How to Calculate Slab Heat Loss
I’ve been dancing around this issue, searching, yakking, etc, and I STILL don’t know how, exactly, to calculate heat loss out of a slab on grade. If anyone can disclose how the fancy software programs do this, I’d love to know.
It may be way too simplistic-thinking to break that out of a complex software program, and if so, then so be it. It is probably illegal/immoral, too. Thus, my question is, are any of the 3 ways I have calc’d said losses reasonably accurate?
How do folks that don’t have access to $800 software calc this? Here is what I have done; does any method sound better than the others?
(1) I used Siegenthaler’s equation, from his “Mod Hyd Heat” book. However, the book says “for R0-R11” at the slab edge. My R50 is therefore pushing the model, maybe to the point of being useless. This route gave me a 2.4 million annual btu loss.
(2) I used the standard heat loss equation with a delta T of 15 F. That 55 F was based on a few reports I’ve read here and elsewhere of under slab temps about 55 F in Finland and Canada (I think). 55 F sounds reasonable, based on what I’ve seen for soil temps here. Further, I assumed the 15 F delta T for 365 days, giving a healthy 5475 HDD. That method gives me a 15.8 million annual btu loss. Is it likely more accurate to use the 55 F for 270 days, which is more in line w/ a typical heating season? Too, this ignores edge losses.
(3) I used a sub-slab delta T of 10 F and a HDD of 3650, adding to this an edge-of-slab delta T of 116 and 14,000 HDD. This gave me an 11 million annual btu loss.
I am trying to figure out the optimum thickness of foam under my slab, so it would be nice to have an honest heat loss estimate. If that means paying someone to run my data through their fancy software, contact me and we’ll talk. I can be reached at three four seven – five four six nine, area code nine zero seven.
Thanks for any input. john
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John,
You have raised one of the $64,000 questions of superinsulated home design. (I assume you have read my blog on the topic, Can Foam Insulation Be Too Thick?)
The short answer to your question is, "Using Passive House Planning Package (PHPP) software." Understandably, you aren't interested in buying $800 software; how does $225 grab you? You can purchase PHPP from the Passive House Institute US; for more information, click here.
Building scientists are still arguing this question; it's a complicated one, because isotherms in the soil complicate the heat loss calculations. Suffice it to say that many North American building scientists recommend 4 to 6 inches of rigid foam under a slab, with comparable or slightly higher amounts of vertical insulation at slab perimeters; slabs that are deep underground need less insulation, while slabs with hydronic tubing might need more. (Gary Proskiw's calculations, reported in this month's Solplan Review, call for even less insulation, even in northern Canada.)
Passivhaus designers defend much higher insulation levels under slabs. This argument is unlikely to go away soon.
Martin: Thanks. I have been assuming that I was looking for an illusive dude w/ this one, as there just does not seem to be much info on it. I'm glad to hear from your end that there is a reason for the lack. $225 does not sound bad at all, esp since, I assume, it pretty well looks at the whole house. I will check into it. Upon looking at my calcs for the 30th time while grappling w/ this, I realized that although you don't have a huge delta T under the slab, its (potential) duration somewhat compensates for the small dT, and I had not previously respected the impact of that. The search continues. john
John Klingel,
I don't have any answers for you but I always thought these thermal cartoons were interesting.
Also Robert's Comments from the "Murky discussion" about heat storage.
The area below the house is not as cold as the "ground temperature" and the further you get from the edge..the warmer
Brooks: What? You don't have the silver bullet? And I was counting on you... Yes, I've read Robert's take on this before, and agree w/ that generalized look at things. It would still be nice to have more than a gut feel, but that may just be asking too much. I was planning on 6", but now think 8" is more likely, and I could not argue one second about 10". But, that is all "gut". I looked at the PH software Martin linked above, and will have to inquire about their assumptions, etc, before I plunk down the cash for it. (BTW: For you Mac users, Maclinks opened their free xl spread sheet easily.) Thanks for the reply. john
So I am waiting. What do you folks who calc slab heat losses DO? I'd like to hear the details of your take on this quagmire. Riversong? Et al?
