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I have been proponent of thinking of a slab as a heat sink that needs to be entirely isolated from the ground and completely wrapped in the building envelope (with or without radiant infloor heating)with 100% edge and under slab insulation.
But, perhaps there is a geothermal gain from leaving the underside of the slab uninsulated as long as there is good horizontal insulation value.
More importantly, my assumptions about losing heat to ground in winter have been challenged by some who say that an underslab heat bubble is formed — so the slab will not lose BTU’s once the earth is warmed up. Kind of like creating a heat sink (significantly larger than the slab) that re-conducts heat back into the house.
These may be season by season issues: fall and spring heat gain, winter and summer heat loss.
Replies
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Unless you locate the house over a seasonally active volcano, most of your theories won't work.
*Tedd,My cellar slab is at 55 degrees...Not a good thing for winter time...I think slabs could be eliminated...What are their purpose? to keep out bugs maybe...to walk on and to deal with ground water...I like simple science solutions aimed at getting the results desired....The dimpled plastic floor covering looks like a good idea. Maybe this...2" stone w/ drainage leveled, ground use plastic, and a special SIP subfloor panel made with PT material. I did see an insulated wire mesh panel for concrete at a show recently...costly I think.
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Gabe ... what a useful reply to my questions :-) Which theories don't work, why and what would you replace them with ?
*Jack ... I was referring to slab-on-grade. So I take it that you would isolate the slab from the ground by full under-slab insulation.
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I have a full mostly unheated cellar...the slab center is at 55 degrees and the edge is a little colder...Some say the insulation is not necessary in the center if your heating a slab on grade...I don't know enough to say not to insulate the center so I am still for it.....
J
*Tedd, the entire equation is dependant on whether or not you're using radiant heat.If you use radiant heat, you insulate the perimeter and under the slab, in it's entirety. The benefits are that the slab will come up to temperature faster.If you don't intend to use radiant heat, you insulate the perimeter and 4' into the slab (Frost protection only). There is not enough benefits from the insulation to make the slab any more comfortable. The thermal resistance of the styrofoam resist rapid changes in temperature, it has no effect from the constant of ground temperature. You cannot significantly affect the temperature of a slab of concrete, unless it contains the heat source or has heat applied to it directly.As I have stated in the past, this is why we routinely install cork floors in basements. To cushion the effects of concrete and to insulate from the coldness.
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Yea, I always took position that the slab-on-grade with or without in-floor radiant heat serves as a heat sink.
I guess my question really has to do with conduction -- the slab would conduct into the ground continually if the ground temp is lower. This applies to a slab with or without radiant in-floor heating. Losses into the ground would be less in an unheated slab, but still happen.
Why would you waste the heating/cooling benefits of a slab-on-grade if it is to be insulated from the interior of the building ?
Where are these people getting the idea of an underslab "bubble" that alleviates the need for underslab insulation ?
What R value of edge and under-slab insulation do you recommend ? XPS is damned expensive.
I am told that Type 2 steam expanded EPS in Canada is a valid replacement for XPS. Anyone have experience with this Type 2 foam board.
Tedd
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Tedd,
An ideal home system to me would gather the roof heat and use it, and gather the underslab earth for cooling and heatsink uses..And most systems should serve multiple purposes along with be inexpensive, low maintenance, low labor, and last but not least fun and artistic in nature...changeable and re-arrangeable....And incorporate lots of outdoor or as close as one can get to outdoor spaces that are habitable while sleeping, during rain and thunderstorms, while snowing...all the exciting events of nature with closer access...
On the Mountain near the stream,
J
*Tedd,Trying to conduct heat into the ground via the slab would be a loosing battle at best. This would be the most inefficient method that I know of.What part of the country are you in? It would make giving an opinion on the insulation requirements a little easier.You haven't indicated either "if" you are going to use radiant, in-slab heating.You can even cut down on the ground temperature effect on the slab with proper drainage. ie 6" of compacted crushed stone as opposed to damp wet clay in contact with the concrete.
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Jack ... slab-on-grade has only one purpose for me anyway -- if its not a controlled heat sink wrapped
into the building envelope, its a liability in my view. I was asking my question to see if anyone could challenge my views and substantiate the
assertions of others who have spoken against insulating slabs.
