Any air barrier is improved by a second air barrier. And while an exterior barrier can prevent most air passing through, it doesn't prevent air from coming from the interior, approaching the exterior wall, creating high humidity or even condensation (some conditions, even with external foam) and then returning to the interior.

On the other hand, cellulose helps (as compared to fiberglass) and so does EPS (as compared to something like foil covered PU).

If you feel strongly about "no interior vapor barrier", then you could use interior housewrap (like Typar) as a second air but not vapor barrier.

It is as Jonr says.
The air in the stud bay will cool somewhat regardless of exterior air barrier or the type of insulation. This will start a convectional air pattern in the stud bay. Now if you have both a high (light outlet maybe?) and a low (receptacle) rupture in the drywall, you will circulate the interior air through the stud bay where the moisture will condense on the cold surfaces. Hence a paint barrier only works if there is no openings in the drywall.
Now of course some insulations are better at slowing down this convectional air then others and some buffer the moisture load better then others but you still want to do your best to prevent any opening in the drywall. This would mean gasketed plastic electrical boxes or an electrical box with a plastic vapour shield around it that can be sealed to the back side of the drywall and of course the drywall sealed to the bottom plate with acoustic caulk and all electrical holes between stud bays seal etc., etc.

I see that there are concerns about the effectiveness of some types of EPS, OSB panels and Tyvek as air barriers. Caulk and spray foam are also questionable.

What are the sure things - ICF, taped Zip, plywood, XPS, polyurethane and plastic films?

Vapor barrier was the best investment on my 40 x 48 shop it stays cool in summer and warm in winter with very little cost.

Spending around 2 bucks a day to heat it right now.

13 kw on demand water heater set at 60 percent running on a timer for 3 hours a day at different times during the night and morning. Paying .086 per kw. Keeps shop around 60.

Posted By uerling on 15 Dec 2014 08:05 AM
Vapor barrier was the best investment on my 40 x 48 shop it stays cool in summer and warm in winter with very little cost.

Spending around 2 bucks a day to heat it right now.

13 kw on demand water heater set at 60 percent running on a timer for 3 hours a day at different times during the night and morning. Paying .086 per kw. Keeps shop around 60.

Methinks you mean "air barrier".   There is no efficiency to be gained by blocking water vapor diffusion, but there is by blocking air movement.  Vapor barriers are often detailed as air barriers (and should be, to limit air-transported moisture migration, which is a higher volume mechanism than vapor diffusion), but air barriers need not (and often SHOULD not) be vapor barriers.

True vapor barriers need to be carefully considered in any stackup, since they are double-edged swords, since they limit the drying paths.

It directional and perforated on one side.

http://www.archiexpo.com/prod/insul...em_1226543

It's the radiant shield.  What ever it is it works really well under the metal. 

I've got r19 in the walls.

r 50 in the attic and untapped sheet rock interior.  NO holes all my electric is conduit running outside the sheet rock.

Comes in a 10' wide roll and overlaps to make a continuous seal.

If it's perforated radiant barriers are not vapor barriers OR air barriers, though they can be both air and vapor retardent. Most perforated versions run about 5 perms vapor permeance.

The thermal benefits of reflective materials only occur when there is an air space between the reflective layer and adjacent layers, and the orientation matters. If installed between fiberglass batts and gypsum board it's thermal performance is about as good as a sheet of Tyvek or kraft paper. With the infra-red translucency of low-density fiberglass (like R19s, which are no more than a "fluffed up" R13 batt) there may be a very modest improvement due to the aluminum measurable in a laboratory, but not in the heating/cooling bill.

Must be doing something out there under the metal and a good 2 inches from the bat insulation considering in 0 degree temps I can keep in 50 degrees in there for 3 bucks a day.

What ever you want to call it; I know for a fact it is making a difference on my heating and cooling bill.

BTW did you really think I had install vapor barrier between the r 19 and sheet rock????  It's right up against the metal.

 

I have a similar situation in the office side of my shop with a 1/2" of Thermax between posts and a 3/4" air between foam and the corrugated steel ground to the ridge vent. No AC was needed summer when we finished. Glass blown in the walls with 5/8 drywall perforated with receptacles on inside of this and cellulose above to 18".

