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Passive Solar House design for Cloudy Climates?  RSS feed

 
Matthew Connors
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Location: Acworth, New Hampshire
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I have been planning an addition (timber frame with strawbale or slipstraw infill) to my tiny home with passive solar at the forefront. I was talking with my neighbor about it and he wondered if a passive solar design is the best for our region (SW New Hampshire). That got me thinking. I can only think of maybe 5 or 6 days since mid November where the sun shone fully. Most of the winter is mostly cloudy. If I design for solar gain, I may lose in the long run by having too much glass! My wife and I aren't crazy about a trombe wall.

What do you think? Are there other design elements to take advantage of the little sun we get for a net gain heating solution? Are there regions that just are not suited for passive solar design?

Thanks!
 
Will Meginley
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I came across a series of videos on Youtube a while back that you might find helpful. The series was called Passive Solar Simplified. Here's a link to video 1 (there are 6 total):

https://www.youtube.com/watch?v=Prx6rJPZFIE&list=PLHXG5QjHwatIM89bsMOSEcNNkZD2eGTas&index=1

Some strategies he talks about that you may find useful include thermosiphons and sun spaces (discussed near the end of video 3.) A lot of the strategies discussed in videos 4 and 5 also look like they would be very helpful to incorporate even if you decide not to go the passive solar route. I thought all of the videos were very helpful and apparently he's also written a book on the subject. Haven't read that, though... yet.
 
Troy Rhodes
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Trombe walls are generally awful in most climates, the exception being climates where the average daily temperature is 65f-75f. Ideal for SW desert areas that get relatively cold at night, and relatively hot during the day.

In all other climates, you produce a nice hot thermal mass, and trap it right next to a cold window all night and every cold cloudy day, absolutely insuring it will lose heat as fast as possible to the great outdoors.


The performance of a trombe wall can be drastically improved if you insulate the glass at night and on cloudy days, thus preventing that mass from bleeding to death against a cold window.


I had pretty good luck by building a house in Ontario canada which is not overly sunny, with very high levels of insulation (think r-50 in the walls, and r-70 in the ceiling, and the basement walls r-30 and the basement floor r-20, 4" of high density styrofoam, ie insulated freaking everywhere.

The modest square footage of normal windows could then provide a significant % of the heat load, and the incidental heat from appliances and people also made up a significant % of the heat load. Then the propane could make up the modest difference on long cold cloudy stretches.


The Superinsulated Home Book by Nissan and Dutt have all the data and plans and drawings you would ever need, along with pretty straightforward math to get your house to perform well.

It' out of print, but pretty available and not stupidly expensive for a very professional yet accessible book.

troy
 
Cj Sloane
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It's true the sun is often absent during the winter, but the coldest days are also the sunniest. The sun is out today but right now it's 14°. I'm pretty toasty in my passive solar home. You need mass to store that excess heat you get while the sun is out.
 
Terry Ruth
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Check out this thermal mass homes specs.....http://www.sirewall.com/portfolio/residential-projects/otter-limits/

"With no heat for 4.5 days in freezing weather, all that was necessary to be comfortable was to put on a sweater".....

It reacts slow to store heat or cold and buffer it. It works in any climate zone if you know what you are doing. So yes to OP question it is worth it in cold climate zones as proven.
 
Troy Rhodes
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The only thing to keep in mind is that thermal mass doesn't make heat.

Thermal mass only helps you if you have a surplus of heat (from fire, or sun) to start with.

If you don't get a surplus of sun, enough to overheat your space, more mass will just get cold, stay cold, and keep you cold longer.

That's one of the real advantages of rocket mass heaters, the bench/mass soaks up excess heat and continues to release it long after the fire goes out.


finest regards,

troy
 
Cj Sloane
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Troy Rhodes wrote:Thermal mass only helps you if you have a surplus of heat (from fire, or sun) to start with.


My passive solar house is R-38 on 6 sides. The designer told us there would be some days that will be a little chilly, but not really cold enough to make a fire. He suggested having a party or baking some pie and I have to say, that does work.

You do need back up in cloudy climates. Our only source of heat besides body heat & using the oven is our wood stove. We burn a little over 2 cords for 2400 sq ft and I do like it to be around 70°.

ps - the passive solar part really shines during early fall & spring or very cold sunny winter days (not nights) when other homes have to heat & we don't.
 
Troy Rhodes
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High levels of insulation and appropriate thermal mass are the dynamic duo!

High levels of insulation can cause rapid overheating if you don't have good mass to soak it up.

High levels of thermal mass with poor insulation just makes a house that is difficult or impossible to get up to comfortably warm levels.

Together, they are unstoppable.

finest regards,

troy
 
Terry Ruth
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"If you don't get a surplus of sun, enough to overheat your space, more mass will just get cold, stay cold, and keep you cold longer".