I agree with you -- capture the latent heat from the attic, the furnace/boiler and from the ventilation system (HRV/ERV) and recycle it. Keep the slab dry and isolated from external conducted and convected losses (ie; no outdoor exposure at all) and it will be a giant climate regulator and central radiator.
The only time I would want the slab to have conductive ground exposure and exterior heat loss would be for summer cooling. I guess a low voltage heat pump could draw heat out of slab's radiant tubing which inturn draws heat out of the interior air -- as long as the temp differential is keep above the dew point. Otherwise the slab will sweat.
I have no use at all for basements -- fancy evolutions of root cellars that don't do anything well and do lots of things badly. In fact, concrete foundations are a primative idea and useless overbuild in residential applications. The only good thing about basements is that they ... ah ... I can't remember ...
Tedd
*Gabe ... I am in two locations; western NY just below Lake Ontario and Toronto, just above the lake. Where I build, clay is common and wet ground is the norm in spring up 'til early summer.I include radiant in-floor heating as a component of the slab. Wouldn't consider a slab on grade without it doing more than just sitting there. I think my views are the same as yours. I was just trying to elicit any pros on the so-called unerslab heat sink cavity that some say remove the need for underslab insualtion.Tedd
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Tedd,
Okay that clears up a little fog.
I'm not a fan of slabs on grade for residential house construction. The cost to do a proper slab on grade here with adequate protection, insulation and frost footings is such that it's far more cost effective to install a full basement.
All of the basements that I design are minimum 9'-0" clear. All are structured to make comfortable use of the available footprint of the main house.
Too many builders minimize foundations and as a result, they are only good for storage space.
But that's them....
*
tedd,
I am also in WNY and have just dug a basement under my 140 yr. old former one room school. I am planning on radiant heat and will pour the slab this week. How much foam? Will foil-faced work or is Dow-type better? The foil-faced is less expensive for the same R-value, but will it break down under the slab?
Thanks in advance,
bear
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Bear,
In your location, 2" styrofoam would be the norm.
Haven't seen any tests results on foil-faced to comment on.
I will emphazise the crushed stone though, as a base for your slab and make absolutely sure that it is properly compacted. Cover with a thin layer of sand, install the poly and cover with the styrofoam.
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Thanks, Gabe
I was planning on 2" of foam, so I'm glad to hear that's OK. I have about 4" of stone down already, but hesitate to add much more due to headroom issues. This will be shop/storage space, not a rec room or anything. 4" slab is planned.
As for compaction, I own a mechanical plate tamper, but am afraid to use it down there because of clay/mud pumping up through the stone as I tamp. My newly placed sump pump runs every 10-20 minutes, depending on the weather, and the soil under the stone is still a little squishy. Was thinking of hand tamping the stone after raking it out nicely.
Maybe I should have placed some geo-tex under the stone, but I didn't think it was necessary at the time.
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Bear,
If using a plate tamper causes the clay to come through, then you have a problem that has to be solved BEFORE you do the slab. You may have to install a drainage pipe prior to the concrete to relieve the water, or at least dig out the soft spots if you don't want to go this route.
Tamping down on soil covered with 4 inches of stone has no business pushing up clay. A Jumping Jack will displace, a plate shouldn't.
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Bear ...
Use a back-up water barrier:
As Gabe has said earlier in this string, the most important is to ensure the slab/foundation is isolated from water. Concrete is a sponge so you must use a water barrier under the slab and wrapped up the walls. Caulk seal the poly joints and laps. If you think you have a lot of mechanical action in the soil from frost, double the poly barrier. Every other method I know of for sealing a foundation is expensive and or unreliable and unnecessary if you stop water from arriving at the foundation/slab in the first place.
Prevent water access to the slab/foundation:
In the WNY fruit belt, clay soil causes water to travel on the surface and pool in low spots. The water will evaporate before it is absorbed into the ground.
Your project will be creating a water dam (the foundation) on your property and a vertical channel (the basement wall) inviting surface water (rain and snow melt) to travel down and around your slab. The foundation will be the low spot where the water pools. The up-hill side of your foundation, in particular, will stop and migrate water down around your foundation.
If you have clay dense soil, gravel fill may make the water pooling problem worse as it creates a porous space for a lake to form around the foundation. If you have reasonable drainage on the site, gravel back fill is ok. If not, I would return the excavated soil to the trench and tamp it. This view will raise some hackles.
