Helical Pier Foundation Details
Let’s say you buy into the embodied carbon discussion, and try to eliminate concrete by building atop these new-to-residential helical steel piers, which promise to set you up structurally with zero excavation.
What else do you need to do, for a PGH in a mixed-humid climate?
Are vented crawlspaces still verboten? Do you try to condition the crawlspace? What’s the perimeter look like – do we have to put in concrete *there*? If so, how do you connect that thermal barrier to the exterior walls? Are you trying to thermally break these piers (anchored deep in the Earth and a near-perfect thermal conductor)? What’s the best sort of insulation scheme for the floor generally? Are there vapor barriers used anywhere?
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I'm going through the same process as I'm building a house on a spot where I have to use piers. My thread about it is here:
https://www.greenbuildingadvisor.com/question/best-way-to-insulate-floor-of-addition-on-piers
I don't like vented crawlspaces, I'd want it either completely open or completely enclosed.
"Are vented crawlspaces still verboten?"
>Crawlspaces that look like basements but with a few holes to the exterior and no insulation are not a good idea in any climate, as they are too prone to developing moisture problems. Building on piers also results in a crawlspace, but because the structure is sealed and insulated well above grade (the IRC requires at least 18" of clearance; having crawled through tighter spaces, I like 18"), and the entire perimeter can be essentially open for good air flow, they can be safe (assuming good detailing.)
"Do you try to condition the crawlspace?"
>You can, but in that case you need to follow all of the advice for building a warm, dry basement--just a basement with limited headroom. https://www.greenbuildingadvisor.com/article/building-an-unvented-crawl-space
"What’s the perimeter look like – do we have to put in concrete *there*?"
>There are many ways to build. You don't have to use any concrete, or the entire house can be concrete. Considering the climate crisis, the less concrete we use, the better. But sometimes concrete still makes sense, and there are ways to reduce its carbon emissions.
"If so, how do you connect that thermal barrier to the exterior walls?"
>It depends where the thermal barrier is. If you're talking about building on piers, there should be a thermal control layer at the first floor. Depending on your construction details there are a variety of ways to maintain a thermal control layer.
"Are you trying to thermally break these piers (anchored deep in the Earth and a near-perfect thermal conductor)?"
>A thermal break is good, but hard to do with piers. The surface area of the piers is small enough that in most cases the framing lumber above the pier provides enough insulation. Add more depth at the floor joists to create a high average R-value, even with some thermal bridging.
"What’s the best sort of insulation scheme for the floor generally?"
>This should cover it: https://www.finehomebuilding.com/2012/03/08/how-to-insulate-a-cold-floor
"Are there vapor barriers used anywhere?"
>No vapor barriers. You can include a variable permeance membrane under the floor joists, but plywood or OSB do the same thing. Cellulose is great for this. Sprayfoam fanboys will recommend foam, of course, and it's a good insulator, but definitely not critical in this situation.
>Coincidentally, my next FHB article will include my details for a Pretty Good house on helical piers. Not super detailed, but it should answer some of your questions. I think it will be in the next issue but I'm not sure.
Hi Michael - was that FHB article ever put up, with helical pier info? Would love to read it!
Laura, I just saw your comment. Here is my article: https://www.finehomebuilding.com/2020/04/07/the-evolution-of-a-pretty-good-house. It's behind a paywall, sorry!
I'm on FHB, so I can access. Thanks!
In my situation I have to do it as a raised floor, I can't do a crawlspace. Insulating it should be straightforward, any sort of fluffy insulation should give R35-40 with common joist sizes. What I'm having trouble with is vapor.
I've been learning the teachings of Dr. Joe Lstiburek, who says:
* moisture moves from wet to dry
* moisture moves from warm to cold
In the winter the inside will be significantly colder than the outside. In the summer the outside will be warmer, although possibly not by much. All year long the outside will be wetter, it's a humid climate. That makes me think that I want a moisture barrier on the bottom of the assembly and to configure it so it can dry upwards. What I don't see is how to prevent condensation in the insulation during the heating season, or what is going to drive that upward drying.
