Hybrid Earth Sheltered/Passive Solar Design
Last Post 24 May 2018 06:28 PM by sailawayrb. 17 Replies.
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thoner7User is Offline
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21 Oct 2016 11:19 PM
I am thinking of building a home as a second/vacation/snowbird home. This home will be in either the Austin Tx area or Nashville TN area. I am hoping to achieve little to no heating or cooling load. Here is my line of thinking, please tell me if this is plausible, or if I am right/wrong/what I am missing. The idea would be to have a rectangular house, say 25 feet wide and 60-80 feet long. I altered and attached a passive solar illustration to show what I mean. (I may or may not use the clearstory roof design, any roof design input is appreciated.) The ground water temps in Austin and Nashville are about 70 and 60 degrees respectively. This is a good indicator of average soil temps. By having a basement and an earth sheltered north wall, I could theoretically get my basement slab to 16-18 feet deep on that north side. I am thinking this will provide a very constant temperature. I believe the basement is key, as that air could be circulated to the upper floor for a more constant temperature. Austin I believe would need no heating but what about AC? Nashville may need more heat than passive solar could provide but, I think no AC would be needed, and any supplemental heat could be provided with solar hot water. I plan to use ICF construction for the strength and thermal mass. Is this a worthwhile plan?? Am I missing something? I believe that, on the basement north wall, I should have very little if any insulation (on either side of the wall) so that the constant temps of the earth can easily migrate into the space. Potentially removing the exterior foam after the ICF pour or, even pouring a bare concrete wall and going ICFs on top of that. As the wall gets closer to grade, the insulation should increase, particularly on the outside. Is this theory correct? Quad Lock has a nice system where you can step up to 4" of exterior foam insulation (from 2"0 and stay 2" on the interior throughout. Are all my ideas correct? Would love to hear some expert advice on the engineering behind this stuff

Attachment: passive-solar-diagram.jpg

Dana1User is Offline
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23 Oct 2016 02:55 PM
With that much glass you'll have a significant mid-day cooling load even in winter.

Moving air to distribute heat between floors with air handlers is grossly inefficient unless the temperature difference is high (say, 100F upstairs, 60F downstairs). At a 10F difference it takes a LOT of cfm to bring it down to a 5F difference.

A better approach would be to build high-R, but with the normal amount of glass/ to floor ratio (say, 12-15% of floor area= window area), but distribute it carefully. To lower peak cooling loads, minimize or eliminate west facing glass, and design seasonal overhangs to severely limit mid-day sun on the south side windows at the equinoxes for the Nashville house or even March 1, for the Austin house.

The cooling & heating degee-days are pretty diffferent between Nashville & Austiin so different levels of house performance are called for. Nashville is in US climate zone 4A, whereas Austin is zone 2A. (see: http://www.greenbuildingadvisor.com/sites/default/files/images/DOE%20climate%20zone%20map.preview.jpg )

Also, in Austin local code also requires that you build to "Net Zero Energy Ready" for all new housing. If you build to at least these "whole assembly R" numbers (which include all thermal bridging of framing & structural elements) you would be able to hit Net Zero Energy with a PV array that fits on the house:

Location.....Wall ........Vented.Attic....Compact.Roof.......Basement.Wall.....Exposed.floor....Slab..edge.... Windows.(U/SHGC).....Subslab

Austin..........15............. 50.................... 40 ......................10 .........................20 .................5 .....................0.35/<.25............... none

Nashville.......25............. 60 ............. ......45 ................. ....15......................... 30 .................7.5.................. 0.30/<.35............... 7.5

See: https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf

Note, an R15 wall isn't R15 batts in a 2x4 wall- the thermal bridging timbers bring that down to about R10-R11 "whole assembly", but a minimalist 2" + 2" EPS ICF makes it. Similarly, an R25 framed wall would be a 2x6/R20 with 2" of continuous polyisocyanurate foam sheathing, but a 3" + 3" ICF or a 2.5" + 3.5" assymetric ICF (fat side on the exterior) makes it.

There is no advantage to earth-coupling the house to the subsoil soil in Nashville, which has a deep subsoil temp below comfortble conditioned space temperatures, but it would be fine to do that in Austin.

http://www.earthrivergeo.com/img/geothermal-article/geothermal-subterrainean-temperature-contour-map.jpg

But if building an ICF foundation, stripping the wall foam below grade does very little for earth coupling, and creates a significant thermal bridge from conditioned space into the above grade portion. Stripping the exterior foam below grade creates a thermal bridge from the near-grade soil into the middle of the above grade wall, reducing it's performance. In short, even in Austin, if you build with ICF, leaving the foam will better. With no sub-slab foam you have plenty of earth coupling.