Glad you are rethinking your ROI calculation in regards to your ground insulation…there are many essential factors flowing into figuring out good values and coordinating ROIs for the whole equation. The reality of 34 degree soil temperatures for 365 days is certainly one part of the puzzle.
IMO heat loss through the ground is widely misunderstood, neglected and underestimated in our current insulation requirements and building practices.
If you really want to get a better understanding about this topic I recommend you start by purchasing and studying the ISO 13370 reference book.
The ISO 13370 standard http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=40965 was established in 1998 and revised and updated in 2007-or the Passiv Haus publication #27 which lays out the scientific bases of heat loss to the ground and thermal bridge free foundation construction and means to properly calculate this particular heat loss. Delta-T isn't the only equation in this. The listed publications show very good isothermal graphs which illustrate this visually rather well. Then you should download the free THERM program and go through the exercise to calculate and model this yourself to come to the same conclusion. PHPP07 is a good means to calculate this particular heat loss and establish a good insulation ratio for the overall building performance…I have not found a better modeling software and at $200 it is a steal.
I have seen very high real world gains in energy efficiency in our homes by increasing the under slab insulation in accordance with the calculated heat loss thought the ground - and adjusting the Insulation ratios between the overall building envelope: wall insulation, roof insulation and under slab or floor insulation to a balanced level (crucial IMO!). Our standard slabs are R-50 (more than double the recommended R-value) - more if the client can afford to put more money into insulation. Forget about the flip-flops and do not neglect your ground insulation, you will not regret it.
As Martin stated this is a very hot and controversial topic amongst experts with very different arguments…one is that our soils are very different and that we cannot use the ISO formulas to calculate heat loss values. The SunRise home has many thermo couplers throughout the foundation, wing insulation and adjoining baseline reference points with sensors in 2 feet intervals down to 12 feet. Data loggers work away right now and I am very curious to compare my PHPP07 calculations to real world data. At least for me this will lay this particular question to rest…
Now – happy number crunching, you will have endless hours of entertainment with THERM guaranteed ;- ) TC
PS: John B. – since your ever curious about building science I really recommend you follow the exercise and see what you come up with. You might discover the fundamental difference between a heat storage and heat loss…murky stuff indeed.
Viel Danke, Herr Chlupp. Ich studieren werden. (Bitte, ich erinnnern mich nicht so viel von 1969.) When you get your data ready, we are waiting with bated breath. I am about convinced that 8" is a minimum, but going the full 12" like you do is rather tough on a ranch-style house. Thanks very much for the input. john
Thorsten, great comments, your input here is very welcome. You're way ahead of the curve figuring things out.
This paper gives some of the basic calculations for steady state heat loss:
http://www.thailand-energy.info/Article/Ts-3%20steady-state%20component%20of%20three-dimensional%20slab-on-grade%20foundation%20heat%20transfer.pdf
It's a basic diffusion calculation with boundary conditions at the soil surface and water table.
Thorsten Chlupp wrote:
I'd be interested to know how you have "seen" these gains in performance. Did you build identical homes but with varying levels of slab insulation and then closely monitor them for a year? Maybe you built 50 homes with 2 inches of slab insulation and another 50 with 6 inches and another 50 with 10 inches and then compared their utility bills? I'm just curious how you have seen the gains in performance.
I'm also curious about any data to support your claim that foundation heat loss is underestimated. I would guess that it's over-estimated, but that's just conjecture based on comparing modeled and measured whole house energy use for many relatively inefficient homes (with no insulation anywhere in the basement -- slab, walls or ceiling) and finding that whole house heating energy use is typically dramatically over-estimated.
