I have a customer who I very interested in using a solar slab. He has done a bit of reading prior to contacting me and is using James Kachadorian's The Passive Solar House, The Complete Guide to Heating and Cooling your Home, primarily. It looks like the book is fairly old, is the information still relevant or are there better ways to accomplish this? Any advice would be appreciated.

A ducted slab needs just the right location. My passive solar house in so central Pa doesn't get enough sunlight in December to warm the slab appreciably, let alone heat any other part of the house. Roughly half the homes here have radon issues as well, which would be a deal breaker in Kachadorian's approach. Proponents say that condensation and mold aren't issues, but slabs are permanent enough that I wouldn't try it in a humid climate. Like Pa.
Google UCLA HEED. The school offers a couple of free design tools that use weather data downloadable from the nearest NWS station. Climate Consultant presents that weather data in a series of charts and graphs, one of which shows how many hours per year that specific energy strategies will maintain comfort. I'd want to see a big number for passive solar/high mass before I went any further on ducted slabs. If your client has a specific design, you can do a rough model using UCLA's HEED, putting the windows in the proper orientation and cranking it through 365 days of local average weather.

I built a hydronic slab that picks up excess heat from a 24-tube collector solar hot water system in the shoulder seasons, and from a wood stove boiler in dead winter. But again, I'd want to see a fairly low number of hours for passive solar/high mass in Climate Consultant before incurring the extra expense.

Yeah, it's still relevant as a way to build a passive solar, but you need to know what you are doing to avoid making a BIG mistake. Location is important. Once you start reaching for an increasingly large solar fraction, a lot of your other flexibility starts to go away. For example, Kachadorian says you can use the same building materials that you would have otherwise used, but just rearranged. The first "rearrangement" requires you to lose the full basement you might have planned.....

What is your client's goal?  If it is an energy efficient home than there are lots of ways to get there.  If it is home in which 50% of the heat is supplied by passive solar, then you have a serious project on your hands.

The main goal is a generally energy efficient house. The owner just really wanted to focus on the passive solar. I am in Western NY. Definitely not the best location for winter sun. I'll see what the climate consultant has to say. Thanks for passing that along toddm.
The concrete people that I talked to are saying about an additional 50% for the solar slab over a conventional slab. I don't have specific expertise when it comes to designing a truly passive solar house, so I want to make sure that this is going to be worthwhile for the cost. I should add that the budget is extremely modest. The construction will be timber frame with SIPS envelope. R25 on walls R40 on roof. The primary heat source will be a high efficiency wood stove. We are planning to use insulated shutters for night time. It's a relatively small house at under 1500 sf, so I know it's going to be extremely easy to heat with the stove.
My main concern is that high solar heat gain/ low U value windows are out of the window budget. So the plan was to use double pane non low E windows to maximize solar gain and use the insulated shutters at night. I'm afraid that with so many cloudy winter days this is going to be a net loss. I am guessing we might be better putting the solar slab money into better performing windows. Thanks guys, I appreciate the feedback.

Insulative shades can be far superior to coated windows. http://www.1windowquilts.com/ Even diy curtains incorporating a radiant barrier should outperform coated windows as long as they are closed religiously. Trouble is, IRC 2009 and later requires low e windows in northern climates IIRC. I'm no expert, but I think you also need hvac that can maintain livable temps automatically. Your BI may give you a pass based on common sense, or ny may have an alternative energy compliance procedure.
But nothing passive will deal with humidity. I ended up with a mini split and more expense.

Definitely not the best location for winter sun

I'm in the Pacific Northwest. Our cloudy day factor may be quite similar. How are you planning to get heat into the "solar slab"? By circulating room air through the passages?

What about a regular old solar slab that gets warmed by the sun shining on it? Being willing to do the night time shutters is going to help quite a bit.

To be clear, the sun in my aggressive passive solar house shines on a plain ole 4 inch slab. You'll see in Climate Consultant that passive solar/low mass (no concrete) is the recommended approach in the northeast, but I poured concrete to insure against overheating. Works perfectly although occasionally you must switch chairs because the sun is heating you directly. But, as I said, there's too much concrete to heat in dead winter, and forget about using the slab to warm the rest of the house.

forget about using the slab to warm the rest of the house.

I've got a sunroom that rapidly gets warm when the sun comes out in the winter. If you play with the hydronic heating system valves, you can cut the sunroom slab in and out of the system. When you cut in the sunroom, the temperature of the returning fluid increases.

Posted By ICFHybrid on 25 Mar 2013 05:30 PM

forget about using the slab to warm the rest of the house.

I've got a sunroom that rapidly gets warm when the sun comes out in the winter. If you play with the hydronic heating system valves, you can cut the sunroom slab in and out of the system. When you cut in the sunroom, the temperature of the returning fluid increases.

One - you need to play with it. Two - to be of any use, would the return temp not have to come from a slab that is beyond the comfort of my tender tootsie?

One - You could use either the slab sensor or the room thermostat to control a valve that does the same thing. The valve I am turning is actually motorized.
Two - I don't know what your tootsies can tolerate, but my return temp is in the range 75F - 78F

I'm not knocking passive solar. Our exterior decorator did an extraordinary job for us today:



Multiply it by another nine 4x6 windows (40 linear feet.)

That said, energy savings are entirely a function of climate. In my part of the world, overcast days are plentiful but relatively warm. Thanks to lapse conditions, clear days tend to be cold. You'd have to be tougher than me to depend entirely on passive solar in December. But with slooow changes in a hydronic radiant slab and intermittent sun, control isn't that difficult. There was a long discussion here a while back about the wisdom of radiant heat combined with passive solar. I can say, definitively, not a problem in south central pa. to turn off the heat on a relatively few afternoons. Even so, my target energy bills of $50/mo seem high at this point.  

So what makes sense and is economically feasible for a cold climate? And follows the KISS principle.

As others have indicated, economics and passive solar performance are largely a function of your design objectives and your specific location/climate. We are big fans of integrating passive solar and hydronic radiant floor heating systems. We also have free software on our website for designing both these types of heating systems. In fact, our hydronic radiant floor heating design software will design a multi zone system that only uses a single circulator pump and a common boiler supply temp for all the circuits/zones…which is about as KISS as you can get and is the preferred design approach for DIYers, especially those in remote areas off the grid. You would of course first need to get educated on these subjects and the software instructions on our website might be a good place to start.