We are planning to build a passive solar home in the cold climate of Midland Michigan. In order to incorporate thermal mass, we would like to have finished concrete floors on the main floor of a ranch home built over a basement. We plan to have 10% south facing windows, floor plan is sunplans.com "Islander 4" 1840 sq ft ranch over basement. Could the thermal mass be just concrete board underlayment, some thickness of poured concrete, or some thickness of poured gypcrete (lite crete)? If it is possible to install concrete floors, how would this best be done, and what would the significant structural and cost implications be? It would also be possible to make the home sun tempered and have only 7% of total sq ft south facing windows and no extra thermal mass. My husband feels it is worth increasing the percentage of south facing windows and putting in the extra thermal mass. Also, is it worth having radiant heat in the floors as we would also have geothermal for heating and cooling? Does anyone on this forum have experience of having poured/installed concrete floors over a basement in their passive solar home in a similar cold climate?

Modeling it with passive solar design tools would be advisable. Just winging it without doing at least the napkin math on the thermal mass and site-specific solar gains is likely to end up with a sub-par result.

Radiant slabs can work well and redistribute heat in passive solar house if you run the loops in a north/south orientation, and slabs are good for keeping water temp requirements low (essential for good efficiency on hydronic geo.) In a Midland MI climate you can probably get comparable efficiency to geo out of the Daikin Altherma air source heat pump for less up-front money, provided your heat loads are within the output range of an Altherma. (NRT.Rob on the radiant heating forum of this site is using one to heat his offices in Maine, and could tell you more about it.) Lower system design risk and lower upfront cost for similar efficiency make it an attractive option if your design heating temp is above -4F, which it almost surely is. http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf

It's hard to tell from their web description what the relative U-factors of the walls/roof would be, and those are more important than the glazing/mass ratio since they determine what the peak and average heat loads would be in your location. With out knowing the heat loads it's hard to say how much glazing area would be appropriate. http://www.sunplans.com/select/plan/__details/Islander_4 Do they give you a BTU/hour per degree F or any other baseline heat load estimation on the design?

you'd want to get the floor engineered, but there are builders that install concrete over wood joists regularly. Typically 1.5 or 2"; can be regular concrete or lightweight. With a well insulated shell, including the basement floor and walls the geothermal and even radiant is overkill - way too much expense for what you'll need. Built right, you'll be able to heat this easily with one or two minisplits. If there was a way to get bigger windows in the basement, that would add even more and give you a very warm & livable 1800 additional sf. This house needs to be modeled with one of the many programs mentioned in these forums, or ideally with the Passive House Planning Program. It will be a fairly straightforward house to superinsulate; the danger is overheating. Some or most of the programs account for overhangs and shading. This could be a very easy house to live in.

You really want to do one of the formed-in place floors for this like Quad-Deck or Insuldeck.

I have a passive solar using an Insuldeck main floor at 49N in WA state and it is working out great!

Don't do the Gypcrete. You lose heating efficiency with it. Concrete is lovely.

Is your plan to have geothermal forced air for the cooling in addition to radiant in the floors?

Posted By Bob I on 02 Nov 2012 08:34 PM
you'd want to get the floor engineered, but there are builders that install concrete over wood joists regularly. Typically 1.5 or 2";

2" or less of concrete is not structural and would be very prone to cracking. 4" is the minimum slab thickness.

With that being said, a wood sub-floor would not hold a 4" + slab. An engineer would be needed.

Ideally you would want a true concrete 2nd floor like an InsulDeck or QuadDeck setup. This requires a complete concrete substructure and support walls. Wood does not support concrete. Concrete or steel supports concrete. For my design I will require a 10" InsulDeck setup with a 4" slab or 14" total floor depth. This will be anchored into an exterior ICF wall on 3 sides with an interior ICF wall carrying the 25 foot span.

Did I mention an engineer will be needed???

Best bet if you are wood structure is to have exposed tile, that provides decent thermal mass without the weight & complexity of a concrete floor.

"2" or less of concrete is not structural and would be very prone to cracking. 4" is the minimum slab thickness"
I'm not a concrete expert; all I said was that there are builders that use 2" concrete over wood framing regularly with no issues. The obvious requirement is that the framing be sufficient to minimize bending & movement, because that is what will cause the cracking. Wood will support concrete.

Posted By Bob I on 03 Nov 2012 01:23 PM

I'm not a concrete expert; all I said was that there are builders that use 2" concrete over wood framing regularly with no issues. The obvious requirement is that the framing be sufficient to minimize bending & movement, because that is what will cause the cracking. Wood will support concrete.

Concrete must have a minimum of 4" thickness to be considered structural and to attain compressive strength. I can guarantee you that a 2" floor slab supported by wood framing will crack and will not support structural capacity of an elevated floor. No engineer would approve a 2" thick structural floor slab.

Every elevated floor experiences deflection or creep. A 2" slab, regardless of deflection or creep will crack.

A 2" concrete slab is not intended to be a structural slab. It is typically poured over wood framing and plywood or OSB floor decking designed to accommodate the added weight of the concrete (~25 lbs per square foot at 2" thick). Or you can use metal framing and decking. This isn't rocket science. A wood floor system, assuming reasonable spans, can easily be designed to accommodate an extra 25#/sq ft.

Best not to combine passive thermal mass with an active radiator. Forced air might be best. Better yet, build a frost protected shallow foundation (a slab) and use the money you save to build more garage space and perhaps some active solar.

