Hi All,

I am considering doing a conditioned crawl space. It sounds appealing because I think it would be easier to keep the critters out and would be more energy efficient.

I am getting close to finishing up the plans on a 1.5 story I am going to build in Dewey, AZ (outside of Prescott). Its in Zone 4 and fairly dry, the relative humidity is usually in the teens or twenties with the exception of the monsoon season in July/August where it gets up over 40% for a while. The building site is on the side of a hill and while there is a fair amount of clay and rocks and drainage is not good for septic (will be needing an alternative septic), I don't think there will be much issues with moisture coming up from the ground, and no concerns of flooding.

I am going overboard with insulation in the house, double stud walls with 10” of cellulose. The entire attic of the house will all be in conditioned air space and I plan on running most of the air ducting up there, I may have to run one or two ducts below to span the great room since it has a vaulted ceiling but for the most part energy savings from running ducts in a conditioned crawl space are a mute point. I plan on sealing the house off fairly well so I am planning on installing an HRV.

If I did the conditioned crawl space I would probably insulate the 4' high 8" CMU stem walls with R-10 ridged foam board vs R-30 fiberglass bats between the floor joists. I am planning on using a heat pump for heating and cooling with central hydronic air handler or several ductless units.

So my questions are:

1. Is a conditioned crawls space right for my area?

2. Is a conditioned crawl space for new construction going to cost more/less than a traditional vented one?

3. Are there any heating/cooling energy savings to be had?

4. What are the best options to consider for plumbing an ERV with a conditioned crawl space? If I were doing a traditional crawl space I was thinking of plumbing fresh air from the ERV to all the bedrooms and pull from the kitchen and bathrooms to be exhausted. If I were to do a conditioned crawl space I was thinking of taking the exhaust aid from the kitchen and bathrooms and pushing it through the crawlspace before it exits out the house. I guess this would put bad air under the house to make its way back into the house. Maybe better to suck the fresh air into the crawlspace before it goes into the house? Or should I just the crawlspace like its own room and provide an intake and exhaust?

We don't build basements or crawl spaces in our area. We don't even dig trenches for footings as we don't have significant frost issue. We pretty much always build a pad using 3-4" crushed rock base and 3/4" crushed rock topping and build the footings directly on top of this engineered pad. Then we fill the interior of footings with pea gravel (after placing the electrical and plumbing chases), place the under-slab insulation/vapor barrier, pour the slab floor (often with embedded PEX for hydronic radiant floor heating), and back-fill with dirt to top of footing height or higher. So I can't comment with any authority about building conditioned crawl spaces... However, I can say that any space that is truly considered "conditioned" always needs to be ventilated to some degree or you may have a significant moisture/mold risk.

Have you looked at ICF? It would probably be cheaper than a double wall and give you excellent insulation.

I'd keep the ERV airflow separate from the crawlspace. Some of your questions will be resolved by reading the code requirements. My guess is that in your dry climate, vented or unvented can work well. And I'd take a good look at a raised, thickened edge slab on gravel with an unheated building frost protected design.

A dehumidifier is sometimes an option for humidity control in sealed spaces.

With an R37-ish center-cavity /R30-ish whole wall double-studwall it makes sense to go with an air tight unvented crawl space, which is FAR easier to make air tight than a floor insulated between the joists. insulate the crawlspace walls to at least R15 (a minimalist ICF would be one approach, 3" of reclaimed rigid polyiso on the interior side of a CMU wall would be another), and R4-R8 EPS (not polyiso) under a 2" rat slab over the ground vapor retarder. Going with 2" of EPS on the exterior (protected by EIFS materials above grade) and 2" of EPS (or 1.5" of polyiso) on the interior side of a CMU foundation would be fine, with fairly stable performance for many decades.

Note: R10 foundation wall would be IRC 2015 code-minimum for zone 4, and R10 using XPS would eventually decay to R8.4 (the same performance as similar density EPS) as the climate damaging HFCs leak out over a few decades.

Using reclaimed rigid foam (even reclaimed XPS) is by far the greenest approach, since it doesn't use any new polymer or blowing agents, just loads up the benefit side of the environmental cost^benefit balances of an environmental hit already taken. I'm not sure how many materials salvage & reclamation companies are trading in used rigid foam in your area, but it's usually 1/4-1/3 the cost of virgin-stock foam board in most areas (often cheaper than fiberglass batts in R per square foot terms.)

See the zone 4 recommendations in table 2, p10 of this document:

https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf

They're suggesting R25 (whole wall) walls, R60 (whole-assembly) attic roof, R15 continuous insulation for basements and crawlspaces, and R7.5 under the slab.

For a vented crawlspace or open pier foundation with the insulation between floor joists the suggestion is R30 "whole assembly", not center-cavity, which for most 16" o.c. framed floors would require 2x12 joists with a full fill plus an inch or two of rigid foam on the underside of the floor. Cost-wise it isn't necessarily much cheaper than the unvented conditioned crawlspace, but the potential for air leakage is much higher.

Air tightness becomes more important for retaining the performance paid for with higher-R assemblies. A well insulated wind tunnel is about as expensive to heat & cool as a barely-insulated wind tunnel. With your high-R walls the air tightness of the insulated conditioned crawlspace is alone going to be "worth it".

robinnc, I have heard of them but not looked into them seriously. I may consider using them for the stem wall. Not having to take another step to insulate the stem wall sounds like a good thing. I'm planning on doing a full traditional brick wainscot to cover the stem well, not sure if ICFs play well with doing this or not. I'm open to ideas. I'm not very interested in doing ICFs for anything more than the stem wall, maybe the next build...

jonr, I think I'll go with your advice and keep the ERV and crawlspace separated. Been diggin through the code, I'm getting closer. Talked to my building inspector, he sounded open to using a dehumidifier to control moisture, I doubt it would ever be needed due to my area though. The only reason I see would be if there were truly no air circulation down there which it sounded like the inspector would be ok with (I thik I misunderstood him) appears to not be permissible by code. There are Ford guys and Chevy guys, I'm not a concrete slab guy...

