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We are about to start building a new home and are planning a hydronic heating system with the tubes installed in a slab on grade foundation with
turned down footings. Even though we are in a pretty mild coastal California climate (CA Energy Commission climate zone 3), it seems like a good idea (maybe even a requirement) to insulate at least the slab perimeter. I need to insulate
from the top of the slab, 6″ above finished grade, to 16″ below grade. I have only found one material, Foamglas from Pittsburg Corning and it is only about R3 per inch, that works both in the trench and above grade. In addition, I’d like to find a foundation/insulation detail that doesn’t require the insulation to stick out beyond the wall siding.
Any suggestions?
Thanks – Gene Massion
Replies
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For my garage (radiant floor heat), I extended the framing (and plates) 1.5" past the edge. With the additional 1/2" sheating thickness, that allowed 2" of rigid EPS (R10) to be placed up along side the foundation (grade beam in this case) and flush with the exterior. I could get away with the overhang, because the walls were 6" framing and it left plenty of wood on solid foundation. I don't know how much, if any, overhang would be allowed using 4" framing. Being a pretty mild climate you probably don't need to go as thick as that, especially at grade level.
Using rigid below ground has many strong opinions, not the least of which is termite damage. As far north as I live, termites are something you only read about in books. You might want to see what others have to say about below ground insulating in some of the other discussions.
*I will let everyone else give their opinions on this issue. I wish to learn but also speak with some air of authority, considering I design/engineer radiant floor heating systems and often hear about the "termite" objection. So let'er blow. Who knows, perhaps we may be privileged to have a response from someone at DOW or AMOCO on this thread? Does anyone out there have a reference to code(national, state, local) prohibiting ground contact insulation? If so, can you quote the code body, requirement and/or local written interpretation?Jeff
*Up here in Alaska, 2 inches of "blueboard" (R-10) is standard around slabs, radiant or not. (My house and garage are both radiant). The hacks jsut leave it exposed and the outer edge weathers away. The better installation is to have flashing made to cover the top and side of the foam and overlap (underlap?) the house siding. The better yet installation adds pressure-treated plywood under the flashing so it can take a hit, kick, whatever. As Scott pointed out, you can overhang the framing to be flush with siding. But up here, if you just cover it you are so far ahead of the Jones' that it's good enough. We're also in seismic zone gazillion so retaining as much framing-foundation connection as possible seems like a good thing.-David, Kenai, AlaskaP.S. A caveat: we have no termites or ants so they are not a concern (Nor scorpions, snakes, sales tax, or income tax) Just squitters and a damn lot of moose crossing the road.
*........ahem....(Jeff , you coward, all those who volunteer take one step forward).....this is what we do....doesn't mean its right..any slab on grade and any basement with utilities in it that the owners have the least interest in using we put minimum 2" Styro -SM, or Foamular or Amoco.....(high compression strength, about R-10 for 2")under the whole damn slab.......figure it out ....the ground under your slab is at what..... 45 deg.say... and your slab is 70 deg. so you have a minimum 25 deg delta-T...now ..if you have a radiant slab....maybe the bottom of the tubing plane is (help me out here Jeff...80..90 deg, so the delta -T goes up to 35 or 45, twenty-four hours a day for the whole heating season...now someone come in and says ...bull , there's a cone of heat that extends under the structure that is 50 deg, or 60deg. or whatever ...taint so.. if you turn off the heat source, the ground will revert to ambient temp...... figure 6 ft. down about 45deg....