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?
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.
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
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
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
At "high noon" you get about 1300 watts / sq meter (not being fully
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
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.
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.
firstname.lastname@example.org 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?
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