Notes ~ not edited.  Conversations about simple foundation systems.  

 

Minimal Shallow Frost Protected Foundation  2008

 

 

Fred McLaughlin:

Architect, East Lansing, Michigan

Building his own office/home from Strawbale. Uses bicycle and mass transportation or carpooling with others.  Has a collection of resources on wild foraging, root cellars, putting food by. 

 

Hi Deanne,

 

I would start with a frost protected shallow foundation version (FPSF) of the rubble trench foundation, which need only go down to 12" below grade as long as 1 1/2" to 2" of blueboard (extruded polystyrene - unless somebody knows of a good equivalent alternate material [please share]) goes down vertically against the exterior face of the rubble trench, continuing from where the wall insulation system above ends to the bottom of the trench (to create continuous insulation package from wall to bottom of footing); no "wing" insulation is required in this detail.  The FPSF is approved in the Michigan Building Code and the Michigan Residential Code. (DB: Note that the code sketch shows wings, but Fred says that the details of the code allow just the vertical 12” of insulation due to our weather in the lower peninsula.) Then, you'd need to top this foundation with a wall system protecting any moisture-susceptible wall elements using distance above ground, detailing that keeps hygroscopic deterioration from getting at the bottom of the wall system and working it's way up, and, finally, providing wall-sheltering roof overhangs of about 2'6" or more depending on the height of the building from grade to soffit (1-story vs 2-story, for example).  You will also need to anchor the bottom of the walls to the foundation assembly somehow, unless horizontal sliding and overturning moment are not structural issues.

 

So I would start with a FPSF rubble trench foundation - only 12" or so of digging, assuming a relatively level site, instead of 42".  I am using a similar FPSF concept on my own small home design, but adapted to a crawlspace foundation approach.

 

Fred McLaughlin 

 

Deanne,

See the PDF wall section drawing from the church addition that I mentioned in my previous email, plus a JPEG image collection of the FPSF construction used there.  As you can see, the floor is a radiant system.  The FPSF detail was carefully worked out by my good friend and architect, Bert Seyfarth.  The detail is a wonderfully simple, minimal detail that consists of only the FPSF perimeter insulation protected by a single layer of ground-contact treated plywood, which is, in turn, attached at it's upper edge to a ground-contact treated 2X6.  This whole insulated assemblage doubles as formwork, with the treated 2X6 serving as form-edge stiffener and concrete screed board.  This entire arrangement remains in place as the finished FPSF construction.

You can zoom way in on the foundation detail and notes in the attachment.  Do keep in mind that, while this info is conceptually useful for thinking through your own foundation detail, the specifics are peculiar to this church addition.

Fred

 

Hey Deanne,

 

Good job searching ...   For reference, I made a couple copies of the attachments you sent.

 

The University of Minnesota (UMinn) started research on the idea of a frost protected shallow foundation (FPSF) concept back in the 80's.  In the early 90's, the National Association of Home Builders (NAHB) discovered that Scandinavian countries had already developed such a concept, extrapolated from a late-1800's/early-1900's precendent used by Frank Lloyd Wright, who, in turn, had gotten inspiration for the idea from detailing that Welsh stone masons were using on some of his projects around that time.  The Scandianavians had completed refinement of the concept and been using it for more than 40years by the time NAHB became aware of their success.  NAHB successfully re-tested the concept here, building test homes in all frost susceptible climates in the lower 48 plus Alaska (of course, Scandinavian research, in addition to 4 decades+ of successful field experience, can't be as good and trustworthy as ours here in the good ol' US of A, right?).   Using worst-case scenario assumptions to number-crunch, such as no protective snow cover and highly conductive clay soils, by the mid-90's, NAHB's Research Center was ready to publish it's results, which confirmed the Scandinavian experience.  Meanwhile, UMinn dropped it's research, which was using far more conservative assumptions than necessary.

