the building science community) have always said was no vapor barrier on either side of the wall that it must be able to dry to both sides (here in the mixed humid south anyway) - seems like we are breaking our own rules. Since I have begun using it, I have not taped the seams and have used a seperate weather resistive barrier thinking this may allow some moisture to escape if it gets in the wall understanding I am probably losing some of my tightness by not taping the joints. What do you think.

    ED - I have had a lot of the same concerns - most homeowners barely do the simple things to take care of their house - clean gutters, change HVAC filter have a maintenance contract on HVAC, etc. How could we expect them them to take care of "advanced features"? I think it really comes down to education and seems like most of the green building programs out there do stress the importance of homeowner education and instruction manuals. I guess our second line of defense it to make sure the home will show signs of a problem before it get sout of hand.

  4. Riversong | | #4

    Danny,

    Untaped foam board joints will do nothing to increase the permeance of the envelope, though it may increase its air permeance which isn't a good thing.

    Air under pressure will find any outlet, which is why air barriers need to be nearly perfect. Vapor under pressure will move one-dimensionally toward lower vapor pressure, so the average permeance of the assembly will determine the flow rate. To create a vapor permeable envelope, the entire surface must be permeable.

  5. Andrew Henry | | #5

    Ed,

    Essentially, your concern or what you are asking is "Will homes (buildings) be resilient enough to handle the indifference and neglect, and occasional "act of God" that they are subjected too?".

    Of all the building envelope implementations that I have read about here, and elsewhere, the ones that have impressed me, the ones that had resilience designed into them were Robert's Modified Larsen Truss approach and Thorsten Schlupp's REMOTE (or PERSIST).

    I regularly check the humidistat in my house to make sure things are optimum. I'd be less anxious with Robert's MLT or Schlupp's REMOTE.

    We had a couple of days without power after a bout of freezing rain brought branches down on the hydro wires. It was interesting watching the humidity climb over the two day period. The house is well insulated so we didn't have to move out, and fortunately the temperature outside remained just below freezing as opposed to the typical -20 Celsius lows. But it reminded me how fundamentally important the HRV in my house is. An HRV does more than just providing fresh air.

    Andrew

  6. Danny Kelly | | #6

    Thanks Robert - I forgot to mention that we are using dense packed insulation which they say is an air barrier which was why I was ok not taping the seams but your explanation makes sense. So if we do tape the seams in the foam which will give us a vapor barrier on the exterior of our wall - do you think this is an ok design? Are there any products that will give us the thermal break, additional R-Value but not a vapor barrier?

  7. Riversong | | #7

    Danny,

    While dense-pack cellulose is highly restrictive to convection, it does not qualify as an air barrier by the standards of the Air Barrier Association of America. Other options for exterior insulation are mineral fiber semi-rigid boards, such as Roxul and ThermaFiber. They have better than R-4 per inch and are fireproof and self-draining as well as being highly vapor permeable.

  8. Ed Welch | | #8

    Yes, perhaps, we just have to do our best, then accept imperfect building systems. I still think we need to build tight buildings then work on homeowner education to hopefully prolong the effectiveness of ventilation systems by getting customers to maintain them.

    Exterior Foam construction is still a mystery to me, primarily because of exterior window and door placement. In traditional construction, you have a housewrap that you cut to overlap the window or door flashing. With exterior foam (or with the ZIP system sheathing), it seems that you have to rely on tape over the windows to the foam....since the housewrap or WRB is installed below the foam. This does not sound like a good idea. What am I missing?

    Drying to the interior...yes, Andrew, an HRV in this system would do much more than just provide fresh air. It is the lifeblood of the system....if it fails or if airflow decreases too much, the whole system crashes. That is my concern. We need some sort of dashboard light, telling customers to clean and maintain your ventilation system which seems to be more important than the heating, cooling....everything!

  9. Riversong | | #9

    "In traditional construction, you have a housewrap that you cut to overlap the window or door flashing."

    I would call that "conventional" not "traditional". Traditional building practice generally called for shingled strips of felt (tar paper) as the weather barrier and secondary drainage plane, and shingled felt gaskets under window trim. It's much simpler to integrate 3' wide strips with door and window flashings, while keeping the weather barrier shingle-lapped. In fact, of the four AAMA approved methods of flanged window installation, the primary two (A & B) require shingled WRB.

