SierraSkier
New Member
Posts:15
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| 23 Nov 2010 03:15 PM |
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There was very little I could find about solar slabs without hydronics when I was designing my house, so here is my experience. I just completed construction of a 1900 square foot two story house in South Lake Tahoe. The house has the long orientation running pure east /west. The south exposure is as much window as I could get and still have the required sheer without going to extra expense with engineering. The solar slab is 400 square feet, 15 feet wide and 27 feet long. The slab was poured into 2 inches of foam board, and is 6 inches deep. The real drag was finding High Solar Heat Gain Coefficient windows. The government got involved with glass, now most glass has a SHGC of .3. I wanted .7 at least. I got that in an inexpensive vinyl window, its old school low E glass, I guess the old school stuff allowed heat in but wouldn't let heat out and would overheat houses. Perfect for my application. The house is 2x6 construction with blown in glass insulation and EIFS siding. EIFS or synthetic stucco has 1 inch of foam and nicely seals the home. My house is heated with a Quadrafire 5700. I also have an HVAC with HRV to move air around and get some fresh air in the house.
The slab works perfectly! But, I dont think I would be as enthusiastic without the wood stove. When the sun is out the concrete gets very warm. The kids run around in bare feet. We have had 4 snowy days and little sun, so the area a long way from the stove is cool. So the stove adds a lot to the slab. We did get a couple of hours of sun the other day and then the whole slab warmed up. I have to be very careful with the wood stove or I can overheat the house.
Here is what I did not do. I did not put hydronics or expensive heat in the house. I did not spend money on fancy windows, just vinyl. I did not spend money on SIPS or ICFs. I did not spend money on fancy insulation.
Heres what I did do. I had my house sealed, every opening was foamed or caulked. I put extra insulation in the attic. I paid for raised heal trusses so I could get full insulation even to the edges of the house. I used EIFS for the 1 inch of foam and I broke the thermal bridge of the 2x6. My house is very very tight, and I had to have an HRV for fresh air.
My wife and I are amazed at how well the house is performing. It is nice and warm and our slab is warm. I have not used my HVAC except to move air around. The temperature is very even between the first and second floor. I guess that is part of the slab, but my first floor stays nice with the second floor not overheated.
Clearly a solar slab works! You do not need hydronics if you have the right glass and put in a wood burning stove. |
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Eric Anderson
Basic Member
Posts:441
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| 24 Nov 2010 04:04 PM |
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Nice Job.
got to love free heat from the sun.
I hear you on the wood burning stove. You can make a well insulated house go from cold to sauna pretty darn quick!
eric |
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| Think Energy CT, LLC Comprehensive Home Performance Energy Auditing |
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ANGELofDEBT
New Member
Posts:60
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| 25 Nov 2010 08:44 AM |
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How did you meet the design heat load for the house? The problem I'm having with my design is that to meet code I have to put in heat for the worst case scenario. But due to the solar heating I'll only use a fraction of it. |
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SierraSkier
New Member
Posts:15
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| 25 Nov 2010 11:52 PM |
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I could not get around that. So my furnace most likely will short cycle. I dont really know because we havent run our furnace but for the first day and we were just warming up the house. I can only get pine easily and my Quadrafire 5700 easily keeps my house warm through the night. The low last night was -5F. We did ask our HVAC contractor to put in the smallest furnace we could get away with. The high today was 25F and we had to let the fire go out and open a window. My point is your HVAC could be a complete waste unless it really is the worst case scenario. |
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SierraSkier
New Member
Posts:15
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| 26 Nov 2010 12:36 AM |
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One thing I think we screwed up; we have bathtubs on outside walls and the plumbing runs through inside walls. Of course the plumbers want the plumbing on the inside walls, but the tub was set before insulation and it is cold and my tile in the upstairs bathrooms is cold. If I were to do this again I would have plumbing in a chaseway on an outside wall and the tub on an inside wall. Or I would have made sure the tub was insulated very well. I blew that, I noticed it, but didnt follow through. |
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Viking House
New Member
Posts:37
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| 28 Nov 2010 04:34 PM |
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Hi Sierra,
Are your 0.7 SHGC windows double glazed? and do they lose heat quicker than 0.3 SHGC windows? How much insulation do you have in the walls and roof? |
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| Can you afford not to build a <A href="http://www.viking-house.ie">Passive House</A>? <a href="http://www.viking-house.co.uk">www.viking-house.co.uk</a> |
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SierraSkier
New Member
Posts:15
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| 02 Dec 2010 02:11 AM |
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Hi Viking, yes the glass is double glazed. It has a little less solar gain than clear glass. The u values were the same as for low gain glass, I think .35 instead of .32 with no argon. Somebody can correct me if I am wrong, but this can be achieved with hard coat E or by putting the soft coat on pane 3. Anyway, it seems there are different e coats to achieve different solar gains but all have low u values. I got r-49 in the attic and r-23 in the walls plus the r-8 for the EIFS siding, so I am up to r-31. But then my south side of the house is a bank of windows, so r-value is really misleading.