 

Fast-forward to today, and we find the concept an accepted part of the Michigan Building Code (MBC) and the Michigan Residential Code (MRC).  The FPSF may be used for any building type, heated or unheated.  The concept is detailed differently for heated-building foundations than for unheated-building foundations.  The detail I cited in my earlier email to you is for a heated building.  The MBC and MRC sections on foundations only picture a basic version of the FPSF concept, which depicts all POSSIBLE features of this simple version, including "wing" insulation.  Our climate in the approximately lower 3/4 of Michigan's lower peninsula is relatively mild enough that Michigan codes require vertical perimeter, but NO wing, insulation for FPSFs used with heated buildings.  A number of variations on detailing the FPSF for slab-on-grade, crawlspace and combinations of scenarios for heated and unheated buildings, is now contained in the American Society of Civil Engineers' document, ASCE Standard 32-01, Design and Construction of Frost-Protected Shallow Foundations (42 pages, including many useful illustrations), which is the approved technical design source cited in Michigan's codes.

 

Attached are an images of the cover of ASCE Standard 32-01, plus a page showing the image of the basic slab-on-grade version of the FPSF lifted for inclusion in the MBC and MRC.  Again, even though the image shows the wing insulation, the depiction is for the purpose of displaying all POSSIBLE elements of the FPSF concept.  Number-crunching for a church addition I did here in Lansing this year was requested by the building department and the calculation results showed NO wing insulation was needed.  The simple calculations are described step-by-step, with accompanying brief text, tables and illustrations, in the ASCE document.  The resulting numbers let you know what R-value to use for the perimeter insulation and how deep to carry it into the ground, whether wing insulation is needed, as well as information about insulation that might be placed under a concrete slab-on-grade floor.  ASCE Standard 32-01 is one of my most valuable reference sources.

 

Hope all this helps rather than confuses.

 

Fred

 

PS  The hand-scribbled notes on the attached ASCE page 5 are reminders of conversations I had with NAHB researchers who clarified for me that  1) the "12 inches maximum" from grade to the top of the concrete slab shown in the illustration is NOT an actual limit on that height for the FPSF concept but is only showing that the perimeter insulation needs to continue down from where the wall insulation system ends, so the wall/perimeter insulation combination forms a continuous insulation layer from the wall system to - in Lansing's case - 12" below grade; and  2) the bottom of the insulation must go below grade the vertical distance determined from the FPSF calculations (again, in Lansing, 12"), but the depth of the foundation element (concrete slab thickened-edge footing, for example) for the building does NOT have to be located at the same depth as the insulation and is determined as a separate structural matter unrelated to the presence or absence of the FPSF (the illustration appears to suggest that the bottom of the FPSF perimeter insulation and the bottom of the concrete footing need to be at the same depth, which is not true, and which is why I asked the question).

 

 

 

Notes from conversation between DB and FM on 11/16/08

 

Heat moves toward cold proportionally to the difference in temperature between the hot and cold.  Therefore when a part of a building is not insulated, and the very cold outside temperature meets warm indoor temperature, the warm indoor air will want to exit, or move toward the cold.  Heat moves in response to the temperature difference. The more the difference in temperature, the harder the heat is going to work to get to the cold.

The “vigor” of the movement or “thermal pressure”, is dependent on the difference in temperature. 

 

The body is radiating.  Radiation is a direct experience.  For instance the light of the sun warms our body almost instantly through space. A fire warms us right away.  This is radiation, the most effective of the heating styles.  That is why radiant heat is so much more effective

 

Another way heat can move, is to pass its temperature along by an “intermediary”.

Air (an intermediary) can carry heat.  Likewise liquids and solids materials (materials that have more mass, or density, or closeness of particles) such as water, stone, cement or metal can carry heat.

 

 

“Convection” is a way that heat moves by passing the heat from air molecule to air molecule. Air is the intermediary.  Since the molecules in air are very far apart, it is a slower process of moving the heat along in space.

”Conduction”, like passing along a rumor, so heat conducts through mass so quickly.  Cement, metal (conducts heat very quickly).  The denser the mass, the quicker heat is transferred, because the molecules are closer together !  Metal conducts quickly.  Earth conducts slowly.  The opposite is true, as well.  The slower that heat moves through mass, it

 

 

Mass can also “store” heat.  In order of heat-storing capacity: Oil, water, stone, cement, earth. 

---

 

Christina Snyder

Architect, Adjunct Professor, Lawrence Technological University, Zero Energy House Classs, Solar Home Contest, Architect of buildings for Manatou Arbor Ecovillage, Kalamazoo, MI. 

