Unvented Crawlspace in PH?

[Benjamin Wolk-Weiss]

Hey Gang,

So I'm currently working on the permit drawings for a PH for my local Habitat for Humanity chapter in Corvallis, OR and am unsure of how to address the crawlspace with regards to vented/unvented. I would like to do an unvented crawlspace, as Joe Lstiburek reccommends, I am just not sure the best and most cost effective way to mechanically ventilate it to comply with the building code and meet PH requirements.

How have others dealt with this issue? Or should I just leave it as a vented crawlspace?

The first floor will be insulated from the crawlspace with blown-in fiberglass and the subfloor will be acting as the air barrier in that location.

[em...@haydenrobinson.com]

Ben, 

If you go the vented route, you will not want to insulate between the 1st floor and crawlspace. Rather, you need to include the crawlspace in the thermal envelope, like it was a small basement. From an energy standpoint, I suspect your best bet will be to ventilate it by linking it to the main floor - since this would run crawlspace ventilation air through the heat exchanger, rather than venting it directly out. Take a look at  IRC R408.3.2.1:

R408.3 Unvented crawl space. 

2.1. Continuously operated mechanical exhaust ventilation at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7m2) of crawlspace floor area, including an air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with Section N1103.2.1 of this code;

-Hayden

On Wed, 4 Nov, 2015 at 2:46:16 PM, Benjamin Wolk-Weiss <be...@zoeticarchitecture.com> wrote:

 

To: Passive House Northwest

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[Jason Quinn]

If you want to go with an unvented crawlspace as Joe Lstiburek has recommended in the past then you will need to treat it as a basement (as Haden noted). Provide fresh air from the H/ERV and the floor of the crawlspace is now your thermal envelope. You will need to make certain that the airtightness layer (which is now on the floor of the crawlspace) is above your condensation temperature or put insulation on the cold side of it. Once you treat the floor of the crawlspace like a wall or roof it makes sense and you know the drill. 

After looking at all that you may want to go to a vented crawlspace where the airtightness/vapor control/thermal layer are the floor of the living space. If so you need to make sure that when moisture condenses on the underside of the floor it does not cause damage. This is the main issue Joe is trying to avoid as in the spring the cold floor of the crawlspace radiation couples with the underside of the floor and then you likely have condensation or at least high surface moisture and the potential for mold. I've looked at this option in several buildings and then went with a slab on grade - once with the insulation on top of the slab - just cheaper and lower risk in this market.

 Luck

Jason Quinn
+64 21 1846 911

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[Adam Cohen]

Both Blake Bilyeu (sp?) and Artisan have done vented crawls (talk to them)

John Semmelhack has done unvented crawls.

The unvented ones add exterior surface area (heat loss) without adding height or SF.

As joe L says" The best crawlspace is one that is filled in can called a slab or dug out and called a besement!😋

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Adam Cohen
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[Ben Wolk-Weiss, RA]

Thanks Adam,

I will contact Blake, Artisan, and John. I've been meaning to get in contact with Blake since he is only 45 mins from me. We do crawlspaces with Habitat here because it is easier for us to build with volunteers and for the homeowners to maintain. I'd prefer a basement myself, but that's extra money we don't have to spend on a habitat budget.

​

Benjamin Wolk-Weiss, RA, SHP, CPHC®

LEED Green Associate

Zoetic Architecture

P:908.507.5737

F: 320.923.2204

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[graham]

If you opt for a crawl space (arguably not the best option anyway) I would suggest you vent the crawlspace, but cover the ground with a vapor retarder and reduce the vented area to 1/1500th of the floor area by code, then insulate and air seal well between the crawl and the conditioned space above. If you're running water supply pipes in the crawl space, I would suggest you put them in the floor cavity to get them well-insulated. Basically, keep your pipes, ducts, etc. inside the envelope.

As others have stated, a "conditioned" crawl space (unvented and unconditioned is against code) is really a short basement that you've got to insulate on all five sides and use energy to ventilate. I'd be hard pressed to think of a justification for the extra heat loss and ventilation energy from a performance standpoint.

I am with Lstiburek that the best crawl space is "filled in and called a slab or dug out (and insulated) and called a basement," as Adam quoted. That said, flood zones, sloped lots and other circumstances may call for it.

Regards,

Graham

[Blake Bilyeu]

Ben,

I look forward to hearing from you -- we just finished a PHIUS+ certified project in Philomath, right next to you. 
I would not do a conditioned crawl.  We're in a limited area of the country where vented crawls actually perform quite well and don't suffer any condensation issues when built correctly.  The last time I spoke with Lstiburek, and pointedly asked him about PH floor systems in the PNW, he said his first choice would be an insulated slab on grade (of course) but his second choice would be a variation of a vented crawlspace.  He said he would not do a conditioned crawl.

Blake

Blake J. Bilyeu
Certified Passive House Consultant
Sustainable Homes Professional

Green Home Design/Build & Consulting
Bilyeu Homes, Inc.
503.508.6183
www.bilyeugreen.com

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[Ben Wolk-Weiss, RA]

Hi Blake,

Yea, I saw that on your website. Really nice work. Id love to hear more about it and the permit approval process you went through with the City of Philomath as I am almost ready to submit the plans for permits.

 I really appreciate the input. I don't want to condition the crawlspace since that brings it into the thermal envelope. It is good to know that traditional vented crawlspaces work well out here. Most of my residential background is from living and working in New Jersey where everyone had a basement so there wasn't really this choice between slab and crawlspace, vented and un-vented.

