House on 5 ft slab to save energy?
So I’m talking to someone building a house. His builder has recomended to him that he build his house with a 5 foot thick slab that will be 1000 sq. ft. in area. The frostline is about 3 ft where he lives, and the slab would go under the line and have it’s sides insulated.
The whole idea of this exercise is energy savings, however that is all they are doing to save energy. I could see using the slab as a thermal mass to soak up heat in the winter and send out but the house is going to have one window per room. With the slab doing all the work they can even save money on the windows, and insulation!
Builders argument is, the slab will pull up heat into the house durring winter and suck it up in the summer. I look at that and think, your going to have a 40 degree floor all winter long pulling heat down into the ground, summer maybe it makes sense. But that house is going to take forever to warm up. I also see someone with a brother in the concrete business.
Any thoughts?
Replies
Builders argument is, the slab will pull up heat into the house durring winter and suck it up in the summer. I look at that and think, your going to have a 40 degree floor all winter long pulling heat down into the ground, summer maybe it makes sense. But that house is going to take forever to warm up.
Passive Annual Heat Storage (PAHS) works, but this ain't it. Major waste of concrete/money. You didn't say where you are, but PAHS has clearly worked from our 4166 degree-days to Hait's 8125 in Missoula. Apparently neither are the extreme.
We have uninsulated floor/buried walls and a house that needs neither much heat nor AC here in central Virginia. Does not take long to raise our 20,000 cu ft from our 65° minimum, to 70° comfort with a modest woodstove even when it's single digits outside. But it's not built/insulated like you're hearing.
Better description from a book excerpt here:
http://www.axwoodfarm.com/PAHS/UmbrellaHouse.html
PAHS Designer/Builder- Bury it!
Tom, with your umbrella concept, how do you protect the insulation from being chewed up or burrowed into by rodents, ants, termites, etc? Been wondering that recently, and too lazy to look it up. Easier to wait till you posted and ask!
Oh, btw, I also agree that a 5' slab is dopey. Many, many, many better uses for his money and concrete!
Tom, with your umbrella concept, how do you protect the insulation from being chewed up or burrowed into by rodents, ants, termites, etc?
Good question. Far as I know, that hasn't been a problem. It wasn't anything I knew to consider when I used the xps, surrounded by 6 mil poly. I haven't dug up much, but what little I had occasion to inspect was undisturbed.
There's been a lot of soils talk on BT recently, very interesting to me. This is mountaintop, no topsoil to speak of. The subsoil amounts to weathered rock (schist). Backfilling left me with something very similar to undisturbed "soil" here, without mechanical compaction, other than tractor tires. Lacking organic amendments, it's not very attractive to much of anything. As there is a foot of organically amended dirt over the umbrella, I'm thinking that the umbrella isn't very attractive. My guess is if there's a more hospitable place nearby, the umbrella is left alone, especially when it's a foot below grade. The ants and the snakes both preferred living in the raised beds on top of that first foot of soil. Moles headed for lower ground, but all were well above the umbrella, which kept us toasty during single digits this winter. The groundhogs have only been visitors, and then only until I made their acquaintance.
Biggest risk has been tulip poplars trying to get going on the roof. Bad place for one.
Oh, btw, I also agree that a 5' slab is dopey. Many, many, many better uses for his money and concrete!
Dirt works great for heat storage. No reason to waste the concrete.PAHS Designer/Builder- Bury it!
I think the idea is completely ridiculous.
The bottom 2/3 of the lab will be roughly the same temp as the soil around it.
The slab won't just "give off" heat - You would have to set up some sort of system of pipes or air ducts to exchange heat with the slab.
If I figured it right, that slab would take 185 yards of concrete - Roughly $13,000 worth around here. (plus labor)
I would think that as you got near thre end of the winter, the slab would be rather cold, and impossible to heat up. That would make it difficult to heat the house, or at least make it feel comfortable.
I've heard some pretty bad ideas over the years, but I think this is one of the worst.
Free advice is worth what you paid for it
just the slab would weigh 231 tons. The soil will not hold that. talk about settlement problem. Just think of how much rebar. It would take double mats of #7 just to hold together. forming, labor, pumping. That would be a $30k just for the slab.
just the slab would weigh 231 tons. The soil will not hold that. talk about settlement problem.
Yeah? I just did a quick calculation of my last effort. Came out at 224 tons sitting on 336 sq ft. for the back wall. Front wall has 150 tons sitting on 96 sq ft. #7's are involved, back footing only, total $135. No settling to date.PAHS Designer/Builder- Bury it!
The others are missing the point.
This is a good idea. Just needs a slight ajustment in the detials and it will work fine.
What you need to do is make it deeper until you reach the point in the earth where the earth starts warming up.
I think that is about 7500-10,000 ft.
It will take just a little more concrete, though.
That won't help him in the summer, though...Why do overlook and oversee mean opposite things?