 

I still don't have any satisfactory answers to "natural" insulation that

can be in contact with the ground without deteriorating or absorbing

moisture and loosing insulation value. There is pumice (or manufactured

equivalent, foamed glass), or perlite and vermiculite, but you have to

design for a well-drained installation of them so the voids between

aren't waterlogged, and still they don't have a very high r-value/ inch,

or r-value/$. If you decide to go with Fred's suggestion of a shallow

frost protected foundation, which is basically a slab on grade, I think

your best bet is the XPS foam insulation (blue or pink boards) in

insulation value. But at any rate, you will need to have some kind of

continuous insulation around all edges of the floor "slab" (even if its

cob, not concrete) including underneath, and either extending down to

frost-line or out horizontally the same distance or farther. Also a cob

or concrete "slab" is high mass, which as you noted is not suitable for

intermitant occupation/ heating.

As you noted, the other possibility is a framed floor structure

suspended over a crawlspace. This framed structure could be built on

uninsulated foundations, either a foundation wall, or pilings, or

piers/columns. The bottom of these foundations must be down at the frost

line and wide/ large enough so that the accumulated loads they carry are

spread over enough area so that you don't exceed the bearing capacity of

the soil. If you don't use a foundation wall, you should also trench in

galvanized metal fencing around the perimeter from the floor down to a

minimum of 2' below the ground, so that skunks, groundhogs, and rodents

are excluded. XPS foamboard has value here too, since again you are

trying to prevent thermal bridging from the inside to the outside of the

structure through the framing, and where the building sits on its

foundations is a big thermal bridge. Foamboard can be exterior perimeter

insulation around the framed floor, and Dow even makes a high-strength

XPS foamboard that can carry 100 pounds/ square inch, which may allow it

to go between the floor framing and the foundations, depending on your

loads. Yet with the crawlspace concept the need for foamboard is

minimized, compared to the slab on grade/ shallow frost protected

foundation fred was recommending. However, if handicapped accessibility

is required, you will likely need a a long ramp, at least 20' long or

more, not including the landings, assuming level ground.

For the framed structure, I recommend using TJI engineered wood

I-joists, since they don't require using up large trees, and they

minimize heat losses due to thermal bridging (max. 3/4" web instead of

nominal 2x joist). They would be supported by two beams (or on top of a

foundation wall), and these beams could also be engineered lumber that

doesn't use large trees. If you have to get building permits for these

buildings, approvals will be easier w/ engineered or dimensional lumber

than w/ harvested  logs from the site, which should be throughly dried

(1-2 years) before using them. Your building should have a continuous

vapor barrier (relatively impermeable to air & moisture) on the interior

of your continuous insulation layer, and layers from that point to the

outside should always increase in permeability so that any moisture

which gets into the building envelope cavities can dry to the outside.

The continuous vapor barrier on the interior should be as air-tight as

you can possibly achieve with natural building materials - cracks ands

leaks produce a concentration of moisture into the building envelope at

the point of the leak. While this is less of a problem with the natural

building materials which can wick moisture away from the air-tight

interior natural plaster that serves as the continuous air barrier, thus

reducing internal humidity. In the floor structure, this continuous

vapor barrier is likely to be an OSB or plywood subfloor, which can even

be the finish floor surface to save money - I've seen some beautiful

stained and finished floors using only these utilitarian materials The

flooring should have interlocking tongue and grove edges, and seams

should be taped to seal leakage, if only from below. If you opt not to

use OSB or plywood, instead only wood flooring boards, it will be

difficult to keep interior moisture from penetrating into your insulated

floor and condensing inside.

With a framed floor, you can use natural materials to insulate the floor

cavities - I'd recommend rice hulls, which are cheap and high silicon

content (like phragmities reed) so they don't tend to rot, even when

damp. They also don't attract insects, and are somewhat fire-resistant,

and have an R-value similar to fiberglass. It is also cheap, when

ordered by the truckload from rice-processing plants. However, they are

small, so the cavity needs to be free of holes that they could pour out

through, important to remember when making alterations in future. Since

you could also have condensation on the underneath side of the framed

floor, from moisture evaporating off of the ground, having insulation

that won't absorb or be damaged by moisture is very important. You may

think of some other good natural insulations you would want to use.

I'm interested in hearing / critiquing building details / systems you

come up with. If you are most comfortable drawing on paper, you can

photograph your sketches and e-mail them to me,

Best wishes on your project,

Christina