Michael: What are you using to model the heat loss? Maybe that program is one possible source of discrepancy??? In what climate(s) are these homes measured? Thanks. john
Good point, bad choice of words…of course you can’t “see” anything like this. I have built over a hundred homes what I back then considered very efficient homes until I started to wonder if the way we were building made much sense when I started to look really into the nuts and bolts of building science and physics within buildings. Since then I have chipped away slowly step by step … one big puzzle piece for me was the way we build and insulated foundations. Over the years I added more insulation, changed designs and as I always do look back at what worked…actual utility expenses, fuel consumption, temp and moisture probing, thermal imaging. And changed accordingly to my findings…and model since a few years with PPHP07 my insulation ratios (and many other things). Since I am comparing apples to oranges and don’t look at 10 identical homes build with more or less insulation in the foundation – with the same homeowners consumptions and life style, in identical soils condition, with identical ground water levels, same rain fall and so forth environmental conditions this is not a scientific acceptable way of quantifying this. Believing in something doesn’t cut it and we need to keep things straight with scientifically validated data….so when I wrote that I “see” big increase in efficiency it is meaningless and not a point of argument.
After many years of tangling with this question for me the only good measure is to data log actual loss with strategically placed sensors and running them against an also recorded baseline temperature which is logged in the ground – and then comparing the dataset to the modeled and predicted heat loses through the ground. This can be used to validate and fine tune the energy model for a specific area which has similar characteristics…it will not be a universal validation. Unfortunately there are just too many variables flowing into this equation. This question will need to be resolved in different regions and climate zones individually to find good and realistic answers. John is building in my hometown and I tried to give him advice for our climate. I don’t have a universal answer to this million dollar question – however from my experience in my climate I encourage to look into this closer in other regions.
Seeing how well the homes we build today perform is to me encouraging that I am on the right track. I heated my house solely by passive solar gain for the last three weeks (in Fairbanks, Alaska) and we are nice and toasty. A ZeroEnergy home in 14000 HDD needs to function extremely well to be able to offer comfort at 50 below with no fossil fuel based heat source.
Could someone provide a link for the THERM software?
Thanks :)
And said John is leaning as far as he can toward Thorsten's advice, as he thinks TC has a very logical and geographically appropriate foundation design. As I mentioned earlier, I am having to tweak things a bit, both for the upfront money and DIY considerations. The freaking ground is COLD, and it will neither give you any solar heat gain (as per a window) nor much seasonal help, I feel. Nuff said. Bring out the numbers (to which I am planning to add).
THERM: http://windows.lbl.gov/software/therm/therm.html
Thanks, John :)
Woolfe: Let me know what you think of THERM. I am about to download it myself. Shoot me an e at jolinak at gci dot net if you'd like. thanks. john
Thorsten (or anyone else) can you advise on what temperature data loggers you have used, and strategy for placement, what data tracking software etc.? I am going to be pouring a slab in our basement in a a few months here in Seattle, and think it would be fun to track the temp. profile over time under the slab.
Thanks,
Dave
Dave: I spoke w/ some folks at the Cold Climate Research Center here, and they said to get thermocouples of the appropriate temp range, stick them in, and then check their resistance whenever I want the "temp". There is a simple formula for converting resistance to temp. I don't have any fancy equipment, but I do have a VOA meter. Hope that helps. j
My error. They are thermistors, not thermocouples. j
Follow the exercise?
Thorsten, Are you talking about following the "exercise" with THERM or PHPP?
John K, are you planning to post your "exercise" input & results?
are you purchasing PHPP?
John Klingel wrote:
I wasn't referring to any one particular model or even climate -- the over-estimation problem has been found using many different modeling tools and in many different climates (Midwest, Northeast, Midatlantic Northwest) although I'm not aware of any such studies in extreme climates like yours. Some of the models have included Rem/Rate, HES (DOE-2 based), the NEAT audit, models based on sunrel, etc. I haven't seen anyone model inefficient homes in PHPP -- I'd be curious if it does any differently. There is currently a working group being convened by NREL that includes many software model developers that is examining this problem and exploring approaches to improving models or their inputs so that they better match empirical data..
