I'm trying to determine whether it is cost effective to pour a 6" slab vs. a 4" slab. My specific question is, will I retain enough extra heat gain from my radiant heat floor to justify the extra cost of the additional 2" of concrete? I understand BTU's in will equal BTU's out, but will the extra concrete mitigate my boiler demand enough WRT cold snaps to offset the extra concrete cost. I'm leaning toward the extra concrete, but some fresh prospective will be appreciated. I'll be doing a monolithic pour using ICF for the footing with 4" of rigid foam under the slab. Location is west slope Colorado at 6,150' elevation, and building facing 15 degrees west of magnetic south. I'll receive 300+ days of sun, but I don't want to depend on any extra sun heat. Climate shows most of the cloudy days in winter.

Not sure if this is for passive solar, hydronic radiant floor heating, or both? 4" is pretty typical and adequate for hydronic radiant floor heating. You normally don't want to exceed 4" for a passive solar thermal mass either. We have a suite of free DIY passive solar design and hydronic radiant floor heating design software on our website. Be sure to read the software instructions. Here's the link to our thermal mass performance software:

Borst Thermal Mass Performance Software

The increased mass will cause more under and overshoot as a radiator, costing you slightly more in energy use. Passive thermal mass has some use but thermal mass in active radiators is generally a negative. As radiant expert John Siegenthaler writes:

"... fast response allows panel radiators to “track” the required heat input requirements of a room far better than higher mass radiant floor panels."

sailawayrb in Colorado you can't get away from some passive solar, and I am orientating the house along the south facing direction. I had to flip my plan and twist its orientation from due south to eliminate the possibility of ice damming in the valley where house and garage roof connect. Im trying to include as many passive techniques as I can use like minimal windows on the north, placing garage on north side, roof overhangs, window quilts, even placing closets along "cold walls". I have used some of your programs, (thank you). I had not considered the greater heat gain/loss lags as jonr mentioned, but they make sense. Thanks jonr.
Dan

Glad to hear that you found our software useful. I wouldn’t get too concerned about using thermal mass for passive solar or hydronic radiant floor heating. Some people understand how to successfully incorporate thermal mass into an efficient building design and some people never will. And like every village has an idiot, every forum has a gibberish power poster. Take the time to do your homework and research, and you will be more than happy with the results.

Posted By DAND on 17 Dec 2013 01:10 PM
sailawayrb in Colorado you can't get away from some passive solar...

That's certainly true in my part of Colorado too. Sounds like a good design for the area. Hopefully you've got some high solar gain windows as part of the package. In this part of Colorado, most window suppliers stock mostly (or only) low solar gain windows for reasons that I cannot understand.

Hey Lee, I'm torn between just using a double paned window and adequate window quilts vs. the added expense of a triple paned gas filled window. Without taking a look at specific windows I do not see the dollar cost over time value outweighing the simple window quilt. I'm open to arguments for and against the triple paned, gas filled windows. I simply don't know enough about windows to make an informed call. From what I have read you will never get much beyond R-6 for the triple pane so I would still need the quilts. What do you think?
Dan

Dan,

We are talking different things here. I am just saying that whatever windows you select, whether double or triple pane, I would suggest using high solar heat gain coefficient (SHGC) low-e windows, as opposed to low SHGC low-e windows. The high SHGC windows should have a SHGC value of greater than about 0.40. Higher values will increase the amount of passive solar heat gain, assuming that is your desire. In my part of Colorado, going with high SHGC rather than low SHGC will save about 15% on energy use annually for a code-insulated house (http://www.residentialenergylaboratory.com/window_codes.html). The energy use difference is significant here because of our high solar insolation in this area. I am not sure of the solar insolation in your area since you have only said western slope, but I assume that it is high there also.

On a little different subject, you had mentioned using gas-filled windows. I am a little higher than you, 7100' versus 6150', but most manufactures do not make gas-filled windows for my altitude. If they were sealed up with a gas fill at lower elevations, and then transported here, the reduced atmospheric pressure would cause the windows to bow outward and possibly crack. A few manufactures make windows with expansion bellows (e.g., Alpen in Boulder, but they are expensive), and they can be gas filled, but you might need to worry about that for your altitude. In my case, I just went with air-filled windows.