To keep surface water away, some drainage options to consider, include:
1. Excavate an uphill trench 12" by 12", line it with gravel and lay in perforated plastic drain pipe; and, run this catch basin around the sides of the house to redirect the surface water. Bury it enough to resume a normal lawn or garden on top. The trench/catch basin should be several yards away from house. Maintain a continuous downhill grade in this trench.
2. The roof gutters must drain through downspouts that lead water to the downhill side of house. The downspouts can be taken below grade and non-perforated drain pipe can be buried below grade; tied into the trench. Make sure you put coarse gravel and rock at end of the pipe to break up the velocity and direction of water flow to reduce underwash of the lawn/topsoil.
3. Landscape a sloping grade away from the building.
4. Traditional footer level drain tiles are virtually useless under the conditions described above. Water should be kept away from the foundation/slab or the heat loss will be significant – with or without a radiant floor.
Insulation:
If your pouring the slab below the frost line, R 10 might be enough for the slab -- its a trade off in cost. I prefer even higher R values for slabs on grade; at least on the first 4 to 8 feet of the underside slab perimeter and vertical slab edges. You will lose more heat by conduction (to the ground, concrete walls and footers) than by radiation (inside the house). This means isolating the whole basement with insulation. The R value should be proportionately higher between the frost line and grade (R 20 or more). You should insulated from grade to sills, too.
Foil backed insulation has no advantage under the slab. Radiant foil faced insulation requires a 3/4" minimum air space immediately adjacent to its foil surface to perform as a radiant reflective barrier. You can’t create this reliably in your slab - and conduction breaks are more important than radiant reflective returns. If you are considering foil faced foam for a bulk water barrier, forget it.
Your concrete walls will suck away more heat from your radiant slab floor than even the ground (if ground is kept dry). Incorporate the walls into your envelope by insulating it from top to bottom.
A final protection against water and frost penetration is a sloped-at-45 degrees, horizontal plane of XPS or Type 2 EPS buried around the entire perimeter with the top edge touching the foundation wall. This perimeter frost barrier needs to travel from grade to frost line. This will slow penetration of frost moving towards your foundation wall and shed water that might reach the area adjacent to the house.
There is a Canadian made, Type 2 steam expanded polystyrene by a company called Plastifab. Its underground rated and a less expensive than XPS (extruded polystyrene foam). Your insulation materials and drainage materials and installation will cost you more than your concrete installation. This is the untold reality of properly insulated and water protected basements.
If you plan to install a raised finished floor, you will waste the benefit and expense of your in-slab radiant heating. You might want to put your radiant tubing in the concrete basement walls instead of the floor in this case. You can stucco or parge the walls and they will still radiate heat. In WNY a finished basement that has not been adequately waterproofed and insulated will sweat behind the interior walls and demand lots of dehumidifying and requires designed-in convection channels backed up with mechanical ventilation. Interior insulation will not stop this problem. If your using concrete block walls, the mortar is even more porous than the blocks and the moisture and sweating problems will be worse.
Tedd
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One tiny add-on Tedd,
I always cover the outside foundation with dampproofing like everyone else but then I cover it with cheap asphalt impregnated tentest material to make sure that backfilling doesn't scrape the tar off the foundation walls rendering it useless.
*If you feel that you need insulation under the slab other than just the first 2-4 feet, you only need 1/2". With no insulation in the center, the slab will be near the average inside temperature in about a year (instead of the normal 55+/- degree ground temp.). With 1/2 inch of foam it will stabilise in only a few months. Use the money that you saved from the center of the slab to beef up the insulation around the perimeter where most of the heat loss is. With radiant floor heat, you might want to go up to 1" in the center but you still don't need as much insulation as the outer 2-4 feet
*Ron,This idea of the center of a slab warming up must have to do9 with heated space only??? My ten year old cellar slab center temperature is 55 degrees on the nose....I think a thermal break is the way to go.Near the stream,J
*Jack,I suspect that your cellar is un-insulated. You would need the exposed part of the foundation and a few feet below grade insulated for the slab to stabalize near the average room temp. (Or is your average cellar air temp 55-60 degrees?)
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Ron ... I think you inadvertently answered the question I posed to begin this string. Are you saying that no insulation is needed under a slab - for either a basement floor and or a slab-on-grade - other than the vertical and first few feet of under-slab perimeter ? Even if it contains in-floor radiant heating ?