Danna1, Wanting to do an unvented crawlspace for the reasons you stated. Considering a rat slab like you mentioned too and will probably do more than R-10 like you suggested. I did not know reclaimed XPS was a thing, thanks for mentioning.

So I am thinking of the “2.1. Continuously operated mechanical exhaust ventilation....” for the unvented crawlspace option. My plan is to put a remote weather station down there and monitor the humidity and run the fan if needed, I'm doubting it will be needed.

The link below is a draft that shows where I plan to run the HRV ducting. Heating/cooling will be from ductless (hydronic) units shown in each room. Any feedback is appreciated.

ERV Ducting Plan--> http://www.goldenfab.com/temp_share/House/House_Ducting_20170822.3dm.pdf

Did you consider using your bath fans/vents as your HRV/ERV exhaust and tying them into your ERV/HRV? If I read your plans correctly you are supplying fresh air to the different locations and have one primary location pulling the air to be exhausted. The bath fans would be redundant, or could be, depending how you wanted to do this. I have all my bath fans ducted individually to the HRV to be exhausted and then have a central supply dropping in fresh air. Seems to work fine.

thescottcav, there are two HRV returns. One at the base of the the wall in the second level (attic loft area that opens up to the great room). The other HRV return is in the main section of bathroom for the bedrooms. This bathroom has an enclosed toilet space as well as an enclosed shower space. IRC 2012 R303.3 says:

"R303.3 Bathrooms. Bathrooms, water closet compartments
and other similar rooms shall be provided with aggregate
glazing area in windows of not less than 3 square feet (0.3
m2), one-half of which must be openable."

I'm not sure what the interpretation of this is but I was guessing that I would need a total of 3 vents for this bathroom since the toilet and shower are separate areas. Does anyone know if this is not the case? Per TABLE M1507.4 I would need 50cfm intermittent or 20cfm continuous. This seems like a lot of air to be sucked out of this small room as I was not planing on having an HRV this big being I only require ~72cfm for the hole house and half of the return air I think I would want coming from another location.

Let me know what you think. I'm open to suggestions which is why I'm here.

You might want to ck with these builders........http://www.bing.com/search?q=icf+builders+in+arizona&qs=AS&pq=icf+builders+in+a&sc=8-17&cvid=48977BEF02A34A8E8CFC3E78B8C3EC35&FORM=QBLH&sp=1&ghc=1

Again.....I'm not with ICF......just strongly believe in them.

Ditto what Dana said.....he always knows what he's talking about.....which is a good thing.

Thinking the title of this post should be Unconditioned Crawl Space - Vented or Sealed? Crawl spaces suck. There's always a way to avoid them completely and use chases to do plumbing and electrical for initial build and accomondate future renovations.

Posted By icfbound on 27 Aug 2017 01:03 AM
Thinking the title of this post should be Unconditioned Crawl Space - Vented or Sealed? Crawl spaces suck. There's always a way to avoid them completely and use chases to do plumbing and electrical for initial build and accomondate future renovations.

No.

Code questions are best answered by your inspector. I had very good cooperation from my inspectors and asked them whenever I had questions.

For this scenario in my house I have an operable window in the WC and in the bathroom and I have vents in both the WC and the main bathroom space. I tied these two vents into the same run back to the HRV. The other bath vents are ducted back to the HRV. I built a plenum that all the 4" ducts from the bath vents are run to and the plenum has a 6" collar that is ducted to the HRV exhaust.

I know this is a few weeks old but I'll toss in my experience. I built a 2000 sq ft (inside the walls area) one story ICF house in 2008. The floor is suspended inside the ICF wall. The crawl is insulated by the ICF wall. I poured a rat slab in there. I use a Daikin mini-split ASHP with the air handler in the crawl space dumping the output into the crawl space and runs 24/7. I have floor registers all around the perimeter and ceiling grills all around the perimeter. The roof is spray foam insulated on the underneath side of the roof. This makes the attic part of the conditioned building envelope and serves as the return plenum for the air handler. A duct draws the attic air down into the crawl space and into the air handler.

The net result is the entire house envelope is conditioned. There are no drafts, period, hot or cold. The floor is warm in the winter and cool in the summer. There is very even heating/cooling all over the house. An ERV is in the attic and its fresh air output goes into the return duct to the air handler. The stale air input is drawn from grills in the ceiling of the bathrooms, laundry room, and kitchen. I have a range hood but no bathroom ventilators. There are timer switches in the bathrooms to turn the ERV to boost mode if the bathroom needs to be ventilated more than the steady draw.

The heat pump normally cycles one time during the day, turning on in the evening and off in the morning. It has a variable speed compressor which varies up and down depending on the heating/cooling load.

The only heating system that works better, IMO, are old fashioned hot water cast iron radiators. Nothing beats it for cooling!

The only reason I used an ERV instead of an HRV is that the Ultimate 2000 ERV was the only unit I could find that offered an additional control input to turn the exchange off and let it operate as a straight ventilator. I use that mode in the summer at night to pull cool air into the house for cooling assistance.