A simple gable roof is easier & cheaper to build, and the overhang on the clerestory indicated would have to be gia-normous to have the desired summertime shading effect. Sufficiently high-R insulated cathedral ceilings are also quite a bit more expensive to build than a ventilated gable roof.

Solar is cheap in Texas, more expensive in Tennessee, but is dropping in price dramatically everywhere. Net metering rules are all over the place and shifting too, but I'd be surprised if going fully Net Zero isn't code-effective on a 20 year net-present-value basis in Austin right now, today. In Nashville it will be before 2020, unless the state utility regulations actively discourage rooftop solar.







thoner7User is Offline
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24 Oct 2016 06:01 PM
Thank you for the response. I'm sorry if my drawing was a middle misleading - I don't intend to have an abnormally high amount of window area. Whatever is normal and would let in the right amount of light. I am glad you mentioned of the engineering behind the ration of window to floor area as something to consider.

"There is no advantage to earth-coupling the house to the subsoil soil in Nashville, which has a deep subsoil temp below comfortable conditioned space temperatures, but it would be fine to do that in Austin. "

Why do you say this? My thinking is that the soil stays approximately 60 degrees year round. So during the 90 degree summer, and 40 degree winter, I have all that soil working to either heat or cool the house. I would then only have to heat or cool the difference right? and 90 degree outdoor temps with 60 degree below grade temps could average 75 degrees, which would be perfect.


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24 Oct 2016 06:41 PM
Note that while soil acts as a seasonal thermal buffer and as insulation (not so much as a heating source or cooling source for the remainder of the building), you can achieve equivalent energy performance with a fully above ground building. Probably with less cost and hassles and more livable space. If you want storage or workshop space - consider a larger garage.

> ...60 degree below grade temps..

But it starts rising as soon as you start adding heat. Soon it's 75F mixed with 90F.


thoner7User is Offline
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25 Oct 2016 06:14 PM
Posted By jonr on 24 Oct 2016 06:41 PM
Note that while soil acts as a seasonal thermal buffer and as insulation (not much as a heating source or cooling source for the remainder of the building), you can achieve equivalent energy performance with a fully above ground building. Probably with less cost and hassles and more livable space.


How do you do that? More insulation and solar panels or something?


jonrUser is Offline
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26 Oct 2016 03:26 PM
Insulation (mostly), solar panels, good air sealing, thermal mass, more efficient heating/cooling sources (even geothermal). A lot will depend on costs and the value you (or future buyers) put on above ground space with windows vs below ground space. I don't have numbers, but building underground for energy reasons seems to be less popular. As Dana says, in the right climate, slab on grade is enough.


Dana1User is Offline
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26 Oct 2016 05:36 PM
jonr is giving you good advice.

The thermal benefits of earth coupling to 60F dirt is zero to negative in (a temperate but still heating dominated) climate such as Nashville. But coupling to ~72F-ish dirt is worth a little something in Austin. But even in Austin it's not worth paying extra for by digging into a hill. Slab on grade is fine, with the slab only modestly insulated only at the slab edge down to the footing of the stem wall (or to the bottom of the grade beam) for earth coupling.

Insulation of the rest of the structure much beyond what I outlined above isn't cost effective against rooftop PV and high efficiency heat pumps sized properly for the loads. Please read the document I linked to, or a least the first chapter. Again, that's:

https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf

When that was written in 2009 PV cost about $7/watt to install, was about 15% efficiency, and better class ductless heat pumps only had an HSPF of 10, and an SEER of 16.

Today the national average cost of PV is half that (it's even cheaper in Austin), the efficiency is about 20% (less roof real estate needed), and better class ductless runs HSPF of 12+ (20% more heat per kwh), and the SEERs are north of 20, all at about the same or slightly lower cost than it was in 2009. This means that the whole-assembly performance levels outlined in Table 2 are even more conservative now than then, which means that last R3 may no longer be "worth it" compared to PV + heat pump on a lifecycle basis.