(I'm doing this from memory so bear with me if I'm a little off)next ....slab edge...why give up the edge loss, break that with 2" min. also...this is where it gets a little convoluted. ...in a commercial bldg. we use a 4x6 P.T. sill bolted to the found. The 4x6 becomes our slab form and thermal edge break. inside that we nail a 2" strip and a 2" piece over the top of the found. wall and down to the bottom 2" foam under the slab. so minimum slab is 3.5 over the top of the found. wall with mesh because this edge is the weak point...the slab is design thickness, but min. 3" all the way to 6" depending on design floor load.......another variant is we do a lot of pole barn designs for addtions and new const. the outer shoe is a P.T. 2x10 that extends from above the slab level to below finish grade on the outside..if you have a grade problem this can be stacked so the only thing you see on the exterior below the bottom of the siding is P.T.....on the inside of this 2x 10 is nailed a piece of 2" Styro -SM which also serves as the screed for finishing the slab.this runs down to a thickened edge and then returns under the entire slab..Now, termites,Scott, I would be very surprised if you DON'T have 'em.My understanding is that you got em unless you have permafrost....they are the second most numerous insect on earth, (or is it first) the other one is ants...I'm not so much worried about termites...I don't allow anything that would attract them... so I'm hoping they'll go someplace else...but Carpenter ants and other larvae really concern me, they think we invented foam so they could make a home in it......Most of my specs include a Boracare application before the interior gets closed up...I like what I hear about the new baits that you stick in the ground too,I don't have a lot of faith in P.T. either, ... I use PT stakes in my vegetable garden, and I can only get 2 years out of them before the termites have destroyed them, which is one year longer tha I get with untreated stakes....When it gets dry, and my broccoli starts falling over like Paul Bunyan was cruisin thru, I go out and pull some up and they're covered with termites....So, come on back and tell me how to change my evil ways...b I love chocolate covered termites..Kermit
*Heat always moves towards a colder mass doesn't it?As Mike mentioned, it doesn't matter what the DeltaT is, 10F or 45F. In a residential or small commercial application full underslab insulation is a no brainer to prevent backlosses. And furthermore to prevent acceleration and deceleration deviations in respect to rapid changing outdoor temperatures and short-term internal structure gains. I am not as ademant as you Mike on 2" under the entire slab. Often, I will design with 2" perimeter and edge with the balance 1". Depends a lot on the type, efficiency and the sizing considerations of the heating appliance. I am one for undersizing the boiler/water heating appliance and at the same time utilizing priority. Simply because the nature of the mass lends itself to a smaller heating plant(which allows it to run closer to the maximum output for the better part of the heating season-thereby realizing greater efficiency) without compromising comfort. Something that a forced-air appliance cannot do. [sorry if I took off on a tangent totally unrelated to the thread]. Thermal sinks in residential construction exhibit a point of diminishing returns.If you want to take advantage of thermal sinks, build yourself a 10,000 sf plus structure where the perimeter to total square footage is a closer ratio.And Mike, I won't allow pressure treated stakes in my veggie patch either. I don't believe in feeding my salads anything that they I can't breath when I make the saw cut. Leachate?I am intimately involved with some of the CA codes and construction/backloss prevention practices as we speak. Given some time and deliberation, I think all will be worked out to satisfy the appropriate method for ground contact insulation and the energy expenditure. It only takes a bit of understanding amongst the parties to come to a reasonable consenscus.[sp]Jeff
*..........I could go with the tapered insulation towards the center...I had more luck with 2" surviving a couple of cement finishers tromping around on it...but with a radiant tube layout, I guess that is the least of your worries...So what can you tell us about foam manufacturers and what they're doing to prevent vermin infestation... it is a mojor concern of mine...I just don't know what my best course of action is....I gotta have the insulation...and those high compression strength foams are the only ones I've found so far.......is the glass fiber foam board any good, does it wick water ?....will it support concrete slabs, ?....what is the R-value per inch?...does it support vermin infestation ?The only industry comments I've ever seen were by some Structural insulated panel mfr. who were putting some anti-vermin additive in .... but those are EPS, and I have never liked EPS....is it just me ?
