Hi All,
I'm working on getting a foundation lined up for a frame I cut this winter. It's for a young couple of modest means and I'm trying hard to get the maximum bang for the buck for them. I figure with the point loads of the posts a frost protected floating slab is out. A slab with full 4' footer is pricier than I like. I'm wondering if a rubble trench foundation with a concrete grade beam would be a good option. I don't see any issues upon initial impression, but what do you think? Have any of you dealt with this method?
My other thought on the same line would regard the main floor surface. Instead of pouring an insulated slab with radiant tubing in it, to lay the tubing in pea gravel for thermal mass (insulated of course) and then wood frame over top of it resting on the concrete grade beam. It seems like that could be a lot cheaper overall, but is it a good idea?
Have you considered using plinths or pilings for the point loads and either pouring an insulated thin slab with radiant tubing or elevating the first floor over and insulated/ventilated crawl space and installing the radiant tubing above the sub-floor? With the radiant tubing embedded in the pea gravel, the air space in the pea gravel will act like an insulator, as well as increased possibility of abraiding the tubing and developing leaks as the tubing expands and contracts. The additional insulating value of that air space may cause quite a bit higher energy consumption, as the EWT will have to be increased.
If you put the radiant tubing in a thin slab or above the sub-floor with a well insulated crawl space (focusing on reducing or eliminating thermal bridging) you can reduce the EWT, providing your customer with long term energy savings. The initial cost may be somewhat higher, but depending on energy costs in your area may be far outweighed by the reduction in heating cost.
I had considered some similar concepts in our house design, and found that the energy consumption for an insulated radiant slab in the basement and above-the-subfloor radiant on joisted floors was far less than other options. We are in a much milder heating climate, though.
-Dan
I think a rubble trench with grade beam would work fine, although you may have a hard time selling it to the building inspector. Putting point loads on a FPSF (Alaskan slab) should be fine, unless there's something crazy going on. The perimeter of FPSFs are typically 2' thick. If you have interior point loads, just dig the gravel out for a thicker, haunched portion of the slab. An engineer can specify what that size will be, but it's usually around 2x2' to 3x3'.
I too don't like the idea of putting the tubing in the gravel.
I had the good fortune of visiting the massive Bern Cathedral last summer, a stunning late Medieval Gothic cathedral, often considered one of the finest examples of this form of architecture.
This massive solid limestone structure is built on, you guessed it, a rubble foundation.
Granted, the rubble foundation extends very very deep.
The point is, rubble trenches aren't week. If done properly, they can hold up anything.
Thanks for the feedback all. The good news is that while the building inspector has never dealt with a rubble trench foundation he is not opposed. I just need to decide if I want to put a slab on it or pour a grade beam and set a wood floor deck on top. One thing that I am considering with a slab is how unyielding that will be to walk on. Even with area rugs it seems like that would be no fun to walk on.
One reason I like the rubble trench idea is that concrete contractors seem to be an everlasting thorn in my side. I haven't worked with anyone yet that I have been happy with. It would be so easy for me to do it this way with my own crew!
Pouring a concrete slab shouldn't require a concrete contractor, if you have the crew to lay down and pack rubble. Just make good forms around a good solid base, put down the reinforcing rod or mesh, and pour. Strike off, float, and finish. Special equipment, if needed, can be rented - at least around here.
Rubble base under treated wood works well for house foundation. Good drainage is important in either case.
Sarah,
We've done a dozen or so rubble trench foundations with timber frames atop them at Heartwood, with no problems. Imagine a slab with posts in the center; what do we do? Thicken the slab into a footing to support the point load. Usually those would be 12" deep, by 18-24" square. We do the same at the perimeter: thicken the edge (with reinforcing steel) and it should be able to handle the point loads. The rubble fill has to be stable and well-drained, of course, and the loading can't be too extreme. Also, we usually insulate the grade beam so frost isn't an issue, but even for uninsulated barns and garages we've done timber frames on rubble trenches with no problems; the drainage must be good.