Still trying to figure out how to drive this house. This morning is was about 20F and I threw a couple logs in the stove, by noon it was 45F and sunny and the house went to 80 degrees. My wife had to open doors aand windows. I should have let the fire die and the passive solar would have done the rest. Blinds are clearly going to be important in april. By June the sun angle is so high it wont be a problem. |
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jonr
Senior Member
Posts:5341
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| 02 Dec 2010 09:41 AM |
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High thermal mass and passive solar houses are very subject to under and overshoot and require it to make use of the thermal storage. If you want steady temperatures or quick response - use an active collection and storage system.
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SierraSkier
New Member
Posts:15
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| 02 Dec 2010 10:08 AM |
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jonr, not sure I agree. I can turn on my HVAC and instantly obtain a higher temp, or because of the design of my windows, I can open two windows and instantly drop the temperature. I have a window very high in a stairwell for that purpose.
Here is what I am finding. To avoid HVAC or overheating, I have to keep an eye on the weather. This morning is the "warm before the storm" and its nearly 40F outside. We closed up the house last night, no fire, and we were at 66F in the house at 6AM. By 8AM the house will warm up by solar. No need for me to do anything, maybe crack a window this afternoon. With the weather forcast I see, I will be attenuating temperature in the house by using windows this week. Next week we have major storms coming in, so the wood stove goes back on. I loose solar and so I will go to wood. Its not that hard, its just a learning process.
What my house is not: my house is not fire and forget. i dont just set the thermostat and walk away. I have to be actively engaged with the house to keep the temperature where I want it. But, that does not actually involve turning on the HVAC. |
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FBBP
Veteran Member
Posts:1215
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| 03 Dec 2010 11:36 PM |
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Posted By jonr on 02 Dec 2010 09:41 AM
High thermal mass and passive solar houses are very subject to under and overshoot and require it to make use of the thermal storage. If you want steady temperatures or quick response - use an active collection and storage system.