*Mike, sorry, no termites. As a kid I lifted every leaf, stone and rotten log I could find, both in the city and outings to the lake, looking for bugs. I could describe bugs most people never knew existed (like those tiny tailess scorpion things) but not a single termite. Even cochroaches (although I've HEARD of people having them) are rare. Carpenter ants I've seen plenty of, but rarely in buildings. Mostly they stick to trees (seen plenty of bad ant damage to trees) and other natural fauna. Regular earth ants we've always got trucking in and out of the house, but they don't cause damage. Maybe it's just too dry (in additional to the seasonal cold) to make it hospitable for the more virulent forms of crunching bugs. God knows it's enough reason for most people not wanting to live here! I think I've seen the occasional wasp dig into styro, but the worst bug plague I've had to live through are fruit flies brought in with the groceries (one black mark against indoor composting). Not to say it couldn't happen (a one season scourge hitched on imported lumber?), but here there be NO termites.Another recommended practice around here for insulating slabs (in addition to insulating underneath) is to bury a horizontal barrier (1.5" - 2" rigid) 2' below ground extending 4' out from the perimeter. The idea is to isolate more of the earth from the outside seasonal changes and create a bigger (more stable) earth temperature "cone" under the slab (that 45 degrees you were talking about). The theory is, it does more good flat under the surface than the same amount of insulation dug vertically deep because you include more of the earth surface to make a buffer zone. And in the end it's cheaper to excavate as well although it's not real common. Money is money.Others are may disagree (particularily the whole cost vs benefit issue), but as construction on my house progress, each foundation wall is being insulated on the exterior down to 2' below grade, then out 4'. You only have to put a hand on the basement walls to feel the difference. The newly insulated walls are warm to the touch. The unrenovated walls are cold.It's also my opinion that if you insulate the ground around your foundation, you're going to raise, if not eliminate, the frost line below that insulation as well. I think anything you can do to stabilize the soil under the foundation is years+ added to the life. But that's just my opinion.
*Thanks for all the help. In addition to the information, I appreciate the lively discussion. The only fiberglass rigid insulation board I could find is Foamglass by Pittsburgh Corning. Other than that I seem to be stuck with Dow Blueboard. Any other specific recommendations about what brand/type of rigid insulation to use?Thanks again - Gene Massion
*GeneConsider this: I'm assuming you're still in the design phase, if not , disregard. How about doing a two-phase pour. Phase one, pour the turned-down slab to spec, excepting the tubing and possibly holding it down in elevation, allowing for an additional pour up to finish elevation.More specifically: phase one is executed typically, except rebar on 16" centers is left protruding 8 inches above the slab 4 inches inside the perimeter. The bottom ends of these lengths should be J-hooked under and tied to a main element of the iron framework. These can be left folded down and then fished out after screeding (leave locating marks on the form.) A day or so later, one course of 8" block is laid around the perimeter and grouted solid, this can be over an untrowelled but well-floated finish. The top 2 inches of the above-mentioned rebar is bent 90 degrees. Anchor bolts or straps can then be placed to interlock with the rebar. At this point, your foundation is done. Next, after curing as well as possible, glue and weight your choice of 2" foam to the slab leaving a 5/8" gap around the perimeter. I'd use Dow Scoreboard which has longitudinal scoring every 16" that allows the material to be flattened more completely against its substrate. The interior face of the block is insulated similarly except that what's to be the top edge of the foam is bevelled 45degrees with the tall edge to be placed against the block. Instead of weight to help form a good bond, use a wale and clamps. At this point you can lay-out and run the tubing. This done, pour a 6 inch fibermesh slab. Another approach might be using 3 inches of tamped AB gravel thereby reducing the second pour by half. The result is a fully insulated fully isolated mass that leaves the exterior plane relationships typical and only a small cold joint between slab 2 and the interior edge of the block to be caulked to (theoretically) exclude termites etc. from access to the foam.
