Rat slab advice for Climate Zone 4 (western NC)
Greenconfusion
| Posted in General Questions on
Existing 1930 pier foundation with block closing it in for walls. The front third of the house has full height access. First question is where to get sump pit box that can be sealed ? In the process of digging back part of crawl space to add foam board . Is it worth it to use foam board with a poly vapor barrier and a pored rat slab ? The last question is should the interior crawlspace have interior drain to sump pit ? It has no standing water now just moist dirt when digging out .
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According Table 2, p10 of this document, a couple inches of EPS under a basement or crawlspace can still make financial sense on a lifecycle basis in a zone 4 climate, and maybe 3.5-4" EPS (or 2.5-3" of polyiso) on the crawlspace walls:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
Going even thicker could make sense if using reclaimed roofing EPS at less than 1/3 the material cost of virgin-stock EPS.
These folks seem to have a stock of foil-faced Type-I (1lb per cubic foot nominal density) EPS that would be suitable for either sub-rat-slab or walls at about half-price:
https://asheville.craigslist.org/mat/d/2-inch-4-ft-8ft-insulation/6476526151.html
Type-I EPS won't be "walkable" without damage prior to installing the rat slab, but a few planks for the working walk ways can keep it from becoming too dinged up.
Some of these outfits probably have access to used Type-VIII (1.25lbs density) or Type-II (1.5lbs density) roofing EPS, which are a bit more rugged, probably for less money.
https://asheville.craigslist.org/search/sss?query=rigid+insulation
Polyiso is fine for the walls, but can't be used under the slab without risking becoming waterlogged over time, taking a huge hit in performance.
Green,
Q. "Where do I get a sump pit box that can be sealed?"
A. Here is the company:
Jackel, Inc.
15314 Harrison Rd.
Mishawaka, IN 46546
574-256-5635
Q. "Is it worth it to use foam board with a poly vapor barrier and a poured rat slab?"
A. I'm not sure what you mean by, "Is it worth it?" What are you looking for? Energy savings? Solving a moisture problem? Resale value?
Q. "Should the interior crawlspace have interior drain to sump pit?"
A. Most crawl spaces don't have a sump, but some crawl spaces need one. Whether or not you need one depends on many factors, including (a) whether the crawl space floor is above or below grade, (b) whether the grade around the house slopes away from the foundation, (c) the soil characteristics, and (d) the height of the water table.
Before you can help you with solutions, you need to describe your problem.
Thank you for the foam board advice. The front 1/3 of the house has been hand dug 7' by original owner and seems to get some water during heavy rains. The house has current gutters with some attention needed of sloping away from house. I have dug down the back of crawlspace to allow for room to intall foam board and concrete. This is where i was questioning if i needed interior drain . Also what type of concrete would be best used without installing wire or mesh ,and what thickness would be ideal. The reasoning for rat slab is to get rid of musty odor from crawl space .
The vapor barrier and foam will reduce the musty smell. The concrete is just there to protect the foam, and only has to be thick enough to keep ground hogs, voles and others from setting up a condo association in the crawlspace. They're called "rat slab" for the critter-control aspects (a primary reason for installing them in many urban settings.)
A typical rat-slab is nominally ~2" thick, and there is no structural specification for the concrete. It may develop some cracks, but that's fine, as long as it doesn't crumble. It doesn't have to be a hard troweled finish or anything- some are just coarsely leveled with a bull float to work out the worst of the bubbles and left to set. The 95 year old rat slab in my basement floor was poured directly onto clay soil, and seems to vary from about an inch to about 4" thick in the few places where I dug out sumps, and where I poured a footing for a lally column, but it would be a lot drier down there if it had a vapor barrier and some foam under the slab.
Great info about the slab thickness. The home currently has field stone supports for center beam , i am curious if its worth trying to replace these to be able to get vapor barrier under them ?
No need to replace the beam supports- just seal the vapor barrier to the fieldstone piers/columns with duct mastic (or tape, if it'll stick), lapped to a point higher than the top of the slab layer. The amount of moisture wicking in fieldstone is tiny compared to concrete, limestone, or brick, and the vapor permeance of most stone is also pretty low.
Moisture transport rates by vapor diffusion is a permeance x area x vapor pressure thing- the total cross sectional area of each column is pretty small compared to the total floor area, and not highly permeable.