I would agree with you as to under over. The only way a "passive solar" house works is to leave room for heat gain (start with a house colder than you want) and than let it get hotter than you want so it won't be to much colder than you want. That is why you can't have hydronics in the slab. If the slab was comfortable there would not be room for gain with out overheating. Passive houses require ACTIVE owners! |
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jonr
Senior Member
Posts:5341
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| 04 Dec 2010 10:07 AM |
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To avoid HVAC or overheating, I have to keep an eye on the weather. Maybe your thermostat could check the weather reports - I'm only 1/2 kidding. |
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sailles
New Member
Posts:7
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| 05 Dec 2010 07:29 AM |
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Very interesting to hear your experiences and others comments. I am thinking of a similar design--passive solar--for a 1500sqf 2-story house but as yet have been receiving a range of advice as to whether to go for high thermal mass (takes longer to heat and cool) or low (quick to heat and cool). I live in the mountains in Japan--cold dry winters and hot humid summers. I have access to free firewood, so am also going for a wood stove. Anyone offer any hints on benefits of high/low thermal mass? Also, I would like to research the solar slab concept further, any good places to start? Many thanks! |
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toddm
Veteran Member
Posts:1152
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| 05 Dec 2010 08:18 AM |
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Yes, FBBP, you can't have hydronic in the slab operating on a thermostat. However, if you watch the weather, as Sierra Skier does, and use hydronic to add supplementary heat to the slab, you have in effect a masonry heater without chewing up all that living room real estate. The trick is finding a modern, efficient wood stove boiler. I would say that what you can't have is radiant wood heat unless you surround it with a few tons of masonry. http://mha-net.org/
The more mass the smaller the temperature swing. |
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jonr
Senior Member
Posts:5341
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| 05 Dec 2010 09:34 AM |
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In my opinion, if you live in an area where the daily outside temperature swings above and below 70F, then exterior mass is beneficial to even this out. For interior mass, I'd use only an active system (like a water tank).
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toddm
Veteran Member
Posts:1152
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| 05 Dec 2010 11:54 AM |
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Jonr, you [perhaps meant to distinguish mass in the building envelope from active storage. For the most impact from the dynamic benefit of mass -- its buffering effect over 24-hour periods -- you still want it inside the insulation rather than outside -- interior rather than exterior.
To clarify my point, more mass would moderate the OP's experience with overheating, particularly if it wrapped the wood stove in a couple of tons of slow heating masonry. The benefit of mass in passive solar is to slow things down and make management easier. You, and most people, would prefer to set a thermostat. Even so, I cling to the hope that my passive solar house will have less temp variability than setback thermostats and conventional HVAC, after I read the slab temperature at three points and fire the stove accordingly. Or not. It's work but it's the least work of any approach delivering free heat. |
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jonr
Senior Member
Posts:5341
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| 05 Dec 2010 07:28 PM |
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In terms of comfort, an interior active thermal storage system will always be able to hold a more constant temperature than a passive one - and it will allow rapid response to changing conditions. The downside is that it will probably cost more.
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FBBP
Veteran Member
Posts:1215
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| 06 Dec 2010 10:32 PM |
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Posted By toddm on 05 Dec 2010 11:54 AM
Jonr, you [perhaps meant to distinguish mass in the building envelope from active storage. For the most impact from the dynamic benefit of mass -- its buffering effect over 24-hour periods -- you still want it inside the insulation rather than outside -- interior rather than exterior.
To clarify my point, more mass would moderate the OP's experience with overheating, particularly if it wrapped the wood stove in a couple of tons of slow heating masonry. The benefit of mass in passive solar is to slow things down and make management easier. You, and most people, would prefer to set a thermostat. Even so, I cling to the hope that my passive solar house will have less temp variability than setback thermostats and conventional HVAC, after I read the slab temperature at three points and fire the stove accordingly. Or not. It's work but it's the least work of any approach delivering free heat.
But once you fire the stove, it is no longer passive. Just less automated. At the end of the day, if you use wood or gas, you are still adding energy. Also while it might be argued the wood is "more" renewable it also is more polluting. |
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SierraSkier
New Member
Posts:15
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| 08 Dec 2010 01:07 AM |
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I think one important point was made: I am lacking mass inside the house and that explains some of the temperature swings. But, in a snowy environment with high gain glass, the sun is active heating. So anyone who designs their home like I did is going to have temperature swings just because of the solar input. I get huge energy from the sun.
Another point, I am really glad I didnt get talked into hydronics in my solar slab, it would have been a complete waste of money. My solar slab is the warmest floor in my house. There is no reason, in this climate, to add energy to a properly poured slab. It would have been interesting to have installed PEX on a circulating pump though. If I had done that then I could have taken energy from the wood stove and moved it around the slab on snowy cloudy days. Nice, but not at all necessary.
