Insulating Steel Skin

[John Nordyke]

Having a 30x40 shop space built next to new house construction. Concrete slab. The building is stick-built but with steel exterior skin and roof, and the contractor is going to spray foam insulation - but only an inch or two. To save some money I will take over after that to run my electrical and finish the walls. I will heat the space with Hot-Dog suspended propane heater, about 65,000 btu. This is in Missouri where we have plenty of subfreezing temps in the winter.

Given a steel skin and a thin coat of foam insulation, I am thinking I should supplement the insulation (foam is very expensive so trying to keep cost down) with fiberglass batts. If I do that, do I need a vapor barrier as well before finishing the walls?

[Chris Padilla]

Is your spray foam open or closed cell? There is a big difference between the two in terms of R-value: open is R 3.5/in and closed is R 7/in. Just curious.

Spray foam seals VERY well assuming it is applied correctly so I wouldn't worry about a vapor barrier. I suppose you go super cheap and install unfaced batts if you like. However, you will be slightly crushing the batts designed for a 3.5" deep stud bay. May or may not be a big deal. Frankly, with spray foam, I'd be inclined to skip the fiberglass.

You may also consider DIY'ing the spray foam. Google 'foam it green' as one example of a place that sells closed cell foam DIY kits. I've used two kits from them on separate occasions. Maybe you'll find it more cost effective and can fill the stud-bay completely full. With closed, that is R21+. Plus it adds structural rigidity as well.

[Jim Becker]

Chris is spot on. If it's closed cell (my preference) a couple inches are going to give you the equivalent of a typical 3.5" thick R15 fiberglass batt plus significant structural stiffness and air infiltration benefits.

[roger wiegand]

if you do "flash and batt" you need to be sure the foam layer is thick enough to prevent formation of a condensing surface inside the walls between the foam and fiberglass. Condensation inside the walls is a Bad Thing. There are published tables that will let you calculate what thickness is required for your particular situation. 2" of rigid foam was enough on our house in MA, so 2" of closed cell foam is probably fine for you. Open cell might not be, depending on how cold it gets.

A shop generally has many fewer moisture sources, so you may well be able to go lower than you would in a house unless you're doing something to add humidity in the shop in the winter.

You might be better off putting money into insulating the ceiling (and air sealing, of course) than beefing up your walls. Closed cell spray foam is by far the easiest way to achieve great air sealing.

Look at the greenbuildingadvisor site for lots of information on this.

[Jeff Bartley]

John,
I looked into doing this, I had planned a 2x4 wall with diagonal bracing and horizontal purlins to catch the vertically installed metal. Then 1 1/2" closed cell followed by batts.
Then the metal supplier told me closed cell against the metal would void the warranty.
Now I'm looking into the ZippR sheathing. It's the regular zipp product with a closed cell foam bonded to the inside. It's cheaper than spray foam and regular sheathing and with the tape they claim it's a tight build.
Please let us know how this works out for you!

[John Nordyke]

thanks all for the ideas and tips

[John Nordyke]

The interior side of roof (also steel) will have the same 1 inch of closed cell foam applied to it. I'll add roll-out batts when I install an OSB ceiling. Flash and batt looked good, but to keep the dew point buried in the closed cell foam I'd have to do a second inch of closed cell foam and then find "skinny" batts somewhere to install between studs.

Two inches of closed cell foam would probably be plenty of insulation in the walls from an R factor, but what, if anything, would I do about still having 1.5 inches of dead space in each stud wall cavity?

[Bill Dufour]

I have read it is cheaper to buy the rigid polyisocyranirate panels. Rip them to be 1/2" narrower then the stud bays, place them in the bays and then use the caned spray foam to seal in place.
All this exposed insulation can be a fire hazard unless it is rockwool which is fireproof. The spray foam can have added fireproofing but you have to have paperwork to show the building inspector to prove it.
Bill

[Jim Andrew]

Fiberglass does not burn either. Only the kraft paper vapor barrier. If you have room and can buy a size batt that fits, personally I would fill it with fiberglass. If you insulate the roofing metal, and put batts on the ceiling, will you not also have to vent the attic to prevent moisture build up in the attic?

[John Nordyke]

Yes, I would vent the attic. Anyone know where I get 1.5 inch fiberglass batts? I don't fancy having to split batts - it's messy and itchy.

[Charles Lent]

If you plan on leaving the foam exposed, look into the combustability of it. Some foams do not support combustion while others burn like kerosene and give off very toxic vapors when they burn. Wouldn't want that Hotdawg heater burning your new shop down.

Charley

[Mike Heidrick]

Run elec before any spray. Also vs splitting save up and spray extra inch.

[Jim Becker]

Yes, I would vent the attic. Anyone know where I get 1.5 inch fiberglass batts? I don't fancy having to split batts - it's messy and itchy.

Try a professional supply house...batts that thin are normally only used for sound abatement based on my personal experience in the industry a number of years ago. Otherwise, you'll unfortunately have to split material manually.

[Mike Heidrick]

Extruded poly styrene comes in 1.5".

[Bill Dufour]

Fiberglass does not burn either. Only the kraft paper vapor barrier.

I do not know what fire code says but I have seen fiberglass in a muffler burn. The glass itself did not burn but the glue and dyes do burn so it becomes a more densely matted white cotton ball.
Bill