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R value balance for slab, footers, foundation, walls, roof.

Last Post 20 Nov 2012 09:34 PM by kb. 42 Replies.

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engineer

Veteran Member

Posts:2749

11 Nov 2012 10:07 AM
At some point consider stirring into the mix the possibilities, costs and ROI of generating some energy onsite via PV vs the incremental cost and diminishing benefit of ever higher R-Value. Investments in r value improvements with paybacks out past 20 years may do better diverted to PV (or more efficient utilization within the home - higher efficiency heating / cooling / kitchen/ laundry / lighting / media) The 10 / 20 / 40 / 60 solution sounds quite a bit like the "pretty good house" concept bandied about at Green Building Advisor. That site may be worth a look if you haven't already. PGH is a swipe at Passive House, intended to provide most of the benefit at a fraction of the cost and aggravation.

Curt Kinder <br><br> The truth is incontrovertible. Malice may attack it, ignorance may deride it, but in the end, there it is - Winston Churchill <br><br><a href="http://www.greenersolutionsair.com">www.greenersolutionsair.com</a>

Lee Dodge

Advanced Member

Posts:714

11 Nov 2012 01:19 PM
Posted By engineer on 11 Nov 2012 10:07 AM At some point consider stirring into the mix the possibilities, costs and ROI of generating some energy onsite via PV vs the incremental cost and diminishing benefit of ever higher R-Value. I agree. Note that the payback for solar PV for my application was 7 years, which was shorter than the payback for improvements in insulation that were similar to, or even lower than, the 10/20/40/60 or PGH guidelines (Details at net-zero payback.) Only switching to high solar gain window coatings had a better payback, but that is geographic location specific. ...snip... PGH is a swipe at Passive House, intended to provide most of the benefit at a fraction of the cost and aggravation. Yes, rather than superinsulation to Passive House levels, the energy and financial tradeoff can be much better for solar PV (and sometimes solar hot water), a high efficiency furnace, high efficiency appliances (esp. refrigerator), etc. in many locations.

Lee Dodge, <a href="http://www.ResidentialEnergyLaboratory.com">Residential Energy Laboratory,</a> in a net-zero source energy modified production house

New Member

Posts:87

11 Nov 2012 03:35 PM
Rather than solar, what about wind? I am actually a few miles from a "wind farm" area with dozens of giant wind turbines. Has anyone actually had experience success with wind energy on a small homeowner size. There are plenty for sale, Honeywell Wind Turbine, manufactured by WindTronics, is one. I know I should be posting this in a different forum, but I haven't seen much there on wind energy.

Lee Dodge

Advanced Member

Posts:714

11 Nov 2012 06:59 PM
The economics of large-scale wind are generally more favorable than large-scale solar. However, that reverses for small-scales, where solar is more favorable. The solar insolation in Manistee, MI is 4.17 kWh/m^2/day annual average, which is decent, and is predicted by pVWatts 2.0 (http://gisatnrel.nrel.gov/PVWatts_Viewer/index.html) to provide about 4500 kWh per year for a 4kW DC rated system. Installed cost for 4 kW is about $5.25/W DC, or about $21000, minus 30% federal rebate that takes it to $14,700, minus state and/or utility rebates that you would have to check into. It is much harder to find average wind speeds for a given location, and predict performance for a wind generator. Large scale wind generators can take advantage of the nocturnal jet, but these are up at 100 meters or so and above, and cannot be reached by small scale generators.

Lee Dodge, <a href="http://www.ResidentialEnergyLaboratory.com">Residential Energy Laboratory,</a> in a net-zero source energy modified production house

engineer

Veteran Member

Posts:2749

11 Nov 2012 07:20 PM
I agree, if for no other reason that the height of the tower needed makes "small wind" a tough row to hoe for a typically homeowner. The complete lack of moving parts and infinite scalability makes PV particularly attractive

Curt Kinder <br><br> The truth is incontrovertible. Malice may attack it, ignorance may deride it, but in the end, there it is - Winston Churchill <br><br><a href="http://www.greenersolutionsair.com">www.greenersolutionsair.com</a>