And again, in Austin you are now REQUIRED by code to build to Net Zero Ready performance, whether you install the PV or not. In Nashville it still makes financial sense to take it that far, but taking it even higher R becomes a philosophical argument. (The PassivHaus and PassiveHouse USA crowds would tell you it's not enough, but the arguments & data they use don't always make sense in the face of the rapidly transitioning and improving power grid, which will be much more benign going forward than in the recent past.)



thoner7User is Offline
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27 Oct 2016 05:24 PM
thanks for your help.


thoner7User is Offline
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04 Mar 2017 10:24 AM
I know I asked this question a while ago but I had another related one.....

Now when comparing water to air say, water will sap the heat right out of you much more than air because of the heat transfer rates. a 65 degree day is nice but a 65 degree pool will freeze you out. (Maybe there is a scientific term for this I am unaware of?)

So, what about air compared to the dirt?? Is that why it may be better to have a wall exposed to say 30 degree air rather than 60 degree dirt during the winter????


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06 Mar 2017 06:47 PM
Basements are rare unto unknown in Austin. Texas is famous for expansive clays. In a soil test for a previous house in the Dallas area, the clay moved 6 inches from bone dry to saturated. You can perhaps get it built but you should ask first if the city allows alternative energy analyses to substitute for prescriptive code lest you be required to ignore earth sheltering anyway. If you build outside the Austin municipal utility grid, take humidity into account. The Austin area is more humid than you imagine. https://weatherspark.com/averages/29672/Austin-Texas-United-States


thoner7User is Offline
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06 Mar 2017 06:55 PM
Yea this would be outside of the city limits


shashaUser is Offline
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18 May 2018 02:19 AM
A house that size would cost a LOT to have a full basement, mostly because circles and squares are the most efficient design. Even though oblong houses are perfect for solar gain, they are not perfect for footprint costs per sq ft. Basements, in particular are notoriously expensive, and that shape might not be worthwhile. Perhaps in your climate you could install earthtubes. Have you heard of them? This book I know about explains in detail how to build them for natural cooling, even in humid climates.

Earthtubes for cooling

https://www.amazon.com/DIY-Conditioning-Conditioner-Non-Electric-Sustainable/dp/1523260106/ref=sr_1_7?ie=UTF8&qid=1526609665&sr=8-7&keywords=sharon+buydens


Dana1User is Offline
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18 May 2018 07:45 PM
You realize that this thread is over a year old, right?

Earth tubes are problematic in locations with outdoor summertime dew points as high as they are in Austin TX or Nashville TN. The amount of design time/energy and expense it takes to do earth tubes RIGHT in a humid climate are better spent just lowering the loads and putting the money into solar & appropriately sized higher efficiency mechanical systems.


newbostonconstUser is Online
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22 May 2018 11:05 AM
I agree with Dana that moving the heat around in your house with air isn't a great idea but water might work better. Just looked it up and water holds 4.23 times the heat of air. You might be able do heated/cooled floors with pex and move the heat around that way.

We chilled our floors last summer with our Geo unit and it worked great. We have a both a geo furnace and a geo water to water. We didn't have a issue with hitting the dew point I am guessing because the furnace took the moisture out at the same time. We ran them together.

This year I am going to have the well water for watering the grass go through the floors first and see if we can cool the whole house just off that and not have to run the geo unit. We are in Michigan and the ground water is 50 degrees.


"Never argue with an idiot. They will only bring you down to their level and beat you with experience." George Carlins
Dana1User is Offline
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22 May 2018 08:52 PM
This thread is still seen more than a year since the original poster chimed in... (just sayin'...)


shashaUser is Offline
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22 May 2018 10:05 PM
The other way of cooling a house naturally is through earth berming, or by including plenty of thermal mass inside the home. "Plenty", however, is relative to the amount of square feet of glazing you have as there is a proper glass-to-mass ratio.

There's some good information on passive solar house design for natural cooling here at this site: www.sunstar-solutions.com

Maybe the original poster is gone, but this might help someone down the road.


Dana1User is Offline
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24 May 2018 06:12 PM
There's a pretty good caveat about how even reasonbly designed earth tube systems can fail even in a not so humid climate here:

http://www.greenbuildingadvisor.com/blogs/dept/musings/belgian-passivhaus-rendered-uninhabitable-bad-indoor-air?page=1

The magnitude of energy savings with earth-tube cooling is pretty small. In most cases the money is better spent elsewhere.


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24 May 2018 06:28 PM
Right, earth tubes were once an interesting concept but it has long since been proven that they are prone to cause health issues and not worth the risk. Just build a well-insulated/sealed envelope, use solar/thermal mass properly and perhaps HRV and you will be way better off.


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