Thorsten-
Thanks for your clarification. If you want to build a very low energy home in a very extreme climate like yours, then you will obviously need a lot of insulation pretty much everywhere you can manage to put it. Even then, it's not clear to me that this is a very good use of resources. For homes in less severe climates, I think it's even more questionable. Detailed temperature measurements may be helpful for assessing models of subslab heat loss, but even that approach doesn't necessarily get us directly to how it affects whole house energy use -- especially in less efficient homes where the basement temperature will float. I think whole house energy use data on lots of homes with varying levels of insulation may be needed to get at those impact better.
Thanks John for thermistor info. Found some at omega.com.
Dave
Dave: I am going to check w/ CCHRC about how they seal theirs so that they don't corrode. Maybe Thorsten can enlighten us???? Contact me a jolinak at gci dot net in a few weeks and I should know something. If you learn anything, zap it to me. thanks. john
John B. - both of courses, with the mio questions you always have you absolutely have to have both programs on your PC and tinker with them ;-)
Temp readings - we use thermistors from OMEGA...if you are only reading out with an ohm meter then you could also use RTDs (1000K) I guess. Unfortunately proper data logging is rather involved and expensive, but some strategically placed DYI sensors would work well I am sure. I recommend placing and using 3/4" PVC electrical conduct from HD to build senor wells in which you place your actual sensors. Epoxy the bottom tight, drill a hole with a mag drill and drill extension in the foam, ground or were you like to place the senor (somewhere on a interior partition or so forth)...pour your foundation and get weathered in and then place you sensor string and seal the top...this way it is accessible, replicable and serviceable. Make sure you use high quality shrink tube on your solder connection on the thermistor strings...
Michael - now you really lost me.
You are saying that even after data logged validation of heat loss through the ground you're not agreeing with results? And you truly believe that superinsulation is not a good usage of resources? Should we rather keep on turning up the heat to stay warm and support the oil companies?
You seriously believe that ISO 13370 is bogus and not based on very elaborate research and validation and that Europe is basing their building requirements for 13 years on unfounded bases and waste resources in requiring higher insulation ratios then really need according to your findings? Almost 20 years of research and 24,000 buildings later the Passiv Haus standard also doesn't get it? Have you used PHPP07 at all - or at least looked into the bases of what makes up this modeling software? Several long term and rather elaborate independent studies in Germany, Austria and Swiss conclude that the PHPP models are within 1% of its predicted usage when compared to actual consumption...Which is something I can confirm on my projects. That is very different then results I have seen from other modeling software I tried before. I honest to god don't believe that we have to reinvent the wheel...and that there are very good tools available to make good and sound decision about where to place how much insulation. If modeled right the only real unknown factors are variations in yearly climate sets and of course the 800 pound gorilla aka Joe Homeowner.
There is absolutely nothing questionable for me about this. We need to fine tune models for various very different climate regions and conditions we have - but by no means do I agree that the model is broken to start off with. Let's not be ignorant...TC
TC: Thanks for the input on the thermistors. That sounds like what Robbin told me a year or two ago; I should have written it down.
Michael: WTH?????
Thorsten,
It's quite possible for both you and Michael Blasnik to be correct.
It's possible that PHPP accurately models heat loss through slabs. But remember -- PHPP provides absolutely no cost-effectiveness feedback.
Just because PHPP accurately tells you that your heat loss through a slab with 14 inches of EPS under the slab is very low, doesn't mean that using 14 inches of foam is a good use of the earth's resources. If the last 7 inches of foam manage to save a very small amount of heat annually, it's perfectly reasonable to argue that the foam should be used elsewhere -- on a different house.
Did anyone else read the linked pdf regarding modeling of slab losses? I wish someone with higher math than me would comment, I can read the graphs but beyond that......I cannot tell if they modeled with no perimeter insulation, it kind of seems like they did. I am a little cloudy today and cannot convert the metric U values they list into something meaningful to me, or if I am correct they seem rather low.
It seems like there is math to describe thermal mass effects, which really seems to be the difference between underslab and wall or roof, as in those other cases we assume that the energy passing through the wall has no effect on Delta T, while it quite obviously does in underslab.