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This study might be of assistance.
https://www.lsuagcenter.com/NR/rdonlyres/D33F711D-DC4B-4E4C-9ED6-A97DCE9DB026/79805/pub3187insulatingraisedfloorsLOWRES.pdf
Thanks, that was very helpful.
The gist of the study is that if you have a floor with an impervious floor covering and permeable insulation below, during air conditioning season moisture can condense in the subfloor to a great enough extent for rot to be a structural threat. The recommendation is to have a permeable floor covering and air sealing below the insulation. The study recommends carpet, which I'm not a fan of, but even hardwood floor should let enough moisture out to avoid problems. The study singles out vinyl floor as particularly problematic.
I guess during cooling season heat is going up through the floor and according to building science principles moisture is driven by heat. The air sealing under the insulation has to keep outside moisture from being driven in while that's happening.
If the floor can't be permeable then the other solution is to use impermeable insulation, which means closed cell foam. Then make the underside breathable and let the moisture be driven out during heating season. That sounds risky to me. In a humid climate the area under a house is going to be very damp.
The moisture content in houses fluctuates throughout the year so the idea is to keep moisture below the point where it causes problems vs. ridding the house of all moisture for 100 percent of the time.
To accomplish that you can either keep moisture off the wood, or provide a path to allow it to dry. A house on piers allows the underside to readily dry. Vented crawlspaces do not allow sufficient airflow and create a temp difference during the summer months which encourages moisture to condense on surfaces.
Oh and other than ccSPF the article also said foil faced insulation worked just as well.
If you are going to have a crawlspace I say condition it.
If you decide you want to skip the concrete understand you are building a strange house that will
1 Make it harder to get permits to build.
2 Make it harder to find contractors to build it.
3 Alternative systems have not yet stood the test of time and you are talking a risk.
4 Make it harder to sell.
Walta
Around here (DC) building on piers without a crawlspace is very common. I'm deconstructing a building right now that has two additions that were done that way.
Without any concrete only steel helix is common? Yes concrete bell footing below the frost line is very common low cost way to build.
Walta
I was talking about about concrete footing piers. But helical piers are rapidly growing in popularity, the city passed a tree preservation law that prohibits digging in the root zone of mature trees, helical piles are the most practical way if you have mature trees nearby.
Burninate,
From a building code standpoint there are two distinct assemblies: Those on piers and crawlspaces. Houses built on piers often muddy the water by installing skirts, but keeping the rest of the assembly and insulation at the floor level. From a code perspective that doesn't really fly. Once you enclose the perimeter the space has all the attributes and needs to meet the requirements of crawlspaces, either conditioned for ventilated, which may include things like vapour retarders ,insulation, clearances, mechanical ventilation and heat.
The big problem with building on piers is usually keeping the services warm as they enter and exit the envelope. The usual solution is to include a very small insulated and heated core from frost level to the floor.
This is a very good video from Cornell Engineers in Australia on the structural issues faced when using helical piers and how to solve them.
https://www.youtube.com/watch?v=7ArReODc8Ws
As to keeping the utilities from freezing, rather than building a core or insulated chase from the ground to the underside of the building I designed a slab on grade utility shed next to my cabin and brought all the pipe work up through the slab into that. Now the pipes stay warm and can enter the main building through the adjoining side walls.
Good idea!!! Do you have a heating element in the shed, or is it just really heavily insulated?
Scott,
Isn't that essentially an insulated core that happens to be situated beside, rather than under the structure? I'm not clear on what the advantages are. It's exactly the same construction, but takes up more space and needs a roof.
One of the advantages of locating it under the building is you can leave the top open negating the need for any supplemental heat.
It has an insulated concrete block wall for the lower 40" and an insulated stud wall on top of that with exterior insulation. There is insulation under the slab as well but no heating element yet although I might add a through the wall direct vent propane heater set on a thermostat to come on in case it gets close to freezing.. In addition to the water storage and purification equipment I also put the laundry and freezer in there to centralize everything.
That sounds like a conditioned crawl space.
No, it is a slab on grade with a 40" tall block wall on top of it and a stud wall on top of that. It is a stand alone shed that is placed against a cabin on piers so that their side wall touch. It is not under the cabin, it is a separate structure.