When building on exposed bedrock, is it better to be right on it or on piers?
We are considering a building site on our property where there is a ~20×30 foot area of exposed bedrock/ledge protruding a couple feet up from the ground in an otherwise fairly flat area. Maybe this is a bad idea, but to me it seems like a solid place to put the house.
We have experience living in a place with stone floors, and it seemed to stay cool during summer days and warmer during winter nights. My thought was to couple the house to the bedrock by anchoring a concrete slab on this bedrock.
My other thought was to use short concrete piers anchored to the bedrock, leaving a crawlspace beneath the building.
I put this post in the energy efficiency category because I am curious if the house directly on a slab on the bedrock would have any energy efficiency pros compared to the house on piers. Or would the giant rock just suck all the heat from the building in winter? Would hardwood flooring significantly reduce the rate of heat sink in the floor mass from south facing glazing?
We are in upstate new york.
Pros of slab on bedrock:
– structurally sound
– potential energy efficiency?
Pros of piers:
– easier to run wiring, plumbing
– potentially less cost
Edit: the bedrock is not shale or similar, it is granite
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In your climate zone, you will save a lot of energy annually, and improve comfort for most of the year, by separating your floor from the granite with a layer of insulation. You can still build on the outcrop, pinning the foundation to the bedrock, but you should thermally isolate the interior.
Thanks Michael. So if I understand you correctly, it would go
- rock outcrop
- pinned concrete foundation
- insulation/vapor barrier
- subfloor
- flooring
Dave, yes, and there are many different ways it can be done. There are dozens if not hundreds of posts here on GBA about different approaches. The article linked here is a good place to start: https://www.greenbuildingadvisor.com/article/insulating-a-slab-on-grade
"I put this post in the energy efficiency category because I am curious if the house directly on a slab on the bedrock would have any energy efficiency pros compared to the house on piers. Or would the giant rock just suck all the heat from the building in winter?"
It wouldn't "suck all the heat," but stone is highly conductive to heat, and it would be exposed to outside air, so you would have a highly conductive pathway to the outside. It would be like having one side of your house completely uninsulated.
davec1808,
A few things to consider when dealing with bedrock, exposed or otherwise. Water, septic, geo-profile and seismic levels. First, the thermal idea of pier or bedrock.
Thermally speaking, a case can be made for homes on piers despite the entire envelope being subject to seasonal temperature variation. It is a bit complicated to do the energy analysis because roofing, window and site choices will strongly affect the various thermal loads. Still, ground coupling via slab or basement means about a third of the total surface area of a 25x40x9 foot box will be facing an almost perpetual 50-55F environment. The typical insulation under slab is R-10 to R-15, so if assuming a 70F interior, R-15/U=0.0666 and a delta T of 20F then one can guesstimate a thermal loss for the 1000 sq ft as 1333.332 btu/hr or for the year, 8760 hrs, a total of 11,679,988 btus. Roughly 3,400 kWh of electric energy or 123 therms of Nat Gas at 95% efficiency. Admittedly a simplistic energy analysis, but useful if one also looks at the option of building on piers.
On piers the 1000 sq ft foot print area is now in play for the full range of seasonal temperature. Clearly the floor now will need much higher R value insulation to control for the winter potential of a delta T exceeding 70F. However, the time scale of such a delta T is now much shorter, so much of the year the higher R value in a pier-ed floor is overkill. However, your annual losses drop dramatically. Gains are minimized compared to roof and walls thanks to the perpetual shade. You would need to analyze the local seasonal temperature profile to decide how much R to put in the floor for acceptable losses and foot comfort. The cost of the additional floor insulation and labor is a one time cost, so economically a case can be made if assuming ever higher energy costs.
Beyond complicating the envelope analysis, the pier plan complicates connections for water, septic, electric and access to the home. If you think the critters in your region enjoy attics, wait until you find out how much they would like another entire neighborhood to explore. The utility drop from home to ground will also necessitate digging, insulation and near bullet proof critter resistance. Getting into the house may mean steps or ramps that will haunt you if aging in is part of the long term plan. Bumping the under slab insulation to R-25 and managing the stem wall insulation could be a better option for your site.