As for predicting within 1 percent , if it was for a 100kbtu heatloss, that is a big enough number to feel confident about, but say, 2000? With virtually unmeasurable daily variations in solar insolation, hell, whether the owner shuts the door before wiping their boots is a factor.
Then there is a concept I cannot find a name for, but basically, there is no 'less than zero'. If your heatloss through a surface is so low, you cannot effectively tell if you have exceeded the practical limit of insulation of that surface. Relating to 'this' topic, I find it likely that the difference between say 8 inches and 12 inches of EPS underslab could still have measured results within one percent, if only because that particular heatloss is less than other much larger variables. Additionally with the code required 4 foot frostwall here, if massively insulated, it makes me wonder if insulating underslab more than modestly has any point at all, given thermal mass effects. IOW after several seasons, does the temperature of that 'x' thousand cubic ft mass change practically at all in such a situation?
At the point at which the r value underslab approaches that of the roof, is an 'earth sheltered' house baloney?
Martin,
I know from your past writings that you don’t believe that the PHPP heatloss via the ground measures are justifiable in a economic sense. The economics of superinsulation to Passiv Haus level is plenty of argument and since we are all working with predictions on energy costs which we in reality all have no clue about it is in the end a totally subjective matter and at the end of the personal choice on how bleak we see our future in regards to energy costs. To me it makes absolutely sense and pays for itself from day one to build to such an efficiency level. And once you see how efficient we really can build everything else just doesn’t makes sense anymore – at least my experience. When I read through LEED, GREEn build it made sense to me and I dove in from the beginning and applied it in our building methods. Why would I use anything else then dual flush toilets or VOC free paint? I looked at Passiv Haus, studied it extensively and travel to Europe every spring and try to listen and learn. And it just makes sense to me. I took on renewable energy and designed our own energy stem – because it just makes sense to me. And if I look at some of the latest projects and see how they perform and function it is just mind boggling to see how efficient homes truly can be – and that even in our very extreme climate we can build homes which sustain itself. I have not burned a fire in 3 weeks and maintain 70F purely by passive heat gain…and have unlimited hot water from my solar. It makes sense to me…and once I have all the R&D done I will be able to build these ZeroEnergy homes very, very feasible. I really believe that the problem is not that this all make no economic sense but rather that we are so stuck in our ways and just have a hard time to step outside the box.
Anyways, point is that we can argue about if it makes sense or not all day long and it is a moot point since we don’t know what the near future will bring. That however is really not the crux of the discussion here – and I have this argument quite often. Our building industry is using a different ratio of insulation levels for foundation/walls/roofs then what many European countries prescribe – with Passiv Haus a key example. And this is were the argument comes in…and we just can’t exclude heatloss via the ground and say on one hand that PHPP makes sense and calculates accurate models and then turn around and say that the foundation insulation however is way to much and has no point of return as prescribed. That really isn’t how it works…and to me one of the most important features on an energy effect design is the equilibrium of the insulation ratios. You simple are not going to stay warm if you put on a fur hat, down parka and flip flops…no matter how many hats you put on. And PHPP comes up with a different balance then what we have been used to. But even there I think the debate has gotten side tracked with some rather extreme foundation details with 14-18 foam placed. You really don’t need that much if you really work with PHPP to figure the IMO key component – the windows. They stand your energy model on its head…and one can really tweak performance considerably…if one isn’t stuck with a north facing site. PHPP gives us an accurate tool to find the best balance and always will be only as good as the designer who is using it. The program goes to the extremes of figuring in your average heatloss through flushing toilets…the amount of thought and work put into its modeling is pretty amazing. I certainly don’t believe that Passiv Haus is the holy grail of a building standard for us – we have so many other issues in our industry that are completely omitted. It is after all only a strictly performance driven standard – but we can learn a lot of very good lessons from it.
As I naturally never believe anything before I see it with my own eyes I have gone through length to validate any new idea or concept – and I am really looking forward to see good datasets on our heatloss though the ground to compare it to my PHPP model. And I am sure that there will be some fine tuning because of our very cold soils temperatures. But I am sure it will not be way of – and not have a no point of return insulation level.
