Does my house need to breathe?
meganwood
| Posted in Green Building Techniques on
We’re building a house in the Southeast and want to try for the tightest shell possible. During the interview process several contractors keep telling us the house can be tight but needs to be breathable. Is there any science to this or are they just stuck in their ways? We’re planning on using some kind of air exchange system. Thank you kindly!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Megan, Others will no doubt chime in, but your contractor has confused air-tightness (a good thing) with the ability of a wall or roof to dry by allowing water vapour to move through it, known as permeability (also a good thing). If what they mean by "breathing" is permeability then they are right, but it's not a useful way to describe it.
Thanks Malcolm. I was also wondering if you could give your opinion or point me in the right direction on a related matter. We're building a 2400 sq ft. home in the southeast and none of the contractors we've interviewed are versed or even interested in green building techniques. So we're making all the decisions and quite frankly it is a bit overwhelming. We decided to go with 2x6 framing, zip sheathing and a 1" polyiso rigid foam with an unvented crawlspace. Unfortunately, I'm not sure the HVAC subs that will be used will have experience in sizing and installing an ERV system if we do manage to get our contractors to build a shell that is 2-3 ach @ 50Pa. A lot of the articles here are related to cold climate. We would be climate zone 3 about 2 miles from the coast. Is exterior rigid foam and zip sheathing cost-effective for our hot climate? And do you think we could get away with a ductless mini split heat pump? It's an open floor plan downstairs and bedrooms upstairs but we very rarely shut doors. Are we even going in the right direction? Thanks in advance!
You need to find a contractor. Is it possible to be your own GC? Yes, but if you don't know how to judge a sub's work, how will you know if they did it right before it's too late? It's a crazy risk.
If you tell us where you are, you might be able to get some referrals. You could try the USGBC chapter map as a starting point. http://www.usgbc.org/community/chapters/map
25 minutes outside of Charleston SC. We've been talking about being our own subs but it sounds like it could cause more problems than benefits. We do have a chapter of usgbc fairly close but it's pretty inactive and so far I haven't found a gc from it. I'll keep looking at it. Thanks!
Megan,
Q. "Does my house have to breathe?"
A. It's hard to answer this question, because the reference to "breathing" is a metaphor, not a clearly defined concept.
We need to distinguish between three possible interpretations of "a house has to breathe":
1. Sometimes people who use this phrase mean that a house needs a mechanical ventilation system to supply fresh air to the occupants. If that's what is meant, they are right.
2. Sometimes people who use this phrase mean that a house needs leaky walls and ceilings so that air can move through the walls and ceilings. If that's what is meant, they are wrong. Builders should strive to makes walls, ceilings, and floors as airtight as possible.
3. Sometimes people who use this phrase mean that a wall or a roof assembly should be vapor-permeable (that is, allow water vapor to pass through it). If that's what is meant, they are sometimes right and sometimes wrong.
You never want a basement slab or a slab on grade to be vapor-permeable.
A wall can be impermeable to water vapor and still perform well. Examples in this category include SIP walls, ICF walls, and PERSIST walls.
On the other hand, some types of walls perform best when some (but not all) of their layers are vapor-permeable. For example, double-stud walls with vapor-permeable exterior sheathing perform better than double-stud walls with vapor-impermeable exterior sheathing.
Thanks Martin. We were talking in the context of make the house air tight so I'm pretty sure they were talking about number two but they were pretty vague so I'll make sure it wasn't number three. Thanks so much!
Megan, the two critical subs on your project are going to be the insulation contractor and the HVAC contractor. A locally-based GC will be desirable even if they are not experienced in high performance construction, but they must be willing to learn and to go beyond their local stable of subs if necessary. Accept that the subs in those two critical areas may have to come from further afield. Your GC should be willing and able to help track down those critical skills, as should your architect/designer. You do have one, yes?
Thanks James! I'm looking into the critical subs in my area right now. I designed the house myself and just had a draftsperson draw it up. We oriented/designed our house to reflect passive solar principles but also want it to be high performance. Thanks for the advice!
Good for you Megan! My own experience building our home taught me to stick to my guns with what I want or make sure the contractor satisfactorily explains why their way is better. Then confirm they know what they're talking about!
http://scgreenbuildingdirectory.org has a few contractors listed in your general area that you might explore, and you could also talk to your local Home Builders Association and see if they have a Green Council or similar. Definitely, talk to former clients of anyone you're considering and make sure the contractor pays attention to your specific concerns. Individual subs are extremely important, but you also want to be sure your contractor is knowledgeable in how everything works together so one specialty doesn't cause problems in another (insulation/air sealing products vs tightness and indoor air quality, etc.
Re your ERV question, you will need about 60 - 80 CFM of air for a home this size, which is a pretty common size. Have your contractor follow the installation instructions and you should be fine. Your ERV will do the "breathing" for your house, so follow the adage "Build it tight and ventilate right".