On the garage, north side, I used 1" XPS in the same fashion since solar gain is not an issue on the N. side of Minnesota buildings.

With a proper rain screen, or metal sheeting, I think the only drying will be to the inside.

Posted By uerling on 17 Dec 2014 07:24 PM
Must be doing something out there under the metal and a good 2 inches from the bat insulation considering in 0 degree temps I can keep in 50 degrees in there for 3 bucks a day.

What ever you want to call it; I know for a fact it is making a difference on my heating and cooling bill.

BTW did you really think I had install vapor barrier between the r 19 and sheet rock????  It's right up against the metal.

 

With the air gaps and the fact that it has an inch or so of fiber in the sandwich (now that you've posted the pictures,and the real link to the actual product it's clearer-what you're talking about.)  It's definitely doing something.

Yes I really did think you had installed the stuff right up against the sheet rock. You didn't indicate otherwise, nor did you mention that it was a multi-layred product with a fiber insulation core. MANY people install radiating barrier in full contact with roof deck or between the siding and sheathing of framed buildings, or even under concrete slabs in full contact with dirt & concrete.

Calling it a "vapor barrier", isn't really the right description, and a bit misleading.  The majority of goods sold as vapor barriers are only 6-15 thousands of an inch thick.  And, if it's perforated, it's vapor permeable and NOT a vapor barrier, though the aluminized layers make it a radiant barrier.

It's not surprising that a reasonably air tight building with R19 fiberglass plus a radiant-barrier layer with air-gap on a least one side and such a tiny window fraction would be cheap to heat & cool.  Having it in direct contact with metal siding cuts it's performance a bit though.

Read it again...

"It directional and perforated on one side.

http://www.archiexpo.com/prod/insul...em_1226543

It's the radiant shield. What ever it is it works really well under the metal.

I've got r19 in the walls.

r 50 in the attic and untapped sheet rock interior. NO holes all my electric is conduit running outside the sheet rock.

Comes in a 10' wide roll and overlaps to make a continuous seal."



Wow!!! Good thing I figured out how to post pictures. lol

Really don't know a damn thing just accidentally figured out how to build a building and pay for it.  And somehow it is efficient.

Also stumbled along and managed to get 2 son through UNL college of engineering.  And wow accidentally got the what every you want to call it under the metal with a inch and a half air space between it and the r 19 bats.

Just lucky I guess.

Also have hot water running through the floor on a timer set not to spend more than 2 bucks a day right at the moment.

In the summer I run the water through that ends up in the auto sprinklers around the house to water the grass.

I think I got off on the wrong foot here, will just quit posting and leave you fellas alone.

Sayranora

The interesting question is exactly how much good is it doing and are there more effective ways to get even better performance (like R22 bats). Anecdotal data is pretty worthless.

like I said sayranora

Who's nora? :-)

Don't stop posting- nobody is out to get you, just trying to understand what you are really talking about, since the details were lacking and the description differs from what most in the biz refer to as "vapor barrier".

If I understand this correctly it sounds like your stackup is:

metal siding | radiant barrier | air gap | R19 batt | wallboard

Unless there is an air-barrier tight to the R19 on both sides (the wallboard is good enough as an air barrier on the interior) it won't perform anything like R19, since there is almost certainly convective air currents in the air gap (to either the exterior or the interior. Even if installed perfectly in a wood studwall with air-barriers tight to both sides you're looking at about R12-R13 average performance for the studwall, due to the higher thermal conductivity of the framing. With the fiber insulation exposed to the air gap it's probably more like R10, maybe R11.

With air gaps on both sides of what appears to be 3/4" rock wool or fiberglass sandwiched between aluminized fabric you'd get about R4-5 performance out of the shiny stuff, maybe another R2-3 out of the fiber for about R6-7, more if the air chamber between the siding and fabric is air tight to prevent infiltration and the interior side of the siding is shiny, not painted.

Did you install it with the perforated side facing the siding, or facing the fiberglass?