My point is, we can’t have it both ways Martin. We can argue endless-which we seem to do all the time anyways, and we can like or not like the idea. But you be very hard pressed to make a real case against the building science behind it.
Ok now, enough of my ranting – we all have better things to do, I for sure do. Michael my apologies for letting my frustration out, I am sure you have very good reason for your believes through your work in your area. Somehow it just seems very different from my own little experience I have up here in the Far North. And this topic has become a sore spot over the years… TC
I did read through that paper before posting, and it confirms the basic idea of soil as insulation with a very low R-value per foot (1 is the figure I've seen mentioned).
Since the r-value is so low, a lot of soil is needed to get R-10 or R-20. However if the soil can be "boxed in" around the edges it should provide a lot of r-value. By "boxed in" I mean that every thermal path should meet the target R-value; for instance if the target is R-20 it should be impossible to draw a path from the slab to the outdoors or deep earth without encountering 20' of (dry-ish) soil, or R-20 of foam, or X feet of soil and (20-X) R's in foam.
Heating this soil initially (to get it to a steady temperature gradient) takes some energy, although I don't think it's prohibitive. For instance I estimate a 20' depth of soil under a 1000 sf slab takes about 400,000 btu per degree F to get up to operating temperature.
This is all, of course, dependent on an accurate idea of the soil R-value per foot.
John, yes Robin is the expert on this and will get you the best info.
Good luck on your project, I gotten myself into enough discussions now for a while.
Happy Building - TC
Thorsten,
I'm afraid you misunderstood me.
You wrote, "We just can’t exclude heat loss via the ground and say on one hand that PHPP makes sense and calculates accurate models and then turn around and say that the foundation insulation however is way too much and has no point of return as prescribed."
1. I am not saying we should exclude heat loss via the ground.
2. I am conceding that it is quite possible, even probable, that PHPP correctly models heat loss to the ground.
3. I am suggesting, however, that even if we all believe both of these propositions, that we must remember that PHPP provides no cost-effectiveness feedback. It just gives the designer a certain goal -- 15 kWh per square meter per year -- and tells the designer that this goal MUST be met in all climates. The software provides no red flags or warning bells when a designer decides to spend $5,000 on subslab foam to save $50 per year.
Of course, some designers, builders, and homeowners may choose to spend $5,000 that way. But I'm just saying that the PHPP software doesn't provide cost-effectiveness feedback. It's up to us to use our heads and decide whether the measures make sense or not.
Thorsten says
Please provide the references for this -- especially for single family homes. The one study everyone seems to cite -- CEPHEUS -- only included two single family homes and the modeling errors for those were both more than 50%!
I say it isn't actually possible to model the energy use of a building with 1% accuracy except by chance for any given building. There are simply much larger sources of uncertainty than that -- no matter how good the models. Just the usable waste heat from internal gains creates much more than 1% uncertainty in a home's heating use, as does uncertainty in solar gain through the windows (how dirty does the glass or screen get? what sort of interior shading is used and on what schedule), etc. For less efficient homes the uncertainties in exterior air film R values, true infiltration rates and their net energy loss, thermal regain from buffer zones, and even the true effective R value of an uninsulated wall or attic are all sources of much larger errors and biases.
I don't know where you got the idea that I don't think superinsulation is a good idea. I just believe that the world has limited resources and we should try to use them as best we can. There is an opportunity cost to every resource decision. It seems that some PH advocates think that reaching certain arbitrary goals is worth any cost and people spend inordinate amounts of time and effort over modeled energy use differences that are smaller than the impact of the occupants' choice of TV technology . I think there are highly diminishing returns for detailed modeling of any given home and that detailed modeling isn't worth the effort unless it's a research project.