Dana1

Senior Member

Posts:6991

12 Nov 2012 11:46 AM
Posted By kb on 10 Nov 2012 12:24 PM Thanks Lee, that makes sense. I wasn't aware that the 10/20/40/60 was based on cost/labor. If it is, then the cheapest labor install would be the EPS in the subslab area - just lay it down! I first used SPIS over 20 years ago after I saw an article in FineHomebuilding and I made my own SIPS. I found the ease of istallation and reduced labor to be quite significant as well as less thermal bridging due to lack of framing. It has stood the test of time so far even though SIPS technology has come a long ways since then. Your comments have also led me to realize that walls and ceiling should be rough and not smooth to minimize surface boundary losses by lowering the velocity of the air currents near the surface. I am not planning on putting shag carpet on the walls but a rough plaster will be more optimal than a shiny gloss painted surface. So I will continue to research the origin of the 10/20/40/60 mark and balance of heat fluxes. Thanks for your input. At just about any legal R-value the boundary losses from wind or force-air heating turbulence become a distant third-order effect (lower than the radiant heating & cooling of the exterior surfaces of the house.) The interior & exterior air-films have combined R-values only ~R1 even under idealized conditions. Code-min stick built walls in MI are R13+5c.i. which works out to about R15 after thermal-bridging. The difference in heat transfer rates of the turbulent vs. laminar flow at the interior & exterior surfaces at worst-case conditions vs. average conditions is less than 3%, and roughing up the surfaces unlikely to regain more than half that, and will usually increase the radiation heating/cooling of the surface offsetting any gains. Under wind loading conditions the infiltration drives on air leakage rates are farmore significant than the loss of the exterior air films, making air-sealing a far more important factor to get right. Contrary to marketing fluff there is still significant thermal bridging in SIPs albeit much less than with timber-framed assemblies. An important place to focus on minimizing thermal bridging in a SIP house is at bottom-plate/subfloor/band-joist and at the top plates of wall assemblies, and at corners. A primary advantage to SIP (and ICF) construction is ease of air-sealing, but unless you take pains on air sealing efforts that advantage is easily wasted. The exterior foam approach to stick built construction can be cheaper than SIP at equivalent whole-wall R (with all thermal bridging factored in), and can be made cheaper if reclaimed roofing foam is used (at 1/4-1/3 the material cost of virgin stock.) It's pretty easy to hit up to ~R40 whole-wall with this approach with 2x4 framing and exterior iso (even in a deep-energy-retrofit). Foam-over stick built is easily designed with thermal breaks on all framing and foundation, eg: aligning the exterior planes of ICF foundation foam and above grade foam thermally breaks the band joist & foundation sill and where the top plates encounter "energy-heel" trusses, etc., and running the sub-slab foam right up to the interior foam of the ICF foundation thermally breaks it from the footing. Getting the same thermal breaks between foundation and bottom plate of SIP walls can be more awkward- it's doable, but I personally haven't spent a lot of time working those details. When the rest of the wall is R40 it doesn't take a lot of thermal bridging to cut into performance even with the low thermal bridging of SIPs. It's hard to beat the speed at which high-R SIPs can be put up compared to high-R foam-overs, and there are fewer air-sealing details to attend to. In a "time is money" situation it can sometimes be a money-saver, but in most markets & situation it's a more expensive way to build.

New Member

Posts:87

12 Nov 2012 01:29 PM
Thanks Dana, you are providing valuable information to me. Regarding the wind situation. The house will be located at the top of a 100+ ft bluff, so the tower situation is null. Also the bluff could help chanell the air movement into wind generators increasing its viability over the local average. I do agree with the moving parts detail that engineer has pointed out. A Honeywell WT6500 wind turbine with 2752 kWh annual output costs about $6400. So on the surface this looks better than solar, but reality is seldom the same as claimed/predicted results. Also the house woulld be in a wooded area, so to take full advantage of the solar input I would need to cut down more trees. )-: On detailing, weather or not I use SIPS, I have decided to investigate/incorporate the "gasketing" now being used more often, even in between SIPS rather than expanding cans of foam or even caulk. EDPM gaskets now being made can seal better immediately as well as when the inevitable cracking/shifting/setteling occurs down the road. A friend who installs commercial room size coolers has told me that just a tiny pinhole in one of his constructions causes a major frost bulge. He uses gaskting to join most sufaces but when the electrician doesn't seal his work, if their are any penetrations added, it becomes very obvious.

