I am starting the engineering phase of my single family home in Poway, near San Diego, which will incorparate passive solar features.  The primary interior thermal mass will be the slab on grade.  The temperature is moderate in San Diego and based on this post I would think I would not need slab insulation.  Any thoughts?

http://greenbuildingtalk.co...aspx#40292

Here's the weather for the last year:  http://www.wunderground.com/weather...&year=2010

In January the average low is 44F and the high is 67F.  In August the average low is 62 and the average high is 82.

Thanks,

John

Passive solar in Poway? I think I was three years into ownership of my first condo in San Diego before I realized the heating plant consisted of a single 1500 W baseboard heater and heat lamps in the bathrooms. (It was during a cold snap)

How are you going to keep from getting smoked the rest of the year following your mini heating season?

It seems like not insulating under the slab will help keep you cool most of the time there in Poway.

It will really be interesting to see what you do with this project.

Caveat:  This is based on my research and experience as an occupant in "non-passive solar houses".  I have not build a passive solar house.

Poway is a bit hotter and colder than San Diego proper.  Especially if you had a place on the beach.  That said, even on the beach I think passive solar would be usefull.  If you look at the bioclimatic chart and the average temps you will see that the mean temp is below "comfortable" for much of the year.  I currently don't use the heat much at my house, but that doesn't mean that it is "comfortable".  My current house is not optimally oreinted and it is cold during the winter without heating.  On the other hand I can see the potential for cooling because the downstairs area, that is also mostly shaded, is cool during the summer.  The upstairs is another story.   San Diego is considered a mixed heating/cooling area with highs often above the comfortable temp and the lows often below on the bioclimatic chart.  This shows potential to keep the temp in the comfortable range using a smart design.   

The way I am planning on keeping from overheating is to orient the house East/West with the windows on the south side and not overgazing.  Currently I am planning on 8.6% glazing as it relates to floor space.   I will also minimize glazing on the East/West and shade these windows.   I have played with the overhang on the South side and a 3' overhang will keep the sun almost completely off the windows during June, July and August.  It is a trade off because in late April and May you may not get quite as much sun as you would like and in September you will get more than you want.  The house will be slab on grade with the mass of the slab dampening out the heating/cooling swings.   If I end up overheating I will go to exterior shading.  I may do that anyway.  If I use exterior shading I would be able to controll the heat gain from the windows 12 months out of the year.

That said, even on the beach I think passive solar would be usefull.

I really doubt it. The unit was in Point Loma and was built massive - like old San Diego adobe architecture with lots of balconies and overhangs to shelter from the sun. Point being that we had no direct sun and stayed nearly perfect year round. Windows stayed open almost all the time except during the 3 day cold snap every other year and the Santa Ana winds. Plenty of heat inside from bodies, cooking and what not.

A quick gander at Weatherspark.com graphic data, it looks like the annual mean temp at Poway is ~ 66F (fully 10F colder than south Florida, Miami, say), which means you'd still want to insulate the slab edge up to something signficant, but anything over R5 center-slab would be overkill unless taking it to PassiveHouse levels (which you probably can still do pretty cheaply in that climate.) Model it in the PassiveHouse tools, but I'd bet R15 under slab, with ~R25-30 (whole wall, thermal bridging included) R60-75-ish roof and a careful balancing of glazing/shading can probably get you there.

Short of a true PassiveHouse design, as long as you design it for low summer gains, heating & cooling it with a single tiny high-efficiency inverter-type mini-split is possible if built air-tight, with code-min x 1.5x R values. But even going PassiveHouse on it might not be much more than the cost of a mini-split if it's a fairly simple design. Using guidelines for Zone 3 in this document and you'd be there, with careful window sizing & placement:

http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones

That would be R5 under slab, R7.5 at the slab edge, R20 whole-wall-R, R50 roof. With 24" o.c. 2x6 framing and spray cellulose cavity fill (or R21 batts, if you must), adding an inch of exterior XPS or iso would deliver the R20 walls for not a huge premium. Air tight techniques on the sheathing (blower door verified before insulating) would be key to ultimate performance.

In that climate with those insulation values, getting to annual Net Zero Energy use with a mini-split for the primary mechanicals + some rooftop photovoltaics + modest solar-thermal (like batch solar HW) can be reasonable. Taking it to PassiveHouse you'd be trading PV costs against higher insulation costs, and it may come down to raw subsidy levels which approach makes more financial sense. (Turning a PassiveHouse into Net Zero wouldn't take much either at San Diego insolation levels.)

The PassiveHouse tools are cheap, yet pretty accurate for high-R buildings if you feed it the correct info. You'll probably be surprise at just how much sub-slab insulation it takes though.

