[Greenbuilding] insulating on sheathed roof

[LarenCorie]

From: "John E. Beeson" <jbeeson at quinnevans.com>

> I can see that removing the roof sheathing and spraying foam
> in the rafter bays could produce a tighter seal

I can not.  For sealing up leaks, the first thing to do is to
decide on an insulation that eliminates the venting of the
roof.  Foam will do that, but is prohibitively expensive.
It is a lot cheaper to just keep paying the higher bills.
However, there is a financially viable solution, that
requires far less work.  That is dense-pack cellulose.
It has an Rvalue of 3.8/in and requires no vent space,
so the increase in Rvalue will be very impressive. It
is also fast and cheap.   It can be blown in, in many
different ways, all that do not require the removal
of the sheathing.  From the inside, it can go through
1" (or 1-1/8") holes about every five feet, or a long
hose can be fed through a single hole at the top of
each joist space, then gradually pulled up. I like the
multiple hole technique, especially when there is
existing fiberglass in the cavities. The cellulose will
compress the fiberglass, and fill in around it. dense
pack cellulose is very good at stopping air leaks.
The same two techniques could also be used, by
accessing the cavities by going through the roof,
instead of the ceiling. The material cost for dense
pack cellulose is about like fiberglass, and with
most purchases a 24hr rental of a blower is free.
Prep before getting the blower, and it should
only take a few hours to fill the whole roof.

>  does it make sense to trash all of the fiberglass batts

Yes.  It is not even close to worth the cost of salvaging it.

> and existing sheathing?

The sheathing, and even the existing shingles can remain.
Another problem with tearing off all that stuff, is that the
interior ceiling (and the whole interior) will be exposed
to potential bad weather.

> I would concur with Ben, the foam will provide a much
> better seal.

So will cellulose, for a fraction of the cost.

> Trashing the fiberglass - please no!

When it is a matter of having to tear off the whole roof
in order to salvage it, just leave it in the roof, and blow
in cellulose around it.

>  Imagine all that energy to make that stuff.

Imagine all the energy to salvage it.
Just let the new insulation compress
it, and leave it in the roof.

-Laren Corie-
Natural Solar Building Designs, Since 1975
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