Also, back to ground heat loss, I said that even if we knew the effective ground heat loss, we wouldn't know the impact on heating energy use in less efficient homes without measuring it somehow. In "normal" efficiency homes, the basement temperature will float at some level based on heat losses and gains and waste heat from appliances and distribution systems, etc. If you change the heat loss to the ground, then you will change that equilibrium temperature which then affects all other types of heat loss and even the efficiency of the distribution system. it's not a straight one-to-one correspondence at all.
Options options and options.... this is one worthy thread.
To me a useful combined direction for some to take from this is to build close to the specs of an existing PH plan. No modeling or certification. Yes to blower door. Yes to used foams.
Another idea never mentioned is to build separately insulated zones into standard (not so) super insulated homes. The biggest change would be adding high R interior doors. High R interior walls is easy. This idea of making a super insulated extreme weather home within a home to me is the absolute least expensive solution to any major rise in energy costs especially for existing housing stock. Doing this saves the cost of getting the foundation to PH levels of R.
Great thread...
I looked at THERM, and it is impressive for detail. I read a lot of the WINDOW manual, which is 366 pages. However, I just don't have the time (weeks?) it would take me to learn it enough to use it intelligently. I wish I had another winter to learn. I ran AKWarm, and its results were similar to what I have already done. According to a gent at CCHRC, it is "pretty accurate". I am awaiting a response as to what the assumptions and algorithm are for the SOG heat loss calcs in AKWarm. The short of the long is, I am going to have to use what info I have (unless I can find someone who wants to run exotic software for pay), and look at the ROI of incremental layers of sub-slab insulation. Regardless of the fact that one can't isolate the losses and hug them, they are there, albeit somewhat obscured and nebulous. My head, at this point, is aligned w/ Thorsten on this; insulate heavily. I don't know if that will be 8" or 10", but it won't be less than 8. Thanks to everyone for the input; I'm glad I asked. j
John,
I just left you a voicemail. If your building is close to Passive House standards, PHPP can give you the answers you are looking for. If you'd like, I can model it for you. My email is [email protected]
Cost optimization is an important part of designing a building. We learned about how to use PHPP as part of the process in Passive House training and there are questions about it on the Passive House certification exam.
I'm bringing this back from the dead to see if there have been any updates, as it still seems difficult to find any info about calculating heat loss from a slab on grade, to figure out how much insulation I want/need
Trevor,
To read my advice on this issue, see "Determining Sub-Slab Rigid Foam Thickness."
There are too many differences in soil types & moisture saturation levels to model it accurately in a generic sort of way. The cost of doing a sufficiently accurate conditions analysis is more than just throwing a bit more R at it.
As for how much you actually "...want/need...", as a starting point I'm partial to Table 2 on p.10 of the now classic BA-1005 analysis done a decade ago at BSC, whether slab-on-grade or basement:
https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
At those levels you won't be insanely overspending, and it will make a comfort difference (particularly in cooler climates), even if local soil and energy market conditions are such that it never fully "pays off" on reduced energy use.
Up in permafrost country they're calling out R20 which "seems about right", in merely cold zone 6 it's half that, and in mixed heating/cooling HDD zone 3 it's halved again, and in cooling dominated climates the thermal mass and temperate temperatures of the subsoil makes insulating the field of the slab (as opposed to slab edge) pretty worthless on either a comfort or financial basis. In any heating dominated climate the first R5 is the most important from a barefoot comfort point of view and "worth it" even if the heating energy were free.
Thanks for the recommendations.
FWIW I'm in upstate NY, cz6, -6° F design temp, and want to be sure I use enough, but not waste $$ on excessive foam with no benefit in comfort or energy use. Will be building this summer unless plans fall apart, doing a "pretty good" house, I'm looking at fine tuning systems, insulation levels, etc and was surprised that manual J really only looks at slab edge.
The one thing I can say about sub slab insulation is the comfort difference between none and a minimum R-10 is remarkable. Had identical townhomes to compare, one without and one with R-10. The unit with the uninsulated slab, the lower level was cool in the winter, the other had even temperature on both levels.
The depth of the slab ranged from 4' to 8' below grade, a slab on grade would need much more insulation.