I have heard this point made before but I am not sure what the rationale and principles are. Can you detail your reasons ? Perhaps you can correct some of my assumptions, which include:
1. Conduction of heat between solids is more efficient than convective or radiant transfer of heat. In otherwords, concrete will give off more heat to the ground and structural members it is in contact with than the air it is contact with.
2. If the ground under and in contact with the slab is cooler than the air above the slab, the in-slab radiant heating will send more heat to the ground than to the air. In effect, you will be heating the ground and drawing heat out of the indoor space as well.
3. I understood the below frostline ambient temp of the earth is around 49 to 51 F. I would therefore presume Jack is heating his slab/basement floor by up to 6F degree if it is actually 55F.
I think I have stated the same point three different ways.
If Bear doesn't isolate the basement floor/slab (which is to be his central heating radiator since he is installing in-floor heat) with a thermal break (ie; insulation) sufficient to prevent the out-flow of heat, he will in effect, be running hot water through his PEX tubing for the expressed purpose of heating the earth.
Probably the fastest way to draw heat off any solid is to run water across it. This makes extremely efficient conditions for loosing heat by adding both turbulence, liquid convection and evaporation. These highly efficient conditions for transferring heat exist as long as the thermal break is omitted... no matter how much water proofing is installed.
Bear needs to calculate the heat transfer rate of his slab in BTU's/hour, comparing insulated verses uninsulated contact with adjacent solids; taking into consideration the average seasonal variances in temperature between inside and outside (his design temperatures). This way he can estimate how much money he will spend to heat the earth (and keep it heated) throughout the cold season and the volume, temperature, flow rate and layout of his PEX as well as the size of his boiler/DHW heater and his gas/oil/electric bill to handle the loads.
Tedd
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OK in all this how is radon accomodated? Or, is it just ignored because of your geographic area?
*I really like this thread. I am learning. That is a GoodThing as far as I am concerned. I was thinking about tackling such a project myself.
*Ron,My cellar has wall insulation done in a strange way so as to protect it and it's 2x4 frame from wall moisture I would guess and all is very dry with humidity in the thirty five percent area. It is held off the poured walls about 2 inches, hangs from above off floor joists and ends about a foot before the floor. (I will probably close the gap at the floor level in the future. I acquired this home recently.) The temperature at the floor and wall intersection of the foundation is about 49-50 degrees in the winter...Last summer, the cellar with windows opened warmed up into the 60's...I think a slabs ability to radiat heat both up into the air and down into the ground through a temperature gradiant would have something to do with the grounds r-value, and the slab temperature from the in slab pex tube heat.The radiant heat companies must understand all this quite well by now.Any more thoughts Ron?Near the stream,Jack
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Hey JJ - woundered when you would mosey on over to this site and straighten us out :-). There is need for a radiant man here.
Tedd
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Jack,
You may be getting cold air washing down the wall to the cellar from that 2" gap. Most of the heat loss in a basement or slab is in the first few feet. Dirt has both a thermal mass and an R value. In a way it is like a super thick Trombe wall in that it makes a delay in movement of heat energy thru it. With less delta T between ground temp and inside temp away from the edges of the slab/foundation, insulation is needed more for a thermal break to speed up response in a radiant floor system than to prevent energy loss into the ground but both are important. An inch or so of insulation away from the edges should be sufficient. I would use 2" to 4" for the exposed part of the foundation and for a few feet down, out or under from grade. If you go out from a slab you have a shallow frost resistant foundation.
Ron
*Always willing to help when I can for what it might be worth. Let the questions fly. Emerging technology creates some very powerful and stimulating debate.
*
I have been proponent of thinking of a slab as a heat sink that needs to be entirely isolated from the ground and completely wrapped in the building envelope (with or without radiant infloor heating)with 100% edge and under slab insulation.
But, perhaps there is a geothermal gain from leaving the underside of the slab uninsulated as long as there is good horizontal insulation value.
More importantly, my assumptions about losing heat to ground in winter have been challenged by some who say that an underslab heat bubble is formed -- so the slab will not lose BTU's once the earth is warmed up. Kind of like creating a heat sink (significantly larger than the slab) that re-conducts heat back into the house.
These may be season by season issues: fall and spring heat gain, winter and summer heat loss.