aksmith42
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| 07/12/2008 1:48 PM |
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| i was wondering how important insulating a basement slab is and if you would definitly do this what type of insulation would you use? in my new home i'm going to have forced air heat. i've heard that leaving an uninsulated spot in the center of your slab uses the ground as a heat sink. thanks for any opinions. |
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PanelCrafters
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| 07/12/2008 5:15 PM |
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Posted By aksmith42 on 07/12/2008 1:48 PM
i was wondering how important insulating a basement slab is and if you would definitly do this what type of insulation would you use? in my new home i'm going to have forced air heat. i've heard that leaving an uninsulated spot in the center of your slab uses the ground as a heat sink. thanks for any opinions.
Where are you building? Ground temperatures vary across the US(continental) from about 37° to about 77°. As far as leaving a 'hole' in your insulation to create a heat sink, that sounds ridiculous. You might have a shot with an uninsulated slab that's heated via radiant, but otherwise I'd punt on that idea.
One of the standards is 2" of genuine Styrofoam(R-10).
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....jc
If you're not building with OSB SIPS(or ICF's), why are you building? |
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wes
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| 07/12/2008 6:51 PM |
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I agree with JC, for most circumstances, 2" of styrofoam is a relatively cheap way to insulate under slabs.
However, I tend to think that if the basement is to be heated and cooled, then the insulation should be used in any climate. Even if the ground temp. is only 5-10 degrees off the conditioned temp. you will have some heat loss thru the slab if it is not insulated.
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Wes Shelby
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Mark Fleming
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| 07/15/2008 5:04 PM |
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R10 insulation under a concrete slab is required by code for occupied space where I'm building. 2" Dow Blueboard is the preferred material. It has higher R value and is more durable on the construction site than the bead type. Contact a real material supplier, not a home center, and get the 4x8 sheets for any larger job. It will cost you $1 per square foot, which is why some contractors say you don't need it in the middle. Some jurisdictions may allow you to tape it and also call it your vapor barrier.
I think that leaving the center uninsulated as a "heat sink" is one of those silly fairytale construction myths. I can't believe that it's still repeated and some jurisdictions allow only the first several feet around the perimeter to be insulated. The earth's temperature under your house isn't what's important. The issue is whether there is any moisture transfer under your house.
Here's how you tell. Are there any live plants growing around your building site? Then there is moisture moving through the soil. Or, try this. Place a plastic tarp on the ground the size of your foundation. Collect the gallons and gallons of water that condenses on it every day. That's how much water is moving through your soil. Now, heat that water and throw it out the window every day. That's what a "heat sink" in the center of your slab is doing. End of rant.
Mark |
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Dan Morrison
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| 07/15/2008 10:31 PM |
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Mark's right, but there's a little more. It's not a heat sink thing, it's a water thing and a comfort thing. Your floor will be cold and wet (or at least clammy) if you don't run some XPS under it (it doesn't have to be blue, the pink stuff works too).
Dan
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Dan Morrison
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dmaceld
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| 07/24/2008 11:30 PM |
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For what it's worth here's a link to a thread I started discussing payback of under slab insulation. Payback is very sensitive to energy cost.
http://greenbuildingtalk.com/Forums/tabid/53/view/topic/forumid/12/postid/32800/Default.aspx
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Building house - what a way to spend retirement! It's done! We're living in it! |
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toddm
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| 07/25/2008 9:26 PM |
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This is a basement, right, as opposed to a slab on grade? A vapor barrier is critical in both, as posters have averred, but that question is entirely independent of insulation. You should insist that the contractor put a six-mil polyethylene plastic sheet under the slab and waterproof the exterior of basement walls, if any, regardless of how it is insulated. It costs pennies per SF, and it addresses a health issue (mold) as well as a comfort issue.
Insulation is a tougher call. Protecting the exterior of basement walls near, at and above grade level is the most important thing. It couild be an advantage to skip insulation under a basement floor that is six feet below grade, but only if the space is finished and heated.
Perimeter insulation is even more important if we are talking about a slab. You'd also want to insulate under it, unless your passive solar design in northern climes calls for maximum thermal mass. |
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wes
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| 07/26/2008 6:58 AM |
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six mil poly under a slab is a given, in fact I use a double layer of poly under basement slabs. But insulation under a slab in conditioned space should never be 'skipped'. Our basic below slab configuration is: undisturbed soil, 4+" clean wash rock (ground water capillary break) with drain tile to daylite or sump, 2 layers of 6 mil poly, 2" foam insulation, concrete slab. This arrangement has worked very well for us for many years now.