Wood is not polluting in my area. Okay, some days it is, but 90% of the time we have a good flow and the smoke moves up and out to the Great Basin. That is not the same for a lot of people though, so wood is only polluting based on a specific locations climate. We do get strong inversions from time to time, but then I volutarily suspend burning. Tonight we have 80MPH winds at the ridges, the stove is going. The wood I burn was cut during a forest fuels reduction project, it was going to be burned in a pile anyway.
Sailles, good luck on finding information. It took my wife and I two years to design this house. My recommendations are simple; face south, get good glass (not expensive windows), pour the slab in at least two inches of foam board, get a Quadrafire 5700 or the largest Lopi or Pacific Energy stove you can find and put it on the concrete slab, center north is ideal. Put all of the living spaces on the south side of your house and the laundry, bathrooms, storage on the north. Get good double cell blinds. Understand from the start you are going to have to moderate the energy inputs into your house. Passive solar houses require active residents.
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sailles
New Member
Posts:7
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| 08 Dec 2010 02:59 AM |
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Many thanks for the insights and pearls of wisdom, SierraSkier, I have taken note of all these points! Your concept sounds very close to my ideal, particularly not going "all-out" on bell and whistles, but keeping things simple and effective.
Along these lines, I have a related query on the use of water in place of a solar slab in a passive solar design. (I posted on this in detail in the design thread, but I think it maybe more seen and relevant to this thread).
I am hoping this solution will get over some of the issues with high thermal mass in a hot summer/cold winter location like mine. Any comments/advice greatly appreciated! :-
The concept is based around shifting water as the thermal mass. As with SierraSkier's design, the current house design has lots of glazing on the south side (maybe even over 25% of the total house--the roof slopes up to the south). The 1st floor is around 1200sqf with 550 sqf or so above in a limited second floor configuration.
So... I want to utilize the south facing glazing to warm inside waterwalls/tanks and maybe some kind of inside waterfall that runs to tank/s that release the thermal mass stored overnight (flowing water inside would also help to humidify in the dry winter). I also have a couple of flat-plate collectors that can supplement the heating of indoor tanks/walls (and can buy 1-2 more) and unlimited free firewood for a wood stove (that could have tanks beside or near it).
I am thinking that one of the tanks will be a solar-fed electric boiler for backup hot water in winter. In the summer I would like to use the collectors for the majority/all of the hot water so perhaps the simplest thing to do would be to use the same electric boiler and insulate it, although this would add an unwanted internal heat source. I was also thinking of a simple backup tankless electric for summer use, fed from a separate outdoor solar storage tank (season switchable).
The inside tanks/waterwalls would be emptied in summer and I would either purchase a dehumidifier or use dehumidifying salts in some capacity. A whole-house fan in the roof and vent/feeds in/from the north facing wall/floor/ground would complete the cooling.
How does this design strike all you experienced builders/experts out there? Will the water solution work as well as SierraSkier's in winter and be of benefit in the hot summer? What are the limitations? Also, anyone seen a waterfall concept of this kind? I like inside running water and it would kind of show-off the energy system at work behind the scenes. |
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FBBP
Veteran Member
Posts:1215
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| 10 Dec 2010 11:10 AM |
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Posted By SierraSkier on 08 Dec 2010 01:07 AM
Wood is not polluting in my area. Okay, some days it is, but 90% of the time we have a good flow and the smoke moves up and out to the Great Basin. That is not the same for a lot of people though, so wood is only polluting based on a specific locations climate. We do get strong inversions from time to time, but then I volutarily suspend burning. Tonight we have 80MPH winds at the ridges, the stove is going. The wood I burn was cut during a forest fuels reduction project, it was going to be burned in a pile anyway.
It almost sounds like you are saying that if the stacks are high enough not to cause local problems it is okay to burn coal without scrubbing it ;-) Okay I get your point, it was going to get burnt anyway. |
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