Returning to the common issues to either choice. My own home foundation is 100% set onto the local bedrock. In my case it is sandstone of varying density and quality. As a former New Englander my guess would be that your local stone is granite. I will hazard a guess that ground water there will likely track on your stone like mine. I put a great deal of effort into managing spring melt that tracks the bedrock. It will help your planning to develop a better view of how the stone lies on your site. This info will become critical to both water control under and around the foundation, but equally critical to your septic system placement if you lack public sewer access. Having a good septic field option on the site can be undone if you plan on the house site first.
Pinning your foundation to the native bedrock certainly has attraction if the soil over the rock has questionable properties. The geo-profile on my roughly 5 feet of overburden showed a layer of expansive clay just 10" above the bedrock. For me it was a no-brainer to simply excavate to bedrock as I planned to have a basement. Your thinking about piers leads me to think a basement or even a utility core is not currently part of your idea range. Seismic conditions vary widely. I live less than a mile from a fault considered dormant, but a few months ago the next fault over (about 7 miles) went off with a bang big enough to send people scurrying.
If you have seismic concerns I would plan to make a choice to either pin to rock or remain separated. If the soil over the rock takes a jump, having half the house pinned to the bedrock could result in partial structural movement. Generally not a good thing. If the bedrock option is at play, pinning with fiberglass dowels rather than steel rebar will better ensure long term success. The ground water may be more acid than you think.
EDIT
Update on wood floors and heat banking of southern window energy. FWIW, I would focus on comfort over the illusion of useful heat gains warming the floor or saving energy costs. As has been noted many times on GBA, a surface needs to be about 80-86F to "feel" warm to your feet. Concrete conducts heat easily and at 72-74F will still be perceived as cool. Thus the arguments over in floor heat and "cozy". Wood and or cork are poor conductors and will be perceived as warm or at least not cold at normal room temperatures.
Concrete is slow to gain heat and penetration of useful energy is limited. I have seen references to a 3-4" limit for usable depth of recoverable energy over a set time lag. Any rugs or furniture will of course soak up the sun before the floor, so unless you plan on a very bare open area that receives long winter sunlight, there is little value in designing for a largely imaginary gain. The same windows that warm you by (very short) winter day will be cooling surfaces for much longer winter nights.
Without careful planning of sun exposures, the same windows will be significant heat loads on your AC come summer. Not saying it is impossible to have a warm cozy sunny room, just that it takes careful planning and a willingness to accept fairly wide temperature swings as a built in feature. Top performing windows, like the Alpens I have used, are a better place to spend money for overall energy management.
"The typical insulation under slab is R-10 to R-15, so if assuming a 70F interior, R-15/U=0.0666 and a delta T of 20F then one can guesstimate a thermal loss for the 1000 sq ft as 1333.332 btu/hr or for the year, 8760 hrs, a total of 11,679,988 btus. Roughly 3,400 kWh of electric energy or 123 therms of Nat Gas at 95% efficiency."
Except that during the cooling part of the year the slab is contributing cooling.
A piece of stone on the surface is not going to be at a constant temperature year round. It's going to be at close to air temperature all the time. Stone is highly conductive, so even under the house it's going to be at close to outdoor temperature.
Foundation are very different form one location to the next because what works in my location is likely a total failure in another.
I think the smart move is to find the grumpy old local guy with thousands of standing foundations and pay him for that experience or at least listen to his free advice.
The other option is to pay in engineer to draw something and put a stamp on it.
Walta
Reply to #5,#6 #7,
DC then Walta,
I will cop to my failure to account for the cooling value of losses to ground with one caveat, my loss estimate remains largely true for basements. I have such a basement and believe me I do need some heat year round despite my R-15 under slab and R-15 wall insulation. My shop would stabilize at about 62 summer and 58 winter without supplemental heat and my poor old hands just don't work at those temperatures. The loss calculation still needs to be considered when deciding on how to insulate the walls of the basement.