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Wes Shelby
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crashkahuna
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| 07/26/2008 10:41 AM |
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| We are in a severe winter/hot and humid summer climate in Iowa and our basement was built without insulation below slab. It is always warmer in winter and cooler in summer than rest of house and we have no mold problem. We are about to build a new house and so I am not sure which way to go with insulation. Any experiences from others out there - as in Canada, Alaska etc?? Thanks |
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toddm
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| 07/26/2008 3:10 PM |
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OK, I looked it up. The ground loop heat pump boys have extensively studied heat transfers in soils. Here is a Virginia Tech site that has a mean soil temperature map and a chart of temperature amplitudes for various types of soils. http://www.geo4va.vt.edu/A1/A1.htm
Bottom line, temperatures in average soil six feet below grade swing about 20 degrees through the year, from 44 degrees to 64 degrees in parts of Iowa, for example. What we can say with certainty is that soil temperatures at depth are almost always more comfortable than air temperatures, and that basement slabs are the last place you should look to spend insulation dollars. Comfort is a different question. You probably wouldn't want to stretch out on an uninsulated, uncarpeted basement floor -- except in July and August.
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PanelCrafters
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| 07/26/2008 9:11 PM |
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Posted By toddm on 07/26/2008 3:10 PM
Bottom line, temperatures in average soil six feet below grade swing about 20 degrees through the year, from 44 degrees to 64 degrees in parts of Iowa, for example. What we can say with certainty is that soil temperatures at depth are almost always more comfortable than air temperatures, and that basement slabs are the last place you should look to spend insulation dollars.
Sorry, that is incorrect, unless your basement is not going to be used as livable space. That sounds great for cold storage, but if you are planning to use the space as a 'rec room', you will be wasting heat to keep it warm. Yup, in Iowa.
Please do a Heat Loss calculation, and you'll see that a copious amount of loss is indeed from the slab(unless you live far South:Texas, Florida, etc.).
Bottom Line: If the space is to be conditioned, and you live in an area with cold winters, the advantages of slab insulation exceed the costs.
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....jc
If you're not building with OSB SIPS(or ICF's), why are you building? |
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toddm
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| 07/27/2008 12:29 PM |
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Alas, I belong in the class comprising 98 percent of American householders who do not have unlimited construction budgets, and won't benefit from the reflexive advice of folks who have products to sell, namely that we should buy more of everything. It is instructive that the only payback analysis in this thread was done by a soon-to-be homeowner. I repeat my advice: If you have to make choices, order your insulation priorities starting at the top of the house and work down. If you can't afford to insulate under the basement floor --as opposed to the foundation perimeter -- it isn't the end of the world.
Better yet, build a frost-protected shallow foundation. Here is a link to a joint HUD/NAHB paper on it: http://www.toolbase.org/Home-Building-Topics/Affordable-Construction/frost-protected-shallow-foundations
As its name suggests, these foundations are not dug to the frostline but rather are protected by insulation. You'll find it in 2006 IRC. The Swedes have been building them for 40 years. Spend some of your savings on concrete on extra insulation and you have a house that costs less to build and to operate. Google the phrase and see how rarely it pops up in this forum. Where is the Whole Earth Catalog now that we really need it?
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wes
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| 07/27/2008 4:53 PM |
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If you can't afford to insulate the basement living area floor slab because you just don't have the money in the budget, then reprioritize your budget.
Leave off the granite countertops or the Viking range in favor of formica or a Hotpoint. These things you can change in a few years when you can afford to do so. The insulation cannot be added back under the slab later on. I am so tired of people basing a budget on 'pretty' instead of function. When are people going to learn that 'bling' doesn't keep you warm in the winter, cool in the summer, and dry when it rains.
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Wes Shelby
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SoCalScott
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| 07/28/2008 1:28 AM |
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Posted By aksmith42 on 07/12/2008 1:48 PM
i was wondering how important insulating a basement slab is and if you would definitly do this what type of insulation would you use? in my new home i'm going to have forced air heat. i've heard that leaving an uninsulated spot in the center of your slab uses the ground as a heat sink. thanks for any opinions.
I don't see anything here about not having enough money in the budget, just wants to know if he does it, will he ba wasting money.
Basically, if ground temperature is less than the ideal room temperature for your basement it makes sense to add an insulation barrier to reduce heat transfer through the slab into the ground. Heat will flow from hot to "cold" objects until an equilibrium temperature is reached.
Insulate the basement slab, it's worth it.
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toddm
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| 07/28/2008 9:53 AM |
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Like it or not, designer kitchens represent a perfectly rational economic decision. Granite countertops and the attendant bragging rights count for more at resale than insulation that can't be seen.
In fact, some of what I read here also seems to be about bragging rights, flannel chic if you will. A few very efficient houses get built, but polar bears still drown in the end. The professionals here would do better to throw their arms around designer kitchens. Show your clients how they can save money now, so they can upgrade to Jennaire from GE, as well as save money later. That can be done with frost protected shallow foundations.
