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Skimcoat of closed cell foam...

jfyrad's picture

Hi Guys,

I have an aircraft hangar here in Northern Wisconsin that I'm in the process of adding some partition space to.  I'm going to have the ceiling done in closed-cell foam, and one of the insulation contractors suggested spraying a skim coat of the same product on the inside of the walls' tin sheathing before I install the fiberglass batting.

The building construction is steel truss, with 2-by wood gerts and purlins, and a tin exterior.  My construction project entailed building partition space inside the structure leaving about a 6" air gap between the original outside wall, and the load bearing walls of the new shop space/storage.

What I'm wondering is, is it a good idea to put a vapor barrier on the inside of the outside sheathing, then have 6" of fiberglass, 6" of dead airspace, another 4" - 6" of fiberglass, and interior vapor barrier, before my drywall?  Seems like a moisture trap to me.



The insulation method you are (post #207020, reply #1 of 11)

The insulation method you are talking about is called "Flash & Fill", which aims to be cheaper that purely foam, but better sealler than batts or fill alone.

I think the inside metal is an excellent condensation surface.  You can either build a structure to keep the vapor away from that surface OR directly coat that surface with foam to keep condensation off.

Instead of 2 layers of insulation with a gap between, why not build the inner wall closer to the outer wall, then after flashing with foam you cover the studs with InsulMesh and fill it with cellulose?  You will get better air sealing and more insulation for less money.  If you need a space to run wire and low voltage, you can run horizontal furring and drywall over that.


Look up "Mooney Wall" for all the details of this method, it might be perfect for you.

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Thanks for the reply. Not (post #207020, reply #2 of 11)

Thanks for the reply.

Not sure if you can tell from the picture I included, but the inner structure is already up, and the exposed concrete between the inside and outside has been etched, and coated with Xypex 

The new shop and storage areas were built to clear the trusses and back wall support columns, eliminating the need to notch my staircase in two spots, and to give me a smooth interior surface.  It also has the benefit of breaking the thermal bridge between the inner and outer walls.

Which leads me back to the original question.  If I do go "Flash & FIll" or in this case, flash and batt, is it wise to put a second vapor barrier under the finished interior wall?  The air space between the two walls could be vented into the hangar proper, (rather than outside), during warmer, moisture laden months to help evaporate anything that manages to get in there.

I missed that pic (post #207020, reply #5 of 11)

I missed that pic before!

Seeing as you are in North Wisconsin, and looking at the insulation map:

I think you have a couple options, depending on how much you want to spend on insulation.  Don't split your insulation layers, you just create an inbetween "outside" zone.

You can flash foam the metal - that will help keep out outside rain, humidity, and other water penetration.  Then you could apply the insulmesh in that 6" gap to the outside of the interior stud wall.  Labor intesive pain in the azz, but it could be done. Then fill the remaining 8.5" space with cells, giving you a total R36.

The easiest, though more expensive material wise, it to flash the wall with foam then fill the ENTIRE cavity with cells.  This gives you R50.

Sell it on smaller more efficient HVAC equipment and lower energy costs, more comfort for the workers inside.

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Thanks for the feedback thus (post #207020, reply #7 of 11)

Thanks for the feedback thus far.  I've been poking around a bit more on sites like Building Science, looking at hybrid wall systems.  The air gap between the walls was kind of unavoidable.  But now I'm really not sure how I'm going to deal with it.  Filling the cavity makes the most sense, but the expense of it... ewww. Lol

The problem with an air gap (post #207020, reply #9 of 11)

The problem with an air gap is mainly that air can circulate, connecting the "weak link" in one layer to the "weak link" in the other.  This is especially a problem for vertical areas.  If you can break up the gap into smaller areas (with some sort of airflow barrier -- kinda like firestops) you eliminate most of this.

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This is something I really (post #207020, reply #10 of 11)

This is something I really never thought about.  Considering that air gap is 20' tall along the side walls, and 30+ in the back, it would only make sense to block it at intervals.  Thanks for the heads up Dan.

Is it impossible to just move (post #207020, reply #8 of 11)

Is it impossible to just move the walls back that 6"... or remove them temporarily while you properly insulate those walls?

It would be easier to run vertical furring and attach insulmest to that, then fill with cells.  After that you could stand your wall back up again and ignore the space.

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Yeah, impossible is about the (post #207020, reply #11 of 11)

Yeah, impossible is about the best definition.  The second level floor system, and some of the walls are already up.  Not to montion the staircase and bridge connecting the two sides.  I'm just going to have to make the best of what's there.  Thanks again for the feedback.

Jeff (post #207020, reply #3 of 11)

Can't tell from the picture, are those plates and in the shot, the two studs that run down to the concrete, treated?


Will there be anything sealing the backside of your interior wall you intend to fill with batts?  If not, you'll be losing alot of R with air moving around between the two walls.

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Exactly!  This is why you (post #207020, reply #4 of 11)

Exactly!  This is why you dont add insulation to the attic floor AND THEN fill between the rafters... you end up with a big space that kills the effectiveness of both insulation layers.

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Hi Calvin, The plates are (post #207020, reply #6 of 11)

Hi Calvin,

The plates are treated. The outer one's have sill seal beneath them and are the original 12 y/o "real" treated 2x6's.

The ones at the base of the new wall are the kinder, gentler variety, that are at times, a pita to identify by anything other than weight and the tag stapled on the end.  The two studs in the shot are only temporary.  I deciced to do a little something different halfway through us framing it up, so I just threw them in there. 

I wasn't planning on sealing the backside of the interior wall.  My intent was to fiberglass the stud cavities, install a plastic vapor barrier, and drywall, or more likely, fiberglass/plastic textured paneling. The open end of the wall the picture is looking down is going to be boxed out and filled with 6" fiberglass as well.