I have peeled open many a basement rec-room to find mold and mildew behind the batt insulation, growth dominating at the deepest below grade part of the wall. This is a combo effect of the cooling of the earth and the frequent presence of a footing drain being placed on top of the footing/wall ledge. The room humidity penetrating behind the batt insulation finds a cold and sometimes pre-saturated concrete wall and thus begins the mold and mildew farm. The growth levels typically fade as one goes up the wall. Even after remediation of all the driving factors and installing proper interior insulation the heat loss to ground remains. It becomes a personal decision on how cool a basement living space can be tolerated.
For a slab house there will be a net energy value on the cooling side of the calculations though likely less than one might expect depending on flooring choices and window placement and local climate. In upstate New York the energy need for humidity extraction will dominate over the simple cooling provided by losses to ground. The ground loss issues primarily turn up at the perimeter in the form of cold sill plates and mild dew at wall bases. This chilling is of course seasonal and can be moderated with appropriate insulation placement with acknowledgement of termite issues being at play for OP.
My flawed response to the OP's thermal question aside, I will disagree that the stone stays at air temperature or that the stone under a house would attain anything close to air temperature thanks to conduction from connection to an exposed area. My arid environment certainly differs from upstate New York, but my prior life in New England area provides plenty of real experience that contradicts that perception. Out east, simply rolling over some large stones should suffice as proof. Out here, I am likely to find noon time rocks hotter than comfortable to handle, so the bottoms might actually be air temperature. Still on a house level scale the rock will be cool under the slab.
Walta, while I agree that engaging local resources can be a good starting point for many building choices, I would only take that advice so far. Many an old codger remains just that and from experience I can say some do not take inquiries or being told about anything new well. "This ain't my first rodeo" is a red flag for me. Alternatively, there is at least one thriving local business doing repair work on foundations that were all dutifully stamped by engineers. Not much consolation to OP, but being made aware of what they are about to dive into can only help.
Thank you for your responses. Given me lots to think about.
I think I need to get it out of my head there is anything to be gained from an energy perspective with this outcrop. The concept is a bit beyond me and perhaps just better for me to see it as a nice structural element and nothing more. Perhaps if it is winter time heat storing/buffering I am after, a nice inside brick wall the sun can shine on in the winter time is a better plan.
Given the winter and summer extremes, and issues well documented on the site with crawlspaces, it sounds a slab in general may be preferred over piers. (I am not necessarily against a full basement type house, it just seems like this big flat-ish rock about the footprint of a house could somehow be a cost saver). A very good point about the steps, as yes, this would be an age-in-place type home.
RE: the windows that heat the building in winter being a concern in summer. The plan is to construct the eave overhang to block the summer solar arc while admitting the winter sun position angle.
I am going to continue searching for examples or warnings of building a slab-on-bedrock-grade home in cold climate.
"Perhaps if it is winter time heat storing/buffering I am after, a nice inside brick wall the sun can shine on in the winter time is a better plan."
A drywall wall will do the job just as well. The idea that there is some sort of "magic masonry" that buffers solar gain is essentially a cruel hoax.
"RE: the windows that heat the building in winter being a concern in summer. The plan is to construct the eave overhang to block the summer solar arc while admitting the winter sun position angle."
That is possible to some extent, but the problem is that heating and cooling seasons don't line up with solar seasons, they're offset by about six weeks. So June 21 is the day of the year when the sun is highest and the day is longest, but in most of the northern hemisphere the warmest day of the year is around August 1. This means that August and September are warm months. But the sun is in the same position in March as it is in September, and in most of North America March is a heating month.
From an architectural point of view, if you like how the outcrop looks, consider building near it, not on it. People often choose the best spot on a property to place their home, ruining the natural beauty of that spot, when building near it would be just as effective and maintain what is special about the property.
I agree with Michael: build near the outcrop, not on it. We built on one outcrop in Canada with a crawlspace and had to deal with bad moisture issues down there ever after. No fun.
Piers create insulation problems under your building, plus invite critters to live under your house. I had a cottage like that also and wouldn't do that again either.
Find a nice, easy dirt site to build on and insulate it well. Don't try to reinvent the wheel.
And this is why I get kind of cranky when people start talking about "thermal mass" or passive solar or any other scheme to add unnecessary masonry into a house. It's a good way to ruin a house in a hurry, and if you did the calculations you'd probably quickly see that it's not going to work.