Crushed Granite and Radon
Hello,
I am designing a home in Maine and we will be laying down and compacting a 6 inch layer of gravel this season in anticipation of doing the foundation next summer. The design is for grade beams and a minimal concrete approach that will necessitate a rather large amount of gravel (possibly crushed granite) filling the area between the beams. After building it up, we will then do rigid foam, a vapor/moisture barrier, sleepers and subfloor.
Here in Maine there is a lot of granite and radon. My excavator wants to use crushed granite as the fill gravel. My question is simple. Does granite continue to off gas radon after it has been crushed ?
I will be doing the standard radon horizontal and vertical venting beneath the vapor barrier, of course. Just want to make sure that building on a bed of crushed granite would be ok as long as I am venting it properly.
Thanks, Stephen
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Replies
Stephen, I have not seen any studies on this but I think it's safe to say that if the material is granite, it could release radon. I doubt that proximity to the surface of the aggregate would mean that the radon escapes quickly. If anything, the additional surface area might mean higher levels of radon. I still spec crushed stone often and a good, passive radon mitigation system. To the best of my knowledge, no project I have designed with that system has needed to add a vent fan.
The system is simply a 4" perforated pipe within 12" of the foundation perimeter (which can double as a footing drain), in a bed of 6" or thicker crushed stone, topped with a 10-mil, fully sealed vapor retarder, with a solid 4" riser terminating near the ridge, and an electrical outlet placed nearby in case testing shows that a vent fan is needed.
Thanks Michael.
Should the perf perimeter pipe be pitched to a single 4 inch solid pipe and sent through the grade beam ? If so, would pitching the pipe at say, a quarter inch per foot mean that the solid water drain pipe part would be on the downhill, while the 4 inch solid vertical radon pipe would be on the uphill (thus, on the other side of the foundation and working WITH the stack effect)?
Also, in specifying three quarter inch stone: would larger stone be acceptable? We are chatting about using 6 inch stone on the base and then topping it with 3/4 and then 2 inches of 3/8 as a scree layer. (this is a low concrete gravel filled foundation within grade beams on helical piles). We have soggy soil.