Truss/Concrete Interface Detail
I have attached a pic of the plans for our Zone 4 NC house. This is the detail where the lower level trusses meet the concrete wall–living space above, crawl space below. The crawl is sealed with 2.5″ of closed cell and 5″ open cell spray against the flooring. Since the trusses will be butted up against concrete, should they get some sort of waterproof gasketing, something like ice and water shield stuck along that interface or this http://www.weatherblocsystems.com/sill_sealers_and_gaskets/Reduce%20Thermal%20Transmittion%20in%20Trusses/Thermal%20Barrier-Thermal%20Break.html
I assume the sill plate will get some sort of sill seal, but the short side against the concrete wall would also need protection. Correct?
Suggestions?
I actually have another question about this area and the insulation. If I am doing this closed/open cell combo, would the open be sprayed against the floor above and then the closed over it or vice versa?
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Replies
If you are sealing the crawl space perimeter, you don't need foam on the underside of the subfloor. Typically untreated wood is protected with builder's felt or a similar material.
I don't see a rat slab in your drawing. Have you discussed adding one with your builder?
I am not insulating the crawl space perimeter--only the floor. The house is on a 40% grade, so you don't see the plunging line past that footing! Rat slab would be tough to do. The dirt floor will be sealed with plastic up onto the concrete walls.
Won't builders felt allow movement of moisture? Wouldn't I want something totally impermeable?
Kevin,
Insulating the perimeter would bring the crawl space into the conditioned part of the home. And a properly detailed liner would be fine. This article address a basement, but it also applies to a crawlspace. https://www.greenbuildingadvisor.com/blogs/dept/musings/how-insulate-basement-wall
I'll let a more knowledgeable person respond to your moisture question.
Kevin,
There should be a capillary break placed over the footer, but if this is already built there probably isn't one, as it's not common practice yet. You should have waterproofing or at minimum damp-proofing on the outside of your foundation walls. Even with those measures, and especially without those measures, some moisture will wick up into the concrete. You want to keep that moisture from getting to any lumber. Standard sill seal material in most places is a lightweight, open cell foam, and in other places it's asphalt-impregnated fiberboard. Either can be ok capillary breaks, but I would want a more robust waterproof layer between any lumber and concrete. Grace Vycor is one product good for this, but there are many options.
As for insulation, like Steve I prefer a conditioned crawlspace. There are too many things that can go wrong with a vented crawl. If you're stuck with that approach, though, and want to use a hybrid of open and closed cell foam, I would put the closed cell against the floor sheathing, and open cell below it. Vapor drive in your location will usually be from the crawlspace toward the living space, which might indicate closed cell on the bottom, but closed cell is hard to seal well around truss webs so some moisture will get by it and trapped in the open cell layer. If the closed cell is against the floor sheathing, the open cell will do a better job of air-sealing the bottom side, and the condensing plane will be inside the closed cell (assuming your proportions of closed cell to open cell are correct).
I didn't put a capillary break at the footing because there is dirt on both sides of the wall--in other words, moisture could go around the footing and damp earth will be against the foundation wall on the inside(as shown in the engineering drawing). It's not already built--still yet to break ground. Soon though! The outside walls will have waterproofing material and dimpled drainboard.
The crawl space is not vented. I spoke with the authors of this publication http://www.advancedenergy.org/portal/crawl_spaces/pdfs/Closed%20Crawl%20Spaces_An%20Introduction%20for%20the%20Southeast.pdf who are also in NC(where this house will be built). One of the options they describe is sealed crawl with plastic on the dirt floor and insulation in the flooring above, with a dehumidifier to control humidity. They said it does not require any air in or out and meets code in NC. He had no preference vs.insulating the walls. All he mentioned is that he preferred R-30 in the floor vs. the R-19 of code.
Thanks for the info on the open/closed cell.
Kevin, I skimmed through that document; it looks like their building science is sound. I agree that crawlspaces vented to the exterior are not a good idea, and I have no doubt that a sealed crawlspace with a dehumidifier can work as they show.
What I don't understand is why, for a similar up-front cost, you would want to run a 750-W dehumidifier instead of a 15-W fan, for what as far as I can tell would be similar results. But I'm sure they, and you, have your reasons.
Here are a couple of details I designed for a sealed crawl space in Dallas, TX. You can adjust for your climate zone.
Kevin,
Your plan seems to have strong similarities to another project that appeared on GBA several months ago. It also was a mid southern location building on a strong slope in what I think was relatively sandy/gravely soils. I commented on that thread about the risk of water from up-slope sources needing an extra level of drainage management, both from the surface run-off as well as through the entire soil cut for the footings going down the slope.
Depending on how your local soils and ground water behave, you might need to address the potential for water to weep out of the hill into your very steep crawl space profile. You said the foundation plan did not show the 22 degree slope just to the right and that a capillary break on the footings would not be relevant since soil would come up the wall on both sides. The plastic sheeting sealed to the perimeter will certainly cut down on moisture transport out of the soil, but I am concerned that ground flow water from levels below the upper footing levels could collect against the down hill side foundation wall/footing.
If you don't already have an inside footing drain planned for the down hill foundation wall, it would seem like a good idea to cover that risk now. Super saturated soil on that kind of pitch seems risky. Steve and Armando might have a much better idea of the soils you are dealing with and I would go with their thinking more. Here where I have built, we have highly stratified soil profiles that are mixed with layers of sandstone so whatever water doesn't race across the surface during spring rains settles into the soil until it hits a layer of sandstone and runs along that.
My entire foundation sits on what is called bedrock here, (actually a very thick layer of fractured sandstone), and with only a 2% slope I still took measures to clear the inside perimeter water that weeps up and through. I took care of moisture transport in the footings and walls with Xypex additive. It was expensive, but my basement is quite dry. I do not worry about placing cardboard directly on the floor and my entire lumber collection is against the walls.
It may be that horizontal water transport does not occur the same way in your area. Another thing I would like to mention is the idea of a dehumidifier. In Illinois, I used one in a soggy basement during the summer months. It did noticeably heat up the basement since the hot half of the system did not reside outside. Will the warming effect increase the pull of humidity from the unwaterproofed walls? Last thing to note, what about radon risk in your area. Any plans for having a negative pressure under the plastic cover for the crawlspace?
A dehumidifer in a sealed crawlspace reduces relative humidity in mild, no AC/heat weather. An exhaust fan pulls warm moist air from above and increases its relative humidity - even to the point of condensation.
Then there are issues like continuous fans being compared to devices with very low duty cycles and the cost of continually pulling conditioned air from a house and blowing it outside.
As I wrote above, I would not ventilate the crawlspace to the exterior, but if the crawlspace is fully insulated, water-proofed and air-sealed, there is no reason it should be more prone to condensation than any other part of the conditioned area. I should have added that although the building code is written to require continuous fan operation, I have had no problem getting inspectors to OK intermittent fan use with a similar or greater total air volume exchange. A fan typically needs to run for an hour or two a day at most to meet prescriptive air volume requirements.
I think Jon R is referring to those times when you aren't using the heat or A/C because of mild weather outside. In that case, you would still be dehumidifying the sealed crawl while a conditioned crawl would be getting filled with humid air.
Kevin, in those cases the crawlspace would be getting filled with the same air as the rest of the conditioned area, so I don't understand why that would put it more at risk. Assuming the crawlspace is properly insulated, air sealed and water-proofed, of course. It should be no different than a closet on the main level. We don't usually place dehumidifiers in closets; we expect the air in the rest of the house to keep it at the same temperature and RH as the rest of the house.
If the crawlspace is NOT properly conditioned, I totally agree with you and Jon.