Dana1

Senior Member

Posts:6991

12 Nov 2012 02:22 PM
Rooftop turbines rarely meet expectations, even at the top of a lakefront bluff. Peak output numbers and marketing representations are meaningless without a site-survey. The wind-shadowing of trees and even the shape & pitch of your roof can affect performance. Analyzing the output expections of PV is much easier and much more reliable. The marketing hype for the WT6500 claim ~2000kwh/year in class-III winds (= 11.5 to 12.5mph average wind speed), but the manufacturer's wind estimator yields barely more than half that.( (See: http://www.windknowledge.com/ ) Bump that to 20mph average windspeed (which is QUITE breezy) and it calculates ~3000kwh, which is roughly the annual output of a ~2.5 kilowatt PV array in a MI climate. The price of the turbine may be $6-6.5K, but that's f.o.b. the distributor's warehouse, not mounted on your roof. Figure on a total installed cost of $10K, maybe even more. The installed cost of PV recently crossed the $4/watt line (on it's way down) before subsidies in my neighborhood, but it has a ways to go before it hits the NJ average of $3.50/watt, or Germany's sub-$2.50/watt installed rates for 2-5kw arrays. But even at $5/watt, where PV might be at parity with the WT6500 (assuming your wind resouce is anywhere NEAR that good), the maintenance & reliability issues with PV are favorable compared to turbines. Bottom line- when building new construction set up your roof pitches with future PV in mind. Even if it doesn't make economic sense this week, it very likely will before Y2025 (or even 2020). The turbine may in fact NEVER pay off. The cost of doing a real wind survey over time isn't cheap, but it's small relative to the cost of installing an expensive roof-ornament that doesn't really deliver. (And that's what most rooftop micro-turbines are.)

New Member

Posts:87

12 Nov 2012 05:56 PM
Yes Dana, wait and see is a good plan. If I design for future possible additions in this area I will be best off ... which is why I am still in the "Design and Planning" forum! Thanks for your input, it is worth every penny I paid ... and then some. ;-) I probably have a genetic predisposition to the possibility of wind energy as I have a Dutch ancestory and my predecessors were using windmills way before my time. The exterior foam approach to stick built construction can be cheaper than SIP at equivalent whole-wall R (with all thermal bridging factored in), and can be made cheaper if reclaimed roofing foam is used (at 1/4-1/3 the material cost of virgin stock.) It's pretty easy to hit up to ~R40 whole-wall with this approach with 2x4 framing and exterior iso (even in a deep-energy-retrofit). So what about a hybrid? Like for instance, thinner(cheaper) SIPS instead of 2x4 for the ease of building and then exterior foam over that?

engineer

Veteran Member

Posts:2749

12 Nov 2012 08:35 PM
The most recent thing I read about small wind in HomePower Mag they were pretty adamant that the bottom of the rotor arc be 30 feet above anything else within 500 feet. That's a tough design requirement.

Curt Kinder <br><br> The truth is incontrovertible. Malice may attack it, ignorance may deride it, but in the end, there it is - Winston Churchill <br><br><a href="http://www.greenersolutionsair.com">www.greenersolutionsair.com</a>

