Replies

1. 
   deadnuts | Aug 21, 2015 08:10am | #1

   Of course Joe Lstiburek makes great sense. He has solid science behind his recommended best practices. You're obviously concerned about the transfer of heat energy within your detached garage structure (I would be too) otherwise you wouldn't  be otherwise insulating it. 

   So, the real question is: Why follow Joe at all if you're not intending to follow his advice?

   The only reason I could see not following his advice is a cost benefit analysis that determines time and money spent on perimeter insulaiton never pays for itself in terms of comfort and protection from the elements. Only you could determine that at this point.

1. 
   damunk | Aug 21, 2015 01:01pm | #7

   Joe chooses 4' on horizontal rigid

   He states in his book that he choose 4' on horizontal under slab around perimeter because 2" rigid is sold in 4' wide, so there is your science that you reference. But of course they are also sold 2' wide.  And the 4' wide has snap lines (I think) on 16"s.....  So I continue to wonder how far vertical ( Frost line never greater than 20" here) and how far horizontal, to insulate........ The edge is a no brainer

1. 
   deadnuts | Aug 21, 2015 01:43pm | #8

   No. I'd say that's Joe's practicality speaking.  His building science has to do with respecting the movement of heat energy, moisture migration, etc.His books are full of practical building details coupled with building science. They're not mutually exclusive of one another.

   The earth's temperature (at least the part we deal with on a building basis) equilabrates to around 52-54 degrees F. That may not seem very cold to you, but if you didn't insulate your 98.6 F body from it, you'd fairly quickly succumb to exposure. If it is sub zero outside temp. then the earth is constantly giving up quite a bit of heat energy to the atmosphere. Some folks find it very valuable to dampen that phenomenon.

   So, in answer to your question of how far you should take establishing your thermal barrier: As far as your wallet can reasonably take you. Some folks in very cold climates go to the lenghts of insulating sub-grade around the outside perimeter of there homes in order to minimize loss of heat energy. Sometimes they extend this thermal barrier 4-8 ft away and anywhere from 2-6" thick. You may not find that particulary cost effective, but some do. If you feel snapping a 2" thick x 4' wide board in thirds offers you the best value you an afford, then go for it. Joe apparently disagrees you. Then again, he's not buying your book.

1. 
   mark122 | Aug 21, 2015 10:08pm | #9

   you sure about this

   like you were about knowels and his theory of 3+ " requirement to meet vapor barrier ?

1. 
   deadnuts | Aug 22, 2015 04:03pm | #12

   It's not his theory, it's his quote.

   And it refers to his best practice advice with regard to meeting requirement, not min.  In fact, the code (R601.3) does not state what the minimum thickness of any specific foam product need to be. It does state certain perm rating ranges required to meet each of the 3 vapor retarder class ratings. However, even zone 8 can be exempted from IRC class I and II vapor retarder requirements under certain conditions based on the work of Joe Lstiburek and other building scientists. The reasoning behind this is a reduction in vapor drive and liquid capillary action possibilities within wall cavites by virtue of properly installed WRB's and reaching min. R-values (not thickness) needed to avoid any possiblity of dew point temperatures being reached within wall cavities of certain climate zones as outlined in TABLE R601.3.1.

   You're so far out of the loop, Mark, that it isn't even funny. That's why someone like you likes to latch on to an ambigous number like 1.5" put out by the chemical industry and cling to it for dear life. I imagine it's due to making your small world managable. Heaven forbid your brain actually working to understand the concepts that lead to construction best practices which are constantly evolving with new technology and building science.

2. 
   damunk | Aug 22, 2015 09:21am | #10

   Quoting Joe directly

   "How far inboard should you insulate horizontally? We pick 4 feet. Yup, that is the width of the typical rigid insulation sheet and we go with it. Everywhere? Pretty much. Is there ever any reason to insulate the entire slab besides the perimeter? Yes, Grasshopper, whenever you have in slab heating or whenever you have hot-humid summers like you have in Maine, Massachusetts, and Michigan. Those “M” states are a pain. The ground under a slab is still often cold in the summer when it finally gets nice outside in the “M” states and the top of the slab can fall below the dew temperature of the outside air-vapor mixture when folks are finally brave enough to open the windows and doors."  So, I love Joe and his writting - and all bourbon drinkers get along real well, but i can't justify insulating the whole thing because my detatched garage slab is going to sweat a little in the summer  I'm hip to keeping the thermal bridge broken at perimeter for what you talk about winter and all, but........

1. 
   deadnuts | Aug 22, 2015 03:11pm | #11

   You might be hip to it, but just insulating at the perimeter doesn't mean you have actually broken the thermal bridge. You may have reduced it, but it's not broken. The only way to truly break the bridge is to insulate continuously. That means under the entire slab and up the inside of knee walls to framing or under footing and up the outside of knee walls to framing. And we haven't even disucssed thermal bringing within wall and ceiling envelope. Have you truly covered thermal bridging there as well?

   For the record, we usually spray closed cell foam under entire concrete slabs for our projects to an average thickness of about 3". We feel this avoids the meticulous grading needed to avoid bridging of sheet good foam installation, though both options are perfectly viable. We always include it if their is radiant floor heat. We discuss the pros and cons of doing so with our clients and are clear about continuous underslab insulation not being required by code. 9 out of 10 opt this for detail and pay accordingly. We even had a client opt for this in a detached 24x36 garage that was very well insulated for a non living space. It was unconditioned save for an occasional portable space heater. To this day, this client still raves about what a good choice is was to insulate the slab of his detached structure. It has help keep the temperature fluctuations within his garage to a minimum with very little energy input.

   
2. 
   damunk | Aug 22, 2015 05:00pm | #13

   But if I do that.....

   insulate the entire slab, wont I be insulating myself from that free heatsink called earth?  The garage is not being heated, if you recall, only insulated at perimeter.......I dunno, for sure, like, that is why I raise the question.................and that is very smart of you to insulate if their (sic) is radiant floor heat.  Your a good contractor!!!

3. 
   deadnuts | Aug 22, 2015 11:35pm | #14

   You're forgetting that a concrete slab can also acts as heat sink. Passive solar practitioners use it all the time.

2. 
   DanH | Aug 21, 2015 08:23am | #2

   Insulating below the slab may prevent the ground from freezing and heaving.

3. 
   damunk | Aug 21, 2015 09:37am | #3

   You really think the ground freezes?

   With perimeter walls going down 42" and a building/envelope above the ground you really feel the earth freezes enough to heave a slab?  I have zero water issues at this location. Ground slopes away and I'm at top of hill with trees all around

4. 
   AndyEngel | Aug 21, 2015 09:54am | #4

   Why would you not?

   The only reason I can think of is cost. But if you ever decide to heat that garage, you'll be glad you did. That said, frost heaves in a garage slab are very unlikely unless ground water gets below the concrete. That shouldn't happen. 

1. 
   damunk | Aug 21, 2015 11:47am | #5

   Very good point.

   I don't see it being heated but maybe after my wife throws me out I'll have a warm (er) slab to sleep on. No, I see ur point. Just because I don't see it heated, maybe down the line. Same reason my friend told me to throw an extra conduit in trench - in case someone decides to run fiber to garage for an Internet connection or whatnot

5. 
   damunk | Aug 21, 2015 12:43pm | #6

   I hear you

   i got "good dirt". Thx