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Two questions about cost/benefit for insulation and ventilation in a mild climate

Colby Pierce | Posted in Energy Efficiency and Durability on

Hello all;

I’m in the early stages of designing a house and would love to get some unbiased input on a couple of sticking points between me and the builder.

1. We live in zone 4A, so we generally have mild winters and warm summers. After spending some time on this site, I was ready to advocate for the whole insulation package – closed cell spray foam and a rigid wrap to minimize thermal bridging. The builder I’m talking with is trying to talk me out of this, saying that for the expense is not really justified in a climate like this. He’s suggesting 2x6s with an inch or so of closed-cell foam and then filling the remaining spaces with blown-in fiberglass with adhesive, and a super light weight house wrap. He’s not generally opposed to a heavier insulation approach, but just doesn’t think it’s worth the added expense in this part of the country.

2. Either way, I’ll end up with a pretty tight house. I’ve read a lot on this site about different approaches to ventilation. While there are a lot of high-tech devices around, again I wonder about the relationship of where I am to what I need. We have windows open most of the year, which I assume would provide pretty good ventilation along with typical kitchen and bath exhaust fans. The builder is also suggesting a couple of cheaper alternatives to an ERV or HRV system, namely some recently introduced high-volume exhaust fans that run most of the time. While I like the low-tech approach, it seems to me that having exhaust-only fans (which essentially depend on air being pulled in through leaks when the windows aren’t open) kind of defeats the purpose of having a tight house. On the other hand, does my situation really justify spending thousands of dollars on an HRV or ERV system?

I’m not necessary looking for a yes or no answer, but if anyone can help provide a way to think through this it would be very much appreciated. Thanks a lot.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    The flash'n'fill 2x6 is code-min for zone 4a under IRC 2012:

    http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm

    Doesn't suck but it's nothing to write home about.

    Saving the foam budget for the EXTERIOR where you get the full benefit of the foam, unbridged by framing is usually a better way to spend the money. An inch of iso on the outside of a 2x6 16" o.c. wall and using cellulose cavity fill should cost no more than the flash'n'fill fiberglass (Spider?) proposed, but has a whole-wall performance of about R20 as compared to R14 after thermal bridging is factored in.

    And it is sufficient exterior R for dew point control at the sheathing to skip interior vapor retarders.

    Air tightness is easier but by no means guaranteed with a flash'n'fill, since that only addresses the leaks into wall cavities via the sheathing. Caulking the sheathing to the framing with (more flexible) acoustic sealant works at LEAST as well as flash'n'fill, but caulking under the stud plates, and between subfloor & band joists etc are all part of the necessary detialing to make the sheathing the primary air barrier. Taping the seams of exterior foam, as well as lapping & taping the seams of the house wrap are also well worth the effort.

    If your builder isn't familiar with how to do insulating sheathing he may over-price it but the costs really should be "in the noise".

    High-R cavity insulation in stud bays are a waste, since the performance is so severely undercut by the low-R framing. Even if you put some magical insulation that gave you R1,000,000 in the wall cavity , the whole-wall R of a typical 2x6 wall with a 25% framing fraction would only reach R23, a performance level achievable at modest expense with not-so magical cellulose cavity fill and 2" of exterior polyiso. The difference in whole-wall R between a damp-sprayed cellulose solution and a flash'h'fill with 1" of R6/inch foam and 4.5" of R4/inch 1.8lb Spider is less than R1(!). So what is that buck-a-foot foam and higher-cost fiber really buying you?

    All houses need ventilation, even leaky ones. Houses that meet IRC 2012 tightness specs (

  2. GBA Editor
    Martin Holladay | | #2

    Colby,
    Energy is cheap right now. Your builder may be right -- in a mild climate, with energy so cheap, it may be hard to justify additional insulation for your house.

    That doesn't mean that additional insulation isn't a good idea. For one thing, energy prices might rise in the future.

    The only way to determine an answer for your climate is to do some energy modeling on your house design. If you're not sure how to do that, you would have to hire an energy consultant to do the modeling for you.

    Every tight house needs a ventilation system. You don't need "high-volume exhaust fans that run most of the time," because most homes only need between 40 cfm and 60 cfm of ventilation. Here's more information: Designing a Good Ventilation System.

    In your climate, HRVs and ERVs probably aren't cost-effective. You may want one anyway, because of the benefits of good fresh air distribution. More information here: Are HRVs Cost-Effective?

  3. Spencer Burnfield | | #3

    A couple quick notes. If you are looking for a low cost insulation/wall upgrade you may find one in the "mooney" wall (try searching for it). You could do it with 2X6's or 2X4's and then choose the strapping thickness you desire. 2x4's plus 2x3 strapping give you a wall thickness of 6" so you don't lose much floor space, but gain ease of running electrical and less thermal bridging.

    There are a lot of factors that go into the HRV/Vent fan debate. There are good vent fans that can be coupled with fresh air vents. I know some people on this site champion the Panasonic Whisper Quiet coupled with a specific fresh air vent. I am sure someone will elaborate. The layout and size of your house and occupancy behavior are factors that will determine if you want to upgrade to an HRV. Whether it will save you money is also another good question. I used to say unequivocally yes it is a good idea....but now I am not so sure in a climate zone 4 Marine or 5 (I live on Washington State) after looking at some of the debates on this website. If you have a simple open floor layout you may be better off with an exhaust only fan system. I would still do it personally as long as I didn't have to pay someone to do it (the materials and the best HRV's available in the US should be less than or around $2,500). Paying someone would probably push me into the exhaust fan only, but that depends on the house design among other things. So I guess....budget, design, occupancy behavior....
    Spencer

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