Thanks for your input. Following are the answers we got from Sun Plans. We only ordered a review set from them, but when we order the construction prints they will prepare custom energy specs to address: After a climate analysis is performed, orientation, magnetic north, insulation values, south glass, thermal mass, and overhang lengths will be reviewed along with the construction details for the particular house by the Sun Plans architect. Both the passive solar heating and cooling elements will be considered prior to preparing the Custom Energy Specs. For the Islander 4, the thermal mass for the main floor is on the lower end since the percentage of south glazing is only 8% of the floor area at the original customer’s request to minimize construction costs. The home only needs a little additional thermal mass to prevent overheating in winter so we have designed in only a little thermal mass in the floors. (Some clients request with similar homes ask that we add the mass in the walls instead in the form of interior brick veneer so that is also an option to consider. Let us know when the order is placed and we can prepare the Custom Energy SPecs to address that alternative. Otherwise, as currently designed the mass to be added (see sheet 8) is tile over 1.5” gypcrete (if client is going with radiant heat) or tile mudset over ½” cement board. It only needs to be in the south-facing rooms as shown on the cover sheet. When we get orders for plans that have not been updated in over 2 years (such as this design), we also run them through our in-house check list and often change various things based on newer codes and energy details. If you have a different way of achieving the same results, feel free to send them to us when the order is placed and we can consider them. In any case, on the Custom Energy Specs, we list several options for achieving various insulation values based on the location and particular plan. My question would be that if were to build to passive house standards or 20/40/60 which is way above the energy star standards would it be overkill to increase the south glazing to more than 8% of the total floor area and therefore have to add more thermal mass to increase our solar heat gain during the winter in the cold climate of Midland Michigan. Would our house be equally efficient without the added glazing and thermal mass?

The higher the R values of the assemblies (and lower the U-factors of the windows) the less payback you get out of oversizing the passive solar gains. At the PassiveHouse extremes they have larger windows sure, but those windows typically have very low U-factors (< U 0.15), and a correspondingly low SHGC, so it TAKES more glazed area to achieve the gains.

At something in-between (like a 10/20/40/60 house, which WOULD make sense in your neighborhood) with
But this is all armchair design- this CAN be done in a rigorous fashion. Design-by-web forum isn't likely to lead you to as favorable a result as doing the design adjustments with good simulation tools.

Hello I just cam e across this thread and I was wondering if you finished your house.? I am planning on a build in Michigan and like the the ISlander as well. Look forward to hearing from you Don

Yes, we would be very interested to hear how this turned out as well. We had a bad experience with Sun Plans several years ago and others have told us that they had similar bad experiences too. In fact, it was this bad experience that actually motivated us to add passive solar design and construction drawings to the engineering services that we provide. We documented our personal experience on Amazon.com under their Sun Inspired House book review section.

Fortunately, our credit card company fully resolved the problem for us so this was only an exercise of frustration and not dollars. One should never send a check to an out-of-state company if there is any question that the services rendered may be less than the services expected/performed.

Posted By jonr on 04 Nov 2012 09:13 PM
Best not to combine passive thermal mass with an active radiator. Forced air might be best. Better yet, build a frost protected shallow foundation (a slab) and use the money you save to build more garage space and perhaps some active solar.

A voice or reason in the wilderness. ("Forced air might be best") does sting a little. One of the foremost and earliest radiant floor pioneers showed me his own suspended slab, 2" thick with the appropriate crack isolation membrane under tile...no cracks. Just lucky I guess. Passive solar is grand, but windows--as Dana points out--are a double-edge sword. Best stick with seasoned professionals for all passive solar design work. You, not surprisingly, will find the vast majority in the southwest. The best you can do here is raise more questions than answers. Bristol Stickney is a good man to start with. http://www.solarlogicllc.com/solar-technology-articles/

That’s right, back in 2012 when this post was created, no one was successfully integrating PS and HR heating systems. That’s not the case these days…check out our logo, eh.

The biggest mistake I see with passive solars is that they let too much sun in at one time.

("Forced air might be best") does sting a little.

Just to be clear, this is referring to spending considerable money to create a suspended concrete slab radiator/thermal storage device. I have no argument with the comfort, quiet and efficiency of radiant, although the advantages decline as houses get better insulated.

Would our house be equally efficient without the added glazing and thermal mass?

The extensively analyzed PassiveHaus designs usually come to that conclusion. Ie, typically lots of insulation, modest window sizes, low cost heating systems, no purposely added thermal mass (just the drywall is sufficient).

It is surely a balancing equation. I would argue that a little radiant floor goes a long way in overcoming comfort issues presented by less-than-perfect envelopes. Trying to determine proper mass in proportion to variable design loads is beyond science and highly weighted to climate, season and any particular year.

Posted By ICFHybrid on 28 Mar 2014 12:37 AM
The biggest mistake I see with passive solars is that they let too much sun in at one time.

True, the other mistakes made are that some PS designs let in too little sun and often don't provide adequate control methods to properly address irradiance or outside temp variability. Our experience with Sun Plans in particular was that they don't adjust the roof overhang geometry, fenestration area/SHGC, or passive thermal mass so as to obtain the proper range of PS heat gain BTUs for the specific location and climate. They suggested that we should hire an engineer to do that and also told us that spending 6% of our building budget just to have them modify their plans was a reasonable ROI too. We initially liked their floor plans, but it became quite apparent that they lacked the competence to create PS plans that we would find acceptable at an affordable cost.