Right mass does not generate heat without an input source, and correct if you are in location where there is no sun for weeks at a time mass may release to the point of becoming equal to the indoor air temperature, not colder. Even cloudy days have UV rays. If the air temps continue to drop inside it will have a slow effect since mass gets coupled with other mass not air. That is why mass homes experience little cold or warm spots. So if you load a floor by passive solar or solar thermal tubes, the floor loads the walls by convection or conduction, not radiant heat from the air in the room. Hope I remember that right been a while, it is complex actually, very difficult to calculate and model. ORNL(Oak Ridge National Lab) has a thermal mass calculator for the USA you can download to get an idea of the bang for the buck. It based on 10 monitored builds and around 1000 WUFI models. I played around with it. It shows what you can expect in utility bills at ten different USA climate zones for a mass home vs well insulated (little mass, stick and batting)...If I remember right Florida and Chicago somewhere in the cold north showed the best returns. I'm close to CO it show good there too.

There are also "smart controllers" now with lag time that control solar thermal.

Most home owners I followed say it takes about a week to load a full mass house and about a week to unload. Some leave the AC on when they go on vacation they say it cost too much to reload. I did some test and got what ORNL did, the heat pretty much stays within a 4-6 inch depth for a Rammed earth never bridging 12" with a delta t of 50F. At a min I'd put at least a mass wall in facing the sun, nothing fancy at least 6" thick. I have also heard of people over heating so you have to be careful with too many windows usually is the issue or size per square area it too high, SHGE (solar heat gain coefficient) or e-coating is not correct. Windows play a large roll in solar passive-mass homes.
 
Troy Rhodes
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Terry, good, accurate thoughtful post. I would add one little expansion on your comment:

"correct if you are in location where there is no sun for weeks at a time mass may release to the point of becoming equal to the indoor air temperature, not colder." That's true IF the mass is isolated from the ground. There are many designs that attempt to use the ground itself as thermal mass. If done poorly, -that- mass will get colder and colder as winter progresses and you end up in a house that's difficult and expensive to heat.

And yes, modeling can only take you so far because there are so many variables that affect the way a mass performs. Like how much moisture is in the soil surrounding a structure.


Finest regards,


troy


 
Terry Ruth
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Troy, I gave you a +1 agree moisture and ground complicate it. I been trying to design a foundation lately and those two drive me nuts! They say earth has a great ability to regulate moisture in addition to hot/cold, and many home owners have reported less allergies from mold and mildew in wet climates. So earth clay's (some more than others) are super special that way. If you can find surface sealers or renders that keep it permeable (breathable) and make it durable at the same time. Keeping the clay content up can be a challenge too, need some aggregate for it to hold on to. Whoever said it was easy?

 
Cj Sloane
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My house (that's R38 on 6 sides) has no basement, just a 2" concrete pad that has a vapor barrier and insulation, then another 10" of concrete on top of that with duct work running thru it. So it is insulated from the ground. The duct work is for using a fan to draw hot air from the top of the house down thru a filter and then thru the concrete which acts as a heat sink, then back up to the house.
 
Matthew Connors
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Location: Acworth, New Hampshire
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I appreciate the debate and conversation. it is good to hear everyone's point of view.

I still have the question though......will there be a net gain or loss of heat? If I design for passive solar, I have windows that will lose heat when the sun is not shining, right? And the sun seems to be absent during the coldest season of all!

Thanks!
 
Cj Sloane
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There are some rules of thumb. Only a certain percentage of the south facing wall should be glazing. For sure you need to cover up those windows at night. I have honeycomb shades and they do a great job but must be a tight fit - like a 1/4" gap on each side.


 
Terry Ruth
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This attached graph shows the superiority of water to air as mass, around 3500 times better at storing and transferring solar energy, not to undermine the need to deal with stack effect.

As far as decoupling the ground temperatures from walls that cause thermal bridging, here some good reads on this complicated science. It can be accomplished with long lasting aerated all natural concretes, rocks, without the use of toxic foams and plastic that can each inside the building.

http://nesea.org/conversation/masters-blog/heat-loss-ground-part-1

http://www.byfy.lth.se/fileadmin/byfy/files/TVBH-1000pdf/CEH-1004PartA.pdf

http://www.buildingphysics.com/manuals/slab.pdf

Depending on local cost a solar thermal (water) roof collector tied to a hydronic radiant mass emitter floor or an air-air heat pump couple to a PV roof rack is well worth the cost. The envelope of the house has to be efficient too. I personal value comfort and do not like air blowing at me nor hot/cold spots and am willing to pay a little more for it. HR's are more DIY friendly.

The other point I forgot to mention is mass due to the lag time when it peaks can shift heating/cooling load to "off peak" hours if you are grid tied to a utility company that has surcharges. They average lag is 6 hours can be as high as 24.
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Troy Rhodes
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You can do some estimating with the following tool:

http://www.susdesign.com/windowheatgain/


They account for all the major factors and you can get an idea of what your heat gain per square foot of window will be.


