AAC block construction and R-values
Hi All,
I have a question regarding building materials and code. I realize that code and inspectors vary from place to place, but I’m curious about methods of determining “true” thermal characteristics and understanding how to use them for getting past the typical need for a high R-value as the only way to determine materials efficiency.
To start, I am planning on building a house in the mountain region of Washington State. Summers getting to the 80’s and 90’s, winters compounding snow and staying below freezing. I have been researching green building and am pretty confident that I would like to try to use AAC block for a below grade and one story above grade structure (i.e. one story with a daylight basement). However, most information that I can find states that AAC blocks only attain an R-value of 1.25 per inch, so a typical 8 inch wall has only an R-value of 10 (not good). Code in the area where we’re building is R-21 for walls.
However, I know that the other dynamic thermal characteristics (specific heat and also thermal mass) of AAC block account for their improved thermal efficiency over stick framing, not to mention their improved resistance to “events” (i.e. fire, flood, insects, etc).
So, my question, is there a formula that one can use that will take into account the other thermal characteristics to one, determine if this is really the building material that I want to use, and, two, use to justify to the building department that an R-10 isn’t the complete picture in thermal characteristics and efficiency (I want to try to compare apples to apples, versus apples to oranges)?
Thanks in advance for any help and/or advice.
Replies
Not that I know of.
There are two things this relates to, though, and it's important that you understand that.
First is your "steady state" heat loss. Picture several cold, cloudy days in a row. Your thermal mass does nothing for you there and you would be in "steady state" heat loss. This is the loss that a heating designer would need to design around to size your heating system.
Second is your overall yearly efficiency. This number very well may be affected by your thermal mass, as those walls soak up passive solar gains all day and offset losses into the night, for example. You couldn't design around it happening on any particular day, but you could assume that it is going to happen some portion of the time at least. It may *perhaps* be acceptable to use an "r-value equivalence" number in a standard degree day calculation in that case, but the equivalence factor would vary based on your solar gain potential and "typical" day to day heating characteristics.. and thus, site by site, region to region, etc etc. Hard to pin down, in other words.
You could go ICF and both get the R value you want and the thermal mass you seem to be looking for, along with the structural improvements, perhaps.
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Ah, I have found some great information thanks to the folks at TruStone America (I believe formerly E-crete), manufactures of AAC Blocks.
It is the Dynamic Benefit for Massive Systems (DBMS), which is an independently researched, code-accepted multiplier to static R-values that gives an R-value equivalence for dynamic systems (i.e. taking into consideration thermal mass and heat conductivity). Probably would be a good resource for rammed earth structures, also?
Using the DBMS (which is climate specific) we get an R-value of 19.53 in the general geographical area without stucco and plaster for a 10" AAC block (20.16 for the climate in Denver, which I would imagine would be more similar to the geographical area where we are building). I'm certainly more interested now that I have something objective to compare too.
Does anyone have anything to add to this?
as long as you don't size your boiler on those numbers and use them only for yearly energy usage estimates, sounds ok to me. I'm not familiar with the group you're referring to though, but I don't find their estimate implausible.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
The 2000 IRC has a different table of required R-values for high mass walls.
Hi,
"So, my question, is there a formula that one can use that will take into account the other thermal characteristics to one, determine if this is really the building material that I want to use, and, two, use to justify to the building department that an R-10 isn't the complete picture in thermal characteristics and efficiency (I want to try to compare apples to apples, versus apples to oranges)?"
You can find some stuff that might be helpful here:
http://www.builditsolar.com/References/energysimsrs.htm
The ORNL study gives R value multipliers for thermal mass as a function of climate.
The HEED software (which is easy to use and free) will allow you to model your house with various levels of thermal mass in the walls, and get some idea of how much that helps.
I have to admit that I'm a bit skeptical of the idea of making up for low R value with more mass in the wall -- why not both? -- as in a layer of exterior insulation?
Gary