Spray on foam

[Steve B]

Anyone know how much they charge per square foot to come shoot that liquid foam onto the underside of a metal roof? The two element foam that expands and then hardens. About 1,100 square feet.

Steve

[DD_BobK]

$1 / inch / sq ft

[willshak]

Steve B wrote the following:

It depends upon the economy where you live. Ask around. Spraying it between wall studs requires an extra step of sawing it flush with the face of the studs so that wallboard can be nailed to the studs. Do you need that step?

[hrhofmann]

He said rooof!!!

[Steve B]

This will be the underside of a metal roof ...................

[clare]

AN experienced installer can make the "sawing" a virtually non-existant step.

[Steve B]

I'm just hoping for three things: ANY R value, absorption of the suns heat before it enters the enclosed space, and some water leakage protection from the spraying mist of water I will use on the top of the roof to cool during the hellish summers.

Steve

[Steve B]

I've already thought of that. It will be sprayed vertically onto a slightly sloped roof, and there will be NO need for trimming.

Sometimes it is hard being smart.

Steve

[clare]

Not if the roof is already there.

[Steve B]

One would have to lay out the roof in the configuration to receive said sheets. Said roof is laid out to receive 30.5" wide sheets, two at a time. To use said sheets would mean their seams would fall on nonsupported space.

Steve

[DD_BobK]

Be careful what you hope for.......

I hit the easy ones first............

1) R Value...... depending on the type of foam you use, you'll get some where in 3.5R per inch (open cell foam) to 6R+ per inch (closed cell). But of course there's a catch, open cell is lower density, lower strength & lower cost per inch

2) I'm not sure it's a good idea to depend on the spray foam to help seal the roof. I know it's pretty dry where you are but any roof leaks will find their way under the metal roof and perhaps give you corrosion problems. YEARS I worked a test project that involved metal roofs that had suffered major corrosion damage either due to water intrusion or water vapor from inside the building getting trapped between the foam (this was foam board, not spray foam) and the underside of the metal roof.

3) absorption of the suns heat before it enters the enclosed space....... I'll get this one started but I think I'm going to need help from some of the experienced and technical group regulars, who, I'm sure, will correct mistakes in my posting.

Insulation does not "absorb heat", it prevents the movement of the thermal energy from a region of high potential to lower potential. That is, it keeps thermal energy from going to a colder (lower temp) region to a hotter (higher temp) region.

In your roof system, sunlight hits the roof. Some (mercifully) reflects off the roof, the rest heats the roof. :(

The hot roof transmits this heat to the space below by:

1) radiation...... my bald suffers out in the shop on hot SoCal with air temps over 100F and roof temps even higher 2) conduction.... heats conducts through the metal roof and heats the air in contact

I'm no roofing insulation expert but I understand the basic principles.

One of the cheapest "insulation" techniques for SoCal tilt-up, high bay warehouse style buildings not are not conventionally insulated is to use a "reflective barrier". They staple aluminized heavy paper to the underside of the rood framing system. This creates an air space that is "somewhat" dead but more importantly the aluminum surface reflects the radiative heat coming in from hot roof deck.

here's a link to radiative barrier info

In your particular situation, I'm not sure spray foam is the best product. If your water spray system keeps the roof temp low you might not have very much radiation or conduction heat transfer...problem solved.

At "high noon" you get about 1300 watts / sq meter (not being fully metricated) I prefer 120 watts per sq foot.

So....... if you're experimentally inclined, you could rig up a little experiment with some reflector style flood of whatever wattage you want, to simulate sunlight on your metal roof.

One problem with this idea is that only a a fraction (like 12%) of the "wattage" for an incandescent flood really appears as light.

you'd have to compensate by boosting the number of flood per square foot. but there's also the issue of light frequency content..... maybe some super duty grow lights are the answer. :)

whether or not this thought experiment is worth turning into hardware..... I don't know.

The correct "insulation" method for your situation has probably already been worked out, it's just a matter of finding it.

cheers Bob

[willshak]

[e-mail] wrote the following:

Even an inexperienced installer, like me, can do the same. Just don't fill the space with enough foam to blossom out to the faces of the studs. I've seen a lot of home improvement shows where those 'experienced' foam installers have to saw the excess off. What's the point if you don't fill the spaces to the maximum?

[Steve B]

And what's the point if it is a metal roof that is not going to be sheetrocked or covered on the inside in any way?

Do you get it now?

No ?

sigh ......................................

Steve