Perimeter perhaps, but when you insulate the middle you also lose the cooling effect of that of deep earth grown temperature. Given the desparate loads I would lean toward cooling unless radiating the floors of course.

ROI hard to find unless the goal is net zero or there abouts.

With a good design on the rest of the envelope the benefit of that ground-cooling effect is still net-negative in Zone-3. Those desperate loads just aren't all that desperate in a low solar gain high-R design- even a tiny mini-split can handle a 2000' house with R50+ in the roof, and R20 whole-wall values, and minimal, well-placed glazing.

It's really not very hot there, despite high insolation- Poway CA is both cooler and less humid than Minneapolis in summer:

http://weatherspark.com/#!graphs;a=USA/CA/Poway

http://weatherspark.com/#!graphs;a=USA/MN/Minneapolis

(But it's a heluva lot warmer and more humid then MN in winter, eh? ;-) )

It's still heating dominated, but mixed, something like ~1250 HDD to ~1000CDD. Nothing too torrid or frigid, most years.

Regarding leaving a slab (or walls) uninsulated so that it provides some cooling - don't do this where there is high humidity. Been there, done that and you don't want the high humidity/condensation/mold that it causes.

Posted By jonr on 03 Jun 2011 11:09 AM
Regarding leaving a slab (or walls) uninsulated so that it provides some cooling - don't do this where there is high humidity. Been there, done that and you don't want the high humidity/condensation/mold that it causes.

Hi, I’m interested to hear more about this comment, if you’re so inclined.

In my daily life I’m frequently in an old dairy barn. During hot humid summer weather the slab in the milk room sweats a lot. I assume there is no insulation and likely no VB under it - but it appears the smooth top was sealed with something.

OK, then there is the slab in my garage a few miles away. The garage is ~22 yrs old, no insulation under the slab - not sure about the VB either way, not sealed. It does not sweat during the summer.

BTW, the barn is about 75 ft from a house which has a year-round spring in the basement and both the house and barn are about 75 ft from a 15’ wide creek - so I’m guessing that there is underground water very near the surface there.

Why does the barn’s slab condense the water out of the humid air while my garage slab does not? The reason I ask is because I’m considering the perimeter (only) insulation for the garage slab in my new house, figureing the center of the earth-connected slab may help keep winter temps in the unconditioned garage above freezing. But we have humid summers and I don't want to get condensation on the slab ... and your comment above got me to thinking I don't really understand what conditions (other than humid air) are needed to set up this problem since I see it in one location (barn) , but not in the other (current garage).

Thanks for your thoughts on this.

I can only guess - perhaps the garage gets some solar heating that warms the slab to the point where it can't cool air below the dew point. I know my garage gets hotter than ambient (from the hot roof above it). Or maybe it has more air flow.

Naturally is all about dew point temperatures. In an radiant cooling design - be it natural or mechanical - dew point has to be taken into consideration and ERV, DX dehumidification, ventilation all have to be factored in. I have designed radiant floor systems for basements here in Minneapolis and find people heating the basement slab a couple of degrees will lower relative humidity and can make for a very comfortable and "relatively" dry living space. In a garage or shed it is more about infiltration and air flow. If the door will be open on humid days a DX system is in order and naturally where humidity is constant say around the ocean humidity an uninsulated slab makes more sense in a dry cool climate.

If you have animals in a barn the humidity will naturally be higher and the cooling effect of an uninsulated slab neglegible. There is no "rule of thumb" for insulating a slab. If you are going to heat ANY slab, perimeter insulation is a must, whether the interior slab is insulated depends on climate, design room temperature, design water temperature and ROI.

Does anyone have any experience with Slab insulation in a desert climate, Palm Springs, Phoenix,etc.. What are the recommendations?

Thank You, SF

Perimeter only, as your main loads will be heating and some gain will occur where sun meets foundation.

There's plenty of contractors that would be interested in installing slab insulation for you.  If it's a rare building material in San Diego, it may cost more to have it shipped there from outside the state.  You will be able to find someone to install this or something just as good in San Diego is look at the classifieds and call around for a contractor for your home remodel.

John,

In San Diego the dirt temperature under the house will maintain a comfortable temperature year around with the smallest amount of perimeter insulation. When I calculated dew points for a different house in San Diego those temperatures never came close to the dew point. Dew point calcs are easy to make for your specific location. In general if you have a slab reaching internal dew point it should have dehumidification designed into the house. If comfort temperature in the house is at 100% humidity is not comfortable and you have other design problems with the house. Since there are very little severe weather high thermal mass in the design is the lowest cost way to get a passive house in San Diego.

Brian