*Freelance,Great idea!! You don't by chance have a drawing of this method do you?If you don't have the time or means to post it here, you could just do a rough drawing and fax it to my 800 fax #. I could pretty it up if need be on the CAD program and include a link here under this thread.I will email you the (800)#.Jeff
*I'm a self admitted dreamer of over-complicated solutions and an excessive user of insulation, but freelance, I think you've got me beat. Not that I think there's anything wrong with your two slab sandwich approach. In fact I think it looks like a solid idea. I'm also quite dumb in the ways of termites (which seems to be one of the considerations). But the cost man!You're bottom slab is basically equivalent to the single slab approach. Same grade concrete, same reinforcement (I've always used 12" O.C. myself, cheap insurance), same work to prepare and place. The only thing you save over the single slab is the time spent finishing the surface. I'm only guessing, but I think that's just a tiny piece of the total slab cost. Finished or not, you're still going to have to work hard to get a flat level surface.I'm not sure why you choose to leave a 5/8" gap around the perimeter. The foam could just as easily be butted up solid and compressed across the width of the floor. I also don't feel it's necessary to glue the boards anywhere (even the sides). The weight of the next surface will be more than enough to keep everything in it's proper place. Gluing board is extremely time consuming (=costly) and you'll more likely end up with the board floating on globs of glue rather than sitting flat against the surface like it needs to be. Bond is not important here, what you need is to keep it in place while the next surface is created. A careful crew can manage.Provided the base has been prepared properly, I don't see much point to building up the second slab any thicker than it has to be. The second slab will add nothing to the strength, it's only there to bed the radiant heat tubes and can be built accordingly. Skip the gravel, skip the thick slab, consider the foam surface just like the subfloor of any other framed structure.In the end though, the problem that I don't see being solved is the short piece of block wall around the perimeter. All that effort will be for not if this section goes un-insulated. Then your back to the original posted question. Inside, outside, what kind and how much.Maybe it's just the "garage" mentality of mine (only garages get built on slabs here, houses are built on full basements) but how's this for a thought:Conventional slab for the region (never skimp on strength here). Make sure it's nice and flat, no need to polish it to a mirror surface. Triple the sill plate (keep reading) and frame using whatever method is typical (or untypical if typical is crappy) for the region. Now, within that supersized sill, lay 1.5" foam (no gluing). 1.5 is plenty sufficient for Gene's location. This covers the first sill and leaves two more standing. Now lay out poly over this. Not so much for a moisture barrier, but to keep the next pour from getting underneath the foam and floating it. I would even suggest foil faced bubble wrap under (or in place of) the poly, but that might be over the edge. Concrete mesh, to which is tied the RF tubing and the whole area gets a final 1.5" pour of self leveling RF floor mix. This covers the second sill and leaves the third sill right where it should be for final wall finishing. The floor on foam is sufficient to support flooring, but not interior walls. These will have to be dealt with in some clever way. The floor perimeter is only isolated from the exterior by the wood's width of the sill plate, but then again so is the wall itself (and the top plate, and the window frames, etc). Any modifications to build a better isolated wall could be easily carried down to through the floor to the slab. What about the termites? Well, if they're chewing up your sill to get to the foam, I think you've got bigger problems.Regarding termites and foam.We don't have em here, so I'm relying on books, A&E and PBS. There's no doubt they can chew through foam like butter. I'd be surprised if they even realize they're chewing it (compared to wood and all). But I don't believe I've heard that they EAT it. As far as I understand it, they only chew through it to get at what they really want, the WOOD. Now if the only thing seperating your wood from their appetites is a piece of foam, your in big trouble. Likewise, I don't believe they're going riddle the foam to nothing in their quest for food. I believe they create a path through the foam to the wood then follow it until there's no more wood or it comes time to find a shorter path. I think that puts foam under a slab in the extremely low risk category. I think the same could be said for any foam in ground, except where it bridges a path between ground and wood. But like I say, we don't have em here so I'm just guessing.
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We are about to start building a new home and are planning a hydronic heating system with the tubes installed in a slab on grade foundation with
turned down footings. Even though we are in a pretty mild coastal California climate (CA Energy Commission climate zone 3), it seems like a good idea (maybe even a requirement) to insulate at least the slab perimeter. I need to insulate
from the top of the slab, 6" above finished grade, to 16" below grade. I have only found one material, Foamglas from Pittsburg Corning and it is only about R3 per inch, that works both in the trench and above grade. In addition, I'd like to find a foundation/insulation detail that doesn't require the insulation to stick out beyond the wall siding.
Any suggestions?
Thanks - Gene Massion