Some buyers will insist on basements anyway as the proper way to build. So, SoCalScott, exactly what is floor insulation worth? If XPS is the answer, what's the optimal depth? What will it cost? What's the payback? That's how we know whether or not we are wasting money.
These questions aren't easily answered. The government's REScheck software ignores basement floors except in walkout situations. The only basement heat loss calculator I can find that addresses floors hasn't been upgraded from Windows 95. It's here if someone can make it work: http://www.sbc.nrcan.gc.ca/software_and_tools/basecalc_e.asp
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PanelCrafters
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| 07/28/2008 10:48 AM |
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Posted By toddm on 07/28/2008 9:53 AM
These questions aren't easily answered. The government's REScheck software ignores basement floors except in walkout situations. The only basement heat loss calculator I can find that addresses floors hasn't been upgraded from Windows 95. It's here if someone can make it work: http://www.sbc.nrcan.gc.ca/software_and_tools/basecalc_e.asp
You can always calculate it manually. Say you have a 24' x 40', 960 sqft basement. Your ground temperature is 60° and your design temperature is 70°. That gives you a Delta T of 10°. So, your Heat Loss per hour is: 1 / Insulation R-Value x Delta T x SqFt. So, if you have an R-1 you got: 1 / 1 x 10 x 960 = 9,600 BTU/Hr. For R-10 it's 1 / 10 x 10 x 960 = 960 BTU/Hr.
This calculation can also be used for other parts of a structure by using the proper material R-Value and the proper Delta T. |
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....jc
If you're not building with OSB SIPS(or ICF's), why are you building? |
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toddm
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| 07/28/2008 2:08 PM |
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Well, no. Your formula is missing a coefficient necessary to adjust heat transfer to the much lower rates through soils.
Think about it. If air and soil rates were the same, there'd be no frost line, you wouldn't find constant temperatures at about 30 feet, and those earth berm guys would be the dumbest folks on the planet. Calculating the coefficient for the particular soil under your house is very complicated. |
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SoCalScott
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| 07/29/2008 12:33 AM |
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The optimal depth? The calculations would be run at at the elevation of the slab based on design. Don't think anyone is going to build a basement 3' deeper or shallower to minimize heat loss through the ground (thats what the insulation is for). Of course there are calculations you can run....if anyone can agree on that formula, but the simple answer is that there is, in most cases, heat loss and insulation will reduce it. The general answer to the very general question we started with.
Oh yeah - with certainty 44 degrees to 64 degrees is more comfortable than air temperatures? what air temperatures are you talking about? I don't know anyone, except one of my ex-girlfriends, that keeps any living space in their home that cold.
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PanelCrafters
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| 07/29/2008 8:22 AM |
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Posted By toddm on 07/28/2008 2:08 PM
Well, no. Your formula is missing a coefficient necessary to adjust heat transfer to the much lower rates through soils.
Um, well yes. The formula is a std Heat Loss calculation formula. This is not a lesson in quantum physics, it's pretty basic stuff. And, this the calculation that is used(Manual J). And, we really don't care about Heat Transfer within the soil, it's the temperature of the soil against the exterior of our structure that matters. But since you seem to like to argue... |
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....jc
If you're not building with OSB SIPS(or ICF's), why are you building? |
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toddm
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| 07/29/2008 10:27 AM |
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Sorry. I meant to ask you for the optimal thickness of XPS, or the point at which spending more gets you less. Obviously, the basement is what it is.
The air I am referencing would be the cold air around your house in December, or the hot air in August, my point being that the differential between inside and outside temperatures, Panelcrafter's Delta T, will almost always be less under the slab. Add the fact that heat transfer rates are much lower in soils than in the selfsame surrounding air and this becomes the least important insulating decision.
I am afraid that the formulas have already been agreed upon. This text is surprisingly readable, except for the differential equations: http://www.amazon.com/Transport-Phenomena-2nd-Byron-Bird/dp/0471410772
Here are the Clif Notes. When conduction is the method of heat transfer, the ability of the medium to diffuse it is key. Soils have poor diffusivity. Dry, dense soils have very poor diffusivity. You add heat to the basement floor faster than the soil can move it away. Eventually the floor reaches a steady state temperature not far off the conditioned temperature (in the middle of the basement anyway, which is the argument for insulating the edges.) Yes, with radiant heat in the slab or passive solar, you can use the soil to store heat.
So we need a coefficient to determine what fraction of Panelcrafter's 9600 Btu we'll actually lose. We're still not done. We need to account for the ambient contribution of the furnace and water heater. We need to offset added heating demand by the energy saved during the cooling season.
Results will vary. You could find that the super wet soil under your house soaks up heat at alarming rates. I would answer that the fatal mistake in such places is digging a basement at all. |
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