    Wide plastic housewraps require slots cut to create a flap over the windows, relying solely on tape to seal them, and tests have shown that half of those taped diagonal cuts leak under pressure. Foam and Zip systems also rely on tape to last the life of the building.

    Just as shingled sidings and roofings have worked for thousands of years, shingled WRBs have worked for more than a hundred years. That's traditional building, and it makes much more sense than what we do today.

  10. Danny Kelly | | #10

    Robert Thanks - will check those products out - very helpful - I love this site.

    Ed - I agree - I hate systems that rely on tape to last 100+ years - I'm not betting on that. Even with the foam which should count as a WRB - I still install felt or DOW housewrap as an additional WRB as a piece on mind - also works much better around windows, etc.

  11. Andrew Henry | | #11

    Hi Ed,

    "We need some sort of dashboard light, telling customers to clean and maintain your ventilation system"

    My VanEE HRV (it's a Venmar brand aimed at contractors) has a light that comes on it's controller when the filter needs to be cleaned. I would hope most HRVs do.

    The HRV can deal with the humidity in my cold climate during the winter but in the early fall, spring and summer I am starting to realize that I need a de-humidifier.

    Andrew

  12. Ed Welch | | #12

    Thanks Robert for that clarification on the wraps. I guess builders stopped using felt when marketing departments sold the Tyvek systems as breathable and better that felt.

    By the way, I'm planning to install a Triangle Tube Indirect WH in my house (I think), connected to the existing oversized boiler. But I am embarrassed by the costs...they want $1500 bucks for the Smart 50 plus installation from a boiler specialist who gets top dollar....another, too-expensive for prime time product. Not sure if it will be worth it but I think I'm going to go for it.

    Danny--Do you put the felt or Tyvek over the top of the foam? I had read several sources that recommended putting in underneath the foam. But I'm not sure how that would work if you did not have sheathing on the framing. If I ever use the foam exterior, I would want to eliminate the sheathing (except for corners), and add diagnol bracing to get shear strength, trying to save money on wrapping the building so many times. I've also read that it is better to divide your foam thickness by 2, then stagger the joints to get better air sealing. Not sure if that is necessary.

    Andrew--Thanks for the tip on the HRV light....sounds like we need 'command central' to keep us informed about maintenance. Or I just need someone to carry a ladder upstairs so I can take the two minutes to clean the bath fan plate!

  13. Danny Kelly | | #13

    ED - only done it a few times so may not have all the answers - this is a new detail for us but tried to research it and have started/followed several strings on this site (will see if I can go back and find it - was last summer some time). I personally am not a big Tyvek fan (perm rating of 60ish) - I use 15# felt or the DOW housewrap (In NC - mixed-humid so they recommend a perm rating between 8-15ish from what I remember - but I do put it on the exterior of the rigid foam - figure that if it is the WRB - might as well put it closer to the weather. I have heard of people putting on the inside of the foam but I think that may make more sense if it is serving as the air barrier - probably ok either way but I prefer the exterior mainly so it can be incorporated into weep hole flashing and getting my WRB over top of window lintels, etc - just easier to make sure all of the flashing detail work.

    I used the 1" DOW SIS which they say is structural - being a little leary on new products - I did install 1x4 let in braces just to make me sleep better. I'm sure staggering the joints is the best way to do it but the stuff is pretty pricey, about $22 per sheet for the 1" and the 1/2" is not 50% less so it would have been much more expensive to do it that way - besides was doing it more for the thermal break than the air sealing so not sure if it would have been worth the extra expense - we still had a pretty tight house - minimized exterior holes and caulked the foam to the stud as we installed it. This was a one story house - getting ready to do a two story house so considering using 1/2" OSB on the corners with 1/2" foam over top with 1" foam in the field.

  14. Steve | | #14

    From the research, I have done on this subject, allowing outside air an avenue of ingress into the interior colder (HVAC in operation) and lower vapor pressure area will cause the exterior hot humid air to precipitate moisture on colder surfaces. It will not be long before mold and fungus spores are the growing and multiplying at these points. There are engineered devices that allow outside air to be conditioned as it enter the device to refresh the interior air quality.

    As far as gutters, new gutters have been designed that are clog free and leaves do not enter the drainage system.

    As far as disabled bath exhaust fans, there is no real cure for STUPID!

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