It would be handy to evaluate the effect of U-value and SHGC for windows in your location. In my case, the SHGC was much more significant than the U-value. You could use RESFEN, which is subject to some problems, or BEopt coupled with EnergyPlus, which is more accurate but more difficult to use, or the Borst Engineering software which may allow such a comparison (you have used Borst but I have not).

A disadvantage of using window quilts as opposed to a lower U-value windows is the greater potential for condensation on the windows with the quilts. If they are vinyl or fiberglass windows, the condensation should not be too much of an issue due to the insensitivity of those frame materials to moisture. Further, in Colorado the low humidity makes condensation less of a problem than in many other places. So the window quilts might be a good approach.

Posted By DAND on 19 Dec 2013 01:40 PM
 Without taking a look at specific windows I do not see the dollar cost over time value outweighing the simple window quilt. I'm open to arguments for and against the triple paned, gas filled windows. I simply don't know enough about windows to make an informed call. From what I have read you will never get much beyond R-6 for the triple pane so I would still need the quilts.

In the end I believe you will be happier with a high-quality triple pane window. Price is of course a factor but if you go with a uPVC triple pane window like Intus, you can get them for around $20-$35 per square foot of glazing. A lot depends on the sizes, amount, and options you get. When on a budget you can simply go with white uPVC on both sides and you are always better off cost-wise going with one larger window than two smaller windows. The operable windows are tilt & turn, which are some of the most airtight windows you can get.

If you visit sites like Green Building Advisor, you will see that Intus uPVC windows are a very popular choice in many of the passive solar and certified passive houses. The window comes in at around R-7 and they are super air-tight, around .01 cfm/ft² and get DP70 ratings. They are top-quality windows and built like a tank. They offer SHGC of up to 0.60 but you will have to run calcs to determine the best SHGC for your home project. As mentioned previously, there are free programs but I have found RESFEN to be quite problematic in its calculation results. I would go with the BEopt or the Borst programs.

Posted By Lbear on 19 Dec 2013 11:46 PM
...snip...
They offer SHGC of up to 0.60 but you will have to run calcs to determine the best SHGC for your home project.
...snip...

The highest solar heat gain coefficient (SHGC) that I see in the NFRC (National Fenestration Rating Council) database (http://www.nfrc.org/ then http://search.nfrc.org/search/searchDefault.aspx) for Intus operable tilt/turn windows is 0.40, and for fixed windows is 0.47. The SHGC of 0.60 might be attractive for heating-only parts of Colorado, but I do not see it listed in the NFRC database. And if you don't have an A/C, common in the mountain areas of Colorado, you do want operable windows. Intus windows have an argon gas fill, but can be used at high altitude, which is an advantage.

It is possible to get operable windows with a SHGC of at least 0.49 that meet local code (are under U=0.35), and they work well for my climate (high solar insolation) and house design.

Dan-
Do you have A/C?

No A/C, I will have ceiling fans only. I considered mini-splits, but 6100 feet knocks the heat down, and the SIP's should keep out the heat, with proper roof overhangs, and cross ventilation. I lived in Denver 5,280 feet, for 25 summers and never had an A/C. And the houses I lived in were NOT insulated. It did get hot for maybe 60 days, but I migrated to the basement to sleep. On the west slope, with proper site orientation, and SIP's I doubt if I will need A/C, but the mini-split can be a retrofit if I need it. I lived in Bangkok for 4 years (I needed A/C ALL the time) and had mini-splits there. I liked them because they are point of use friendly. It took 10 minutes to cool off my condo after a 3 week business trip. (No insulation there, just concrete and aluminum).

Posted By Lee Dodge on 20 Dec 2013 02:49 PM

Posted By Lbear on 19 Dec 2013 11:46 PM
...snip...
They offer SHGC of up to 0.60 but you will have to run calcs to determine the best SHGC for your home project.
...snip...

