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25% rule

rktect 1

rktect 1

Gold Member
Joined
Oct 20, 2009
Messages
1,112
Location
Illinois
Ok, so lets look at a 2 story house. Located in zone 5. It requires R-20 or 13+5 (footnote h). The house is entirely sheathed, there is R-13 batt in the walls, and a note on the building section to use R-2 over all plywood sheathed exterior walls. So you read footnote h. It says if struct. sheath. covers more than 25% of exterior, struct sheath. shall be supplemented with insul. sheath. of at least R-2.

I can't tell if that means a cont. sheathed house can then use the R-2 with R-13 instead or requireing R-20 batt. Any thoughts? I have it in the back of my head that once it is cont. sheathed it has to be R-20.
 
a cont. sheathed house can then use the R-2 with R-13 instead or requireing R-20 batt.
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Correct

JMHO R-20 is for 2x6 studs the 13+5h is for 2x4 studs. The fiberlass insulation should be in contact with the interior wallboard and exterior sheathing. Using R-13 in a 2x6 wall will provide an air space either at the wallboard or the sheathing or both providing a place for moisture to collect.
 
The air space causes moisture to collect?

I don't understand it but at at 2 energy classes they where explaining that if an air space was formed within a batt insulated wall the warm air and moisture that is working its way out thru the wall when it enters the cold airspace void condensation can form on the surfaces creating the voids.

We don't always get the best instructors in this area so if it is off base please let me know.
 
I would say that statement is an oversimplification, , , , vapor inside of a wall cavity, unless it can escape first, is going to condense when the dew point is reached on whatever surface happens to be there, in an air cavity, or inside the batt insulation. If the insulation is not vapor permeable and there is an air space, then yes, the vapor is likely to condense in the air space on the cool surface and run down to the plate/sill. And of course, there is no provision for letting it out generally. The vapor is going to get in, and it needs a way to get out.
 
that's precisely the reason we see a lot of sprayed icenyene insulation around here... you get more bang for the buck and no requirement to use the supplemental insulated sheathing...
 
I wanted to resurrect this thread to see if we had any more thoughts on this.

Zone 5, R-13 batt in the walls, 1/2" plywood sheathing on entire house with R-2 rigid board on exterior side of 1/2" plywood is code compliant.
 
I specifically asked this in an ICC-provided training seminar on the '09 IECC, and what you are suggesting is correct, rktect - if the walls are 25% solidly-sheated (or 100%, as the case may be), then a continuous R-2 is compliant. I haven't seen anyone use this approach; I don't know what product they would propose to use if they did that would make that approach feasible?
 
Code Neophyte said:
I specifically asked this in an ICC-provided training seminar on the '09 IECC, and what you are suggesting is correct, rktect - if the walls are 25% solidly-sheated (or 100%, as the case may be), then a continuous R-2 is compliant. I haven't seen anyone use this approach; I don't know what product they would propose to use if they did that would make that approach feasible?
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Why? What is the problem you see with it?
 
I had also asked about this once at some group discussion and I was told that the word you want to look at in footnote H is "supplemented". For me it doesn't really change anything but for him apparently it does.
 
I don't see a problem, but I likewise don't see a benefit. From a cost standpoint, I would think plywood(osb, etc) plus an insulation product could potentially be more costly than 1 layer of R-5 or a 2X6 wall. Don't know - never really ran the numbers, but I never see the 'spark' in any of the builders' eyes when I mention this as an option.
 
Neophyte:The "energy fanatics" are trying to stop "thermal bridging" by putting the styrofoam on the poutside, what thye are ignoring is the environmental fanatics are trying to bewn styrofoam on a worldwide basis, we not only have a Pacific Gyre twice the size of Texas now, but now an Atlantic Gyre full of styrofoam and other plastics. Many cities are banning styrofoam in plates and coffee cups, the garbage companies don't want to take it in the landfills becasue it blows all over the place and into the oceans.


View attachment 183

Our styrofoam homes ending up in Japan and in the bellies of fish and birds.​
View attachment 183

/monthly_2010_09/foam1.jpg.21549f5cc2757f5c25312e9b13c29126.jpg
 
rktect 1 said:
I wanted to resurrect this thread to see if we had any more thoughts on this.Zone 5, R-13 batt in the walls, 1/2" plywood sheathing on entire house with R-2 rigid board on exterior side of 1/2" plywood is code compliant.
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Correct per footnote h.
 
Conarb,

You're quite right about the intent being to eliminate the thermal bridging. In fact, I understand that is precisely why, in climate zone 5, they increased the frame wall R-value from 19 in the 2006 codes to R-20 in the 2009. Neither fiberglass nor cellulose will achieve the R-20, so it eliminates the former solution of simply framing with 2x6 walls. What I hate about this, is that I'm in a 100% solidly-sheathed market - wall bracing was practically a non-issue for me. Now the builders practically have no choice but to box with styrofoam.

On the upside, it does make things easier for the home burglar industry, in that all they need now to break in to a house is a utility knife - cut through the cheap vinyl siding and the foam and they're in!!
 
good point Neo..

there isn't a good solution to the structural stability/insulation/eliminate the thermal break issue. Account for one - there's another issue.

I've seen solidly sheathed structures will additional foam insulation installed then topped with the exterior cladding.. to correctly install vinyl siding, I haven't found fasteners long enough to maintain the manufacturer's requirements for the siding that will keep the cladding in place with wind.
 
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