I think I will take your's and Graham's advice and just do a vented crawl with a vapor retarder on the ground (which I was planning on having anyways). We can't afford the amount of underslab insulation required to do a slab on grade. We do get some donated foam from Dow but it is not structural grade and thus wouldn't hold up underneath a slab.

I appreciate the advice from everyone. That solves my decision issue with the crawlspace.

​

Benjamin Wolk-Weiss, RA, SHP, CPHC®

LEED Green Associate

Zoetic Architecture

P:908.507.5737

F: 320.923.2204

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[Bronwyn Barry]

You may want to look at the long-term challenges with vented crawl-spaces in PH's...

We've struggled with this at One Sky Homes and have now tried all the options: sealed crawlspace (our preferred choice), insulated slab (cheapest) and vented crawlspace (only for when the previous two are not an option.)  Biggest down-side with vented crawl spaces is that any repair work or future renovations will compromise either your insulation or air-barrier at the floor (and likely both.)  Be sure to provide an alternate access to the bath tub in your design.  This is currently a challenge for our PHCA Laney College project, where (for many other reasons) we opted to go with the vented crawlspace.

Durability and repair access should be a factor in your design, if you have the choice.  

Bronwyn Barry, CPHD

Director - One Sky Homes

m: 415.819.7978

t: @passivehouseBB and @oneskyhomes

http://www.oneskyhomes.com/video/hair-dryer-home

www.oneskyhomes.com

  www.co2zeroca.org   

[em...@haydenrobinson.com]

Ben,

Following on the information from Blake, here a WSU an article on crawlspace design in the Northwest: 

http://www.energy.wsu.edu/documents/AHT_Conditioned%20crawl%20spaces%5B1%5D.pdf

It also concludes that vented crawlspaces perform well in our region.

-Hayden

On Thu, 5 Nov, 2015 at 12:06:55 PM, Blake Bilyeu <bl...@bilyeugreen.com> wrote:

 

To: Passive...@googlegroups.com

Ben,

I look forward to hearing from you -- we just finished a PHIUS+ certified project in Philomath, right next to you. 
I would not do a conditioned crawl.  We're in a limited area of the country where vented crawls actually perform quite well and don't suffer any condensation issues when built correctly.  The last time I spoke with Lstiburek, and pointedly asked him about PH floor systems in the PNW, he said his first choice would be an insulated slab on grade (of course) but his second choice would be a variation of a vented crawlspace.  He said he would not do a conditioned crawl.

Blake

Blake J. Bilyeu
Certified Passive House Consultant
Sustainable Homes Professional

Green Home Design/Build & Consulting
Bilyeu Homes, Inc.
503.508.6183
www.bilyeugreen.com

On Nov 5, 2015 11:48 AM, "graham" <gra...@essentialhabitat.com> wrote:

If you opt for a crawl space (arguably not the best option anyway) I would suggest you vent the crawlspace, but cover the ground with a vapor retarder and reduce the vented area to 1/1500th of the floor area by code, then insulate and air seal well between the crawl and the conditioned space above. If you're running water supply pipes in the crawl space, I would suggest you put them in the floor cavity to get them well-insulated. Basically, keep your pipes, ducts, etc. inside the envelope.

As others have stated, a "conditioned" crawl space (unvented and unconditioned is against code) is really a short basement that you've got to insulate on all five sides and use energy to ventilate. I'd be hard pressed to think of a justification for the extra heat loss and ventilation energy from a performance standpoint.

I am with Lstiburek that the best crawl space is "filled in and called a slab or dug out (and insulated) and called a basement," as Adam quoted.. That said, flood zones, sloped lots and other circumstances may call for it.

Regards,

Graham

On Wednesday, November 4, 2015 at 2:46:01 PM UTC-8, Benjamin Wolk-Weiss wrote:

Hey Gang,

So I'm currently working on the permit drawings for a PH for my local Habitat for Humanity chapter in Corvallis, OR and am unsure of how to address the crawlspace with regards to vented/unvented. I would like to do an unvented crawlspace, as Joe Lstiburek reccommends, I am just not sure the best and most cost effective way to mechanically ventilate it to comply with the building code and meet PH requirements.

How have others dealt with this issue? Or should I just leave it as a vented crawlspace?

The first floor will be insulated from the crawlspace with blown-in fiberglass and the subfloor will be acting as the air barrier in that location.

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[Gavin Tenold]

We have done two conditioned crawl spaces on the Cold side of the mountains.  I have decided they are expensive to detail well.  If budget is of a concern, I wouldn't recommend it unless there are other variables in play (uncompacted fill comes to mind).  They are effectively costly but comfortable places to run mechanicals and store holiday ornaments. 

--Gavin  

[Graham S. Wright]

Discussion has covered the ground pretty well I think. ;)

The only thing I might add is, I would suggest to retain the floor insulation on the bottom with something very vapor open, like netting, hardware cloth (as Artisan Group has done.) If you do OSB or plywood this tends to put the vapor retarder on the cold side of the floor or create a vapor retarder sandwich.  Such floors do not WUFI well in that climate - moisture drive from the inside stacks up on the inside of the plywood during the winter, and then in the spring when it warms up, mold-growing conditions can occur.  Again, theoretically.  Based on simulation I believe vapor-open on the bottom is lower risk in that climate.  

Graham