Passive solar in N GA getting two different opinions.

Do the entire slab, others say just do the perimeter and 4' border

Pros & cons when using the slab as thermal mass, but in a not so cold area

Gosolar,

This is an interesting subject. The issue is the conduction and thermal moderation of the soil around and under the home, the local micro climate, the homes design and the deep earth temperature in the area.

Are you trying to engineer the solution or build a specific design on a random lot in a random climate.

When the thermal path through dry soil to the outside environment exceeds 20 feet there is a disconnect between the inside and outside environment with an isolated moving thermal wave front. The soil near the inside environment will move to the average inside temperature and will act as internal thermally trapped mass stabilizing the inside environment.

Shortening the path length increases the thermal effect with the thermal wave front becoming more and more significant and its velocity increases. Note that when you are only talking about using 2 inches of foam that will not isolate the thermal mass. It will change the thermal flow to a continuous low flow of energy to the infinite heat sink bases on your local deep earth temperature. Moving the insulation to a wing arrangement would buy you more performance assuming dry earth is under the site with the same amount of insulation. Optimum configuration is typically at a 45 degree into the soil and buried. you should also taper the insulation down as it gets further from the home and you need to water proof above the insulation and provide for drainage.

It can take 3 to 5 years to stabilize the temperature of the soil under the house depending on the initial temperature of the soil and the temperature differential between that initial temperature and the desired internal comfort temperature. A house on bedrock without a significant dry soil layer will override the temperature due to direct high conductivity to the deep earth temperature and a relative infinite thermal sink at the deep earth temperature. This can be an advantage to use in many situations but you have to understand the thermal modeling carefully or you may over cool the home.

In Georgia these temperature differentials are low and there fore not a big issue on the stabilization of the mass to a usable temperature as long as it is dry dirt. We have built passive solar homes in similar environments. There we put in a six foot tapered insulation wing outside the home.

The slab temperatures with no applied heating after two years are consistently in the mid sixties and slowly rising. The thermal modeling indicates that the floors should level out at between 68 and 72 degrees in that home with its design coasting. These temperature can be modified with both solar thermal heating and or ground water cooling as well in this home.

This provides over million additional lbs. of internal isolated thermal mass to the design. This provides a much larger buffering mass for the solar input without over heating during thermal absorption. It is pretty easy to control solar in the SE US and match its input to internal and external environmental averages.

Your question really comes down to your design ,specific microclimate and properly modeling the home to eliminate major mistakes. The thermal model of your prospective home will have a maximum solar thermal absorption on the site which should determine the amount of internal thermal mass needed to maintain internal comfort and how much thermal energy the home should be able to dump.

The thickness of the slab, does it have radiant tubing in it for distribution of the temperature within and below the slab. Does the design have a buffering water tank giving it additional capacity. Solar homes designed around most of the Passive House standards are a good idea to start. What is it that you are planning to build and where is it.

Brian

Posted By zehboss on 16 Jul 2015 07:00 PM
Gosolar,

when you are only talking about using 2 inches of foam that will not isolate the thermal mass. It will change the thermal flow to a continuous low flow of energy to the infinite heat sink bases on your local deep earth temperature. Moving the insulation to a wing arrangement would buy you more performance assuming dry earth is under the site with the same amount of insulation. Optimum configuration is typically at a 45 degree into the soil and buried. you should also taper the insulation down as it gets further from the home and you need to water proof above the insulation and provide for drainage.

 
The slab temperatures with no applied heating after two years are consistently in the mid sixties and slowly rising. The thermal modeling indicates that the floors should level out at between 68 and 72 degrees in that home with its design coasting. These temperature can be modified with both solar thermal heating and or ground water cooling as well in this home.


The thickness of the slab, does it have radiant tubing in it for distribution of the temperature within and below the slab. Does the design have a buffering water tank giving it additional capacity. Solar homes designed around most of the Passive House standards are a good idea to start. What is it that you are planning to build and where is it.

Wow now that's an answer, tks

Not sure why you state 2" will not isolate the mass, explain further please
No ground water cooling or radiant heat or buffering tank.

this is a 4" slab on 2,000 ft elv hilltop in NW Ga.

the slab is a monolithic 1,000 25x40 with perfect southern exposure, super insulated with Sips construction, 120' s glazing with cardinal 180 IG.

.

Gosolar,

100% isolation happens at about R-120 or 24 feet of dry earth in most area. continuous R-10 and 2.4 feet of earth will isolate the mass as well in general terms. It is a continuum to balance in the configuration and use of the insulation based on the soil properties as well

120 sf of vertical southern glass at SGHC .69 you will peak around 25,000 BTUs per hour peek. Numbers are off top of head and obviously not based on actual orientations, installs, etc. SIPs have no internal mass, I assume you are thinking of standard internal construction. Why have you decided to use SIPs? Other systems can give you higher internal mass and storage buffers. SIPs tend to be a little pricy for performance.

Generally I would up the slab to an 8 inch slab add $2000 and put in heat pex to help balance out temps in home add $1000 , and eliminate the preparation of the footings labor save a $1000 and increase the solar thermal storage of the home by a factor of about 4. Insulate last 4 feet of permeter with 2 inches and use wing insulation external starting at 4 inches and tapering to 0 at 6 feet. The entire slab will be footing capable and will up the thermal mass of the home substantially. There are lots of other things I would do to optimize costs and performance of a solar home in your area.

Call me if you have some additional questions. Are you near Rome, GA.

Brian

Posted By zehboss on 16 Jul 2015 08:17 PM
Gosolar,

120 sf of vertical southern glass at SGHC .69 you will peak around 25,000 BTUs per hour peek. Numbers are off top of head and obviously not based on actual orientations, installs, etc. SIPs have no internal mass, I assume you are thinking of standard internal construction. Why have you decided to use SIPs? Other systems can give you higher internal mass and storage buffers. SIPs tend to be a little pricy for performance.

Generally I would up the slab to an 8 inch slab add $2000 and put in heat pex to help balance out temps in home add $1000 , and eliminate the preparation of the footings labor save a $1000 and increase the solar thermal storage of the home by a factor of about 4. Insulate last 4 feet of permeter with 2 inches and use wing insulation external starting at 4 inches and tapering to 0 at 6 feet. The entire slab will be footing capable and will up the thermal mass of the home substantially. There are lots of other things I would do to optimize costs and performance of a solar home in your area.

Call me if you have some additional questions. Are you near Rome, GA.

Brian

I think the 25,000 btus is about right, I didn't look at other systems for additional mass ie:concrete due to cost and don't think a small home like this needs it.

The sips are IMO not high in price, about $4 sq'. What costs less and has the ease of construction.

From my research anything over 4" was not cost effective in other words diminishing returns.

I have used radiant in a home I had up north loved it.
But here it would only come into play as a backup if there was a short fall of solar gain, I felt a Mini Split 9K would serve that purpose better after rebates about $5-600.

I don't follow what you mean "eliminate the preparation of the footings labor save a $1000"

The wing insulation. just read some articles on it, good info glad you brought that to my attention

Last ? what is "entire slab will be footing capable" mean

thanks again for your input, the site is a bit east of Rome near Ranger.