Dana1

Senior Member

Posts:6991

13 Nov 2012 11:20 AM
Posted By kb on 12 Nov 2012 05:56 PM Yes Dana, wait and see is a good plan. If I design for future possible additions in this area I will be best off ... which is why I am still in the "Design and Planning" forum! Thanks for your input, it is worth every penny I paid ... and then some. ;-) I probably have a genetic predisposition to the possibility of wind energy as I have a Dutch ancestory and my predecessors were using windmills way before my time. The exterior foam approach to stick built construction can be cheaper than SIP at equivalent whole-wall R (with all thermal bridging factored in), and can be made cheaper if reclaimed roofing foam is used (at 1/4-1/3 the material cost of virgin stock.) It's pretty easy to hit up to ~R40 whole-wall with this approach with 2x4 framing and exterior iso (even in a deep-energy-retrofit). So what about a hybrid? Like for instance, thinner(cheaper) SIPS instead of 2x4 for the ease of building and then exterior foam over that? Are you sure about that? When I invited a Dutch teenager to come along to check out an experimental ground mounted vertical axis windmill in Gaasperplas I was told: "Kijk maar- ik ben nederlands. Ik ben DOOD ZIEK van de godverdomme windmolens!" ("Look, I'm Dutch. I'm sick to death of the goddamn windmills!" ) Even though Holland is pancake-flat and downwind of the North Sea the economics of PV is still better than rooftop windmills. But in the windy coastal areas there is sometimes a financial case for 10-20kw tower mounted versions. SFAIK roof-ornament type windmills aren't really flying off the shelves there, and rarely even offered by the more serious micro-wind vendors in NL. Utility-scale windparks as thick as commercial tulip farms sprouting 2-10megawatt turbines on gia-normous towers are becoming commonplace in the windiest parts of the Netherlands, but piepklein daktop windmolen sightings are rare, yet rooftop PV is found in every nearly neighborhood, evidence that the Dutch can still do math. (With a few exceptions to the contrary.) Tiny turbines on the roof are usually a political statement more than an economic, practical source of electricity. But with even modest subsidy and net-metering PV makes economic sense in higher-priced US electricity markets. There are issues with trying to over-foam a SIP as a hybrid. In order to support the siding on the moment arm of the fasteners on furring through screwed through 4-5" of exterior foam the timber screws need to bite into 1.5" of structural wood, at a maximum fastener spacing of 24" o.c. . The skins of SIPs aren't that thick, and not designed for that type of mechancal loading. Even if that problem were soluble, trapping the exterior SIP skin between layers of permeance material is another consideration that would have to be addressed by design. You might be able to get there with high-perm rigid rock wool, but it's about 3-4x the density of polyiso, and would need to be 50% thicker to achieve a similar R, leading to longer screws with greater moment arm, closer fastener spacing (= more thermal bridging with the screws). I'm just not seeing it as a viable option. A 2x4 timber-frame at 16" o.c. spacing with minimal bump-outs, borrowing OVE type techniques for corner, window/door placement & framing to minimize framing fraction, with cellulose or open cell foam cavity fill and 4.5-5" of exterior iso hits R40 (nearly) every time, and there's plenty of structure for the furring screws to bite into. Every builder in the US knows how to build a stick frame, and learning foam-over techniques doesn't take a rocket science degree. FWIW: If it's windy enough to support a financial argument for micro-wind you might seriously consider going ICF rather than SIP. A SIP is a large tympanic surface and you'd hear 20mph wind pretty loudly through a SIP wall of any thickness, whereas ICF is nearly stone-silent at sub gale-strength winds (if you build a low-noise roof, anyway.

New Member

Posts:87

14 Nov 2012 07:07 PM
There are issues with trying to over-foam a SIP as a hybrid I am aware of the controversy of "outsulating" SIPS. There is more opinion than data in that area. http://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/how-make-sip-roof-better bottom of the rotor arc be 30 feet above anything else within 500 feet Curt Kinder, check out the google images of the Honeywell WT6500 wind turbine http://www.popsci.com/files/imagecache/article_image_large/articles/PSC0911_WN_121.jpg

Alton

Veteran Member

Posts:2164

14 Nov 2012 07:18 PM
I have seen the Honeywell wind turbine at trade shows. I read later that it may not produce as much electricity as advertised. Anyone have experience with this turbine? Many sales yet?

Residential Designer & Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period . 334 826-3979

engineer

Veteran Member

Posts:2749

14 Nov 2012 10:32 PM
kb: I checked the link; very pretty, but I'm not buying it: 1) Too small a swept area for meaningful / useful power generation...pretty pictures can't trump physics and math 2) Too close, vertically and horizontally, to obstructions, precluding smooth high velocity wind. Look at presently operating commercial wind farm characteristics: 1) Just two or three slender blades 2) huge swept areas via long blade lengths 3) high towers 4) flat or gently rolling ground, minimizing turbulence

Curt Kinder <br><br> The truth is incontrovertible. Malice may attack it, ignorance may deride it, but in the end, there it is - Winston Churchill <br><br><a href="http://www.greenersolutionsair.com">www.greenersolutionsair.com</a>

Dana1

Senior Member

Posts:6991

15 Nov 2012 11:12 AM
Posted By kb on 14 Nov 2012 07:07 PM There are issues with trying to over-foam a SIP as a hybrid I am aware of the controversy of "outsulating" SIPS. There is more opinion than data in that area. http://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/how-make-sip-roof-better bottom of the rotor arc be 30 feet above anything else within 500 feet Curt Kinder, check out the google images of the Honeywell WT6500 wind turbine http://www.popsci.com/files/imagecache/article_image_large/articles/PSC0911_WN_121.jpg There is a HUGE difference in the mechanical loading between outsulating tilted roof and a vertical wall, at least at any normal roof pitch. It doesn't take an engineering study or hard math to determine that what works for a roof assembly doesn't always work for a wall: In the roof case the outsulation/nailer decks/roofing/etc are all driving the OSB skin of the SIP toward the EPS core, whereas in the wall case the moment arm of the fasteners are working to peel the OSB away from the core.