The highest solar heat gain coefficient (SHGC) that I see in the NFRC (National Fenestration Rating Council) database (http://www.nfrc.org/ then http://search.nfrc.org/search/searchDefault.aspx) for Intus operable tilt/turn windows is 0.40, and for fixed windows is 0.47. The SHGC of 0.60 might be attractive for heating-only parts of Colorado, but I do not see it listed in the NFRC database. And if you don't have an A/C, common in the mountain areas of Colorado, you do want operable windows. Intus windows have an argon gas fill, but can be used at high altitude, which is an advantage.

Intus Windows does offer operable and fixed triple pane windows with a SHGC of 0.620 and 0.494 and 0.373. These windows have been used in numerous certified Passive Houses throughout the US, Canada and Europe. Intus is in the process of getting the updated window types to be tested by the NFRC but as you know, NRFC testing is completely voluntary and is not a requirement. The expense of the testing falls upon the manufacturer. There are many other Passive House window manufacturers who also did not test with NFRC but the windows were tested by the Passive House Institute. Intus is tested and falls under the EN410 and EN673 European classifications.

So if you are looking for operable windows with a high SHGC, Intus does offer them. You can get 0.494 and even a 0.620 if that is what you are looking for.

When NFRC tests for the SHGC, it combines the entire window assembly; the glazing, the entire frame, and the spacers, in order to attain an average SHGC. The European model is done differently. In Europe they don't combine the framing and spacers into the SHGC number. It's the glazing alone. So a European SHGC of 0.620 would be the glazing only. The argument about which is the better method is up to debate among experts but the argument that it is more accurate to rate the glazing as a unit in and of itself seems to make sense. Since we know that the framing is of course a SHGC of basically zero on a thermally broken uPVC window like Intus. Instead of doing an "average" like the NRFC does, in Europe they rate each individual part of the window assembly in and of itself.

For instance; an Intus window can have a glazing U-Value of 0.088 (R-11), a glazing SHGC of 0.494, and the window frame will come in at U-Value of 0.167. So these numbers are individually inputted into the PHPP software to run the calcs for a Passive House.

Dan
I also do not need A/C at 7100 ft. But you will need operable windows then. For my location, the shgc dominated the u-value for energy savings. You would need to run the numbers for your location. I would take the numbers from the nfrc website, so everything is apples to apples. Do not be led astray by folks quoting glazing-only numbers and calling them window values.

Posted By L Dodge on 21 Dec 2013 09:49 PM
 
Do not be led astray by folks quoting glazing-only numbers and calling them window values.

You are "L Dodge", I assume that is a play off of "Lee Dodge?"

Nobody is leading anyone "astray" because the numbers are legit numbers that have been used and are used by numerous Certified Passive Houses and numerous passive homes. If you visit sites like Green Building Advisor, you will see that the glazing numbers that are used by manufacturers like Intus are substantiated and they are used in scientific modeling like PHPP and the PHIUS.

will I retain enough extra heat gain from my radiant heat floor to justify the extra cost of the additional 2" of concrete?

Very difficult to get enough insolation onto normal residential slabs to justify the additional thickness of concrete. As other posters have indicated, they are best treated as normal radiators and making them any thicker results in sluggishness and increasing loss of efficiency.

An exception to this would be a room designed to be a total heat sink, like a sunroom or a room or hallway containing a trombe wall. In those cases, you want to very carefully calculate the actual amount of insolation that is coming in the room subject to the apertures (windows), and the absorption characteristics of the mass to see if you really can collect enough of it to saturate the slab. As I already noted, in most cases, the answer is no. The daily motion of the sun makes it nearly impossible to warm up anything thicker than 4".

In my sunroom, which has a glass roof and is wide open to insolation, calculations showed that a 6" slab would be occasionally saturated, with the possibility of needing up to an 8" slab. However, we opted to stick with the 6", in order to improve the year-round radiator characteristics of the slab as a normal room heater.

ETA: You might also do something different in the case of a room or hallway that can be isolated from the rest of the house should the need arise.  My sunroom is such a room and, because it has proven to be such an excellent solar collector, I am thinking in retrospect that we should have gone towards the 8".  However, I do have to point out that I have not yet considered any data on how that might have worked to the downside, during regular floor heating operation.