On Sep 22, 7:02*pm, Tony Hwang wrote:
TomYoung wrote:
On Sep 22, 1:55 pm, "HeyBub" wrote:
TomYoung wrote:

The roof joists on this flat roof seem to have been ripped to a 9"
width. *Previous insulation was 4" batts, leaving a large air gap
between insulation and roof deck. *The roof has a continuous vent
around the eves, so there should be plenty of air flow up there.
The batts I bought are listed at 9 1/2" thick, so they completely fill
the joist cavity, ceiling to roof. *Is this acceptable? *If not, can I
simply remove an inch or so of the batt's thickness, creating a 1" -
2" airspace?
If the answer to my question is "Yes", is there any easy way to reduce
the batt's thickness, other than just snipping away with scissors?
TIA, again.
Tom Young
Compressing batts merely reduces the insulating capacity of the batts; it's
okay.

Hint: Not necessary for your application, but an easy way to cut batts (and,
incidentally, foam rubber) is with an electric carving knife.

I know that the slight compression of the fiberglass would cause a
small reduction of the R-value, but that's not what concerns me. *I
thought the idea behind having an airspace between the insulation and
roof deck along with air circulation in this space was to allow the
insulation to dry out if warm moist air comes up against the cold
bottom side of the roof and condenses, getting the insulation wet.
No?

Thanks.

Tom Young

Hi,
Where do you live? Maybe cost is a factor but if you do this with rigid
panels it'd be easier and gets more R value out of it.

I live in the "East Bay" area of the San Francisco bay area.
Generally a pretty warm and dry late-spring/summer/early-fall season
with winters frequently cold, sometimes fairly wet depending on El
Nino, etc. Since this flat-roofed addition to the house (built before
I bought the house) has continuous venting all along the overhang it's
clear that it was designed with ventilation between the insulation and
roof deck in mind. Original insulation was 4" fiberglass, which left
5" of air space above.

Tom Young