Then you can calculate the heat loss through the windows. Their formula assume you never insulate the window, hmmmm......

https://www.e-education.psu.edu/egee102/node/2025


Then you can plug some numbers into a spreadsheet and see if a window on the south side of the house is a winner or a loser. Without movable insulation, it's likely a loser. Windows on the north side are 100% losers under every scenario.

South facing windows are the best case scenario. East and West fall somewhere in between.


That book I mentioned would walk you through all of those calculations as well. The Superinsulated Home Book, by Nissan and Dutt.


Once you get the basic idea for how to calculate how much you gain in a day from best sun, and how much you lose in a day through an uninsulated window, you can play with the math and see what happens when you add movable insulation. That can be low tech like one or two layers of bubble wrap, held up with a few spritzes of water, or a close fitting piece of high density styrofoam, 1", or 2", faced with a nice fabric and put up and down as needed.


Do a few sample problems and then ask questions if you have them.


I can't recommend that book highly enough.



troy
 
Terry Ruth
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"If I design for passive solar, I have windows that will lose heat when the sun is not shining, right? And the sun seems to be absent during the coldest season of all! "

You will have a net loss through windows but that is not the entire story, the annual net loss or gain will depend on the envelope (windows, wall, foundation, roof) efficiency and utility cost. These days the trend is to put alot of $ into the entire envelope or PV instead of a HVAC system especially a low efficient one (80-90%) . If you are doing new design you have a chance to get the envelope right, if not putting money into PV may yield a better return than trying to R&R the envelope, windows included.

Most look at solar design gains and losses on an annual basis not seasonal. PV net metering programs allow customers to gain credits at the utility companies, while some get off grid and battery store to save up for a cold winter or hot summer.

In the USA, DOE and RESNET, others have some calculators that input the entire envelope, you can change r-values, windows, etc.......it gets more complicated with mass.

This program calculates how many KWH will be generated per year for the different US locations. Take that look at your local installation cost for it or HR, envelope, to determine a total net gain or loss for the year.

http://rredc.nrel.gov/solar/calculators/pvwatts/version1/
 
Troy Rhodes
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If you install enough insulation, with good details, you can heat it with cats and a few light bulbs. -slight exaggeration-

Do cats count as solar They are pretty passive.


That's the whole idea of Superinsulation.



hth

troy
 
Terry Ruth
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HA! Have you guys heard there is "new generation" of strawbale homes being generated by the cold cloudy northwest called "Gen 3" ..IRC code min is what walls r-30, roof 49, slab 10? These guys are infilling with strawbale r 30 ish, then doing a framed outer wrap 2 x 4 stud blown in cellulose for a total R-50 wall, roof in the r-60 ish. With slip-clay "Gen 2" less than r-2/inch you will need one thick wall just to get to code min. Air tight and done right, triple pane windows. $$$

I don't see the need to mix methods, all blown in or bales. They say they are trying to incorporate "conventional" methods with old Gen 1 to create this new generation. Sounds like promoting cellulose to me. I'd look at a strawcrete cast around timbers or frames, 12-24 inch thick before I did all that, 3 trades, frame, cast, render, have some mass effect call it good, go active solar with some PV to cut utility bills.

Its always a hard call where to put money in an addition, especially if the rest of the house is poorly insulated.

These guys are behind the NW New Gen 3: http://newframeworks.com/

Not enough mass for my tornado zone. They been doing bales homes (some off grid) for a long time in that region, give them a call.
 
Troy Rhodes
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I just did straightforward double wall construction. Surprisingly inexpensive. The finished wall is 12" thick plus. All blown full of cellulose-twice. The second time a year later. It WILL settle unless blown to very high densities.


Heats like a champ.


4" of high density styro under the slab. Excellent vapor barrier.


That's the 30' x 70' shop behind my house.

And I'm right in the middle of remodling my old farmhouse with similar double wall construction.


I live in very cloudy all winter long michigan.



troy
 
Terry Ruth
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Troy, I'm thinking double stud may be the way to get er done and not get to crazy with what trades don't know like BIC and are hard to find. 12-18" with a limecrete, light aerated concrete cellulose mix in the walls and roof, rockcrete(perlite, vermiculite, pumice, scoria) foundation with good drainage feet and hats. Here, singles, I used hemp. The nice thing about double stud easy to attach formwork to, and cladding or drywall, cabenits, pictures, etc, or stucco and plaster. The inside a lime wash would show the natural wall, so that makes it 2.5 trades.

Around r- 2.5-3 / inch with mass effect...good for an addition, just rent a couple of mortar mixes and get some local lime mortor, wet round bales work no need for dry bales. Lime also breaths and regulates moisture, not as well as clay, but good nuff, also absorbs CO2....you add a clay render your good to go.
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