New Member

Posts:87

15 Nov 2012 04:44 PM
In the roof case the outsulation/nailer decks/roofing/etc are all driving the OSB skin of the SIP toward the EPS core, whereas in the wall case the moment arm of the fasteners are working to peel the OSB away from the core. Yes but it would also depend on how you do it. If the outer EPS "outsulate" on a SIP wall went all the way to the footing the load would be carried there and not on the fasteners. Also there si a considerable amount of friction between the surfaces resisting any movement, even with the slippery side out on the osb, a 4x8 sheet adds up to 32 sqft of frictional area. Point loads are dispersed over large areas of the surface; the stress then at any given point is very small. The fasteners are "spring loaded" as they are driven into the foam. For the SIPS, discussions have revolved around "cantilevering"" the SIPS out over the edge of the sill. To me, this would not make sense as the outer OSB layer has no support making the SIP loose its structural integrity, creating the stresses you are talking about.

New Member

Posts:87

15 Nov 2012 04:49 PM
Kurt and Alton, I'm with you, waiting for some solid data to prove the usefulness of a small wind generator. The honeywell has been out for a couple of years ... so where is the user data? is everyone too embarassed to say they spent their money but have nothing to show for it?

Dana1

Senior Member

Posts:6991

16 Nov 2012 11:07 AM
Posted By kb on 15 Nov 2012 04:44 PM In the roof case the outsulation/nailer decks/roofing/etc are all driving the OSB skin of the SIP toward the EPS core, whereas in the wall case the moment arm of the fasteners are working to peel the OSB away from the core. Yes but it would also depend on how you do it. If the outer EPS "outsulate" on a SIP wall went all the way to the footing the load would be carried there and not on the fasteners. Also there si a considerable amount of friction between the surfaces resisting any movement, even with the slippery side out on the osb, a 4x8 sheet adds up to 32 sqft of frictional area. Point loads are dispersed over large areas of the surface; the stress then at any given point is very small. The fasteners are "spring loaded" as they are driven into the foam. For the SIPS, discussions have revolved around "cantilevering"" the SIPS out over the edge of the sill. To me, this would not make sense as the outer OSB layer has no support making the SIP loose its structural integrity, creating the stresses you are talking about. Unless you did the mechanical engineering design to guarantee that the weight of the siding was carried by the footing rather than the SIP you'll have issues. While there is a modest effect, without known & tested adhesive bonding it you can't really count the exterior foam being structural on friction alone. In more than half the installations of exterior foam the housewrap resides between the foam and the sheathing (for an "innie" window installation.) While it's arguably possible to do what you're suggesting, it's not a well tested method with standard code specs for fastener spacings & depth, and you'd probably need to spend some money engineering it to get it approved. The cantilevered SIP situation may also deserve an engineering treatment at times, but rather than small attachment point loads the OSB is effectively glued over the entire mating surfaces with the EPS with HUGE shear strength, as well as bonded to the bottom sill and other structural timber elements within the SIP. When the bottom plate of the SIP cantilevered most may need additional structure such as a separate sill plate to guarantee edge support of the exterior OSB skin, which is usually required ANYWAY to keep the OSB from wicking moisture from the foundation. In that case it becomes an issue of the ratings of the sill plate timber. ( I'm not sure how well that topic well covered in the manufacturer's specs- never looked at it in detail. ) A SIP resting on the subfloor atop floor joists & band joist is a configuration capable of fairly deep cantilevers (VERY deep relative to the thickness of the exerior EPS on an ICF) without having to do much in the way of engineering it.

New Member

Posts:87

16 Nov 2012 09:10 PM
OK so I have hacked up the pdf you linked to and I added outsulate that cantilevers off the floor joist and adds an actual ventilation layer which must cantilever out, but the given drawing had an unrealistic thickness for the vent space, so I made it more realistic. (yeah, I know I will be looking for an engineer to verify my thesis). I have also added outsulate to the foundation and in the process covered the thermal bridge created by the floor joist. modified SIP fondation detail

New Member

Posts:87

18 Nov 2012 06:20 PM
I think my goal was to make the outer layer of osb on the SIP warmer. "warm wood tends to be dry and cold wood tends to be wet" From this discussion on GBA also there is frequent use of foam over corner bracing weather it be plywood or osb and another recommendation about keeping walls dry So, does foam outsulate over SIP keep it dry or does it trap the water vapor inside the osb layer?