When you compress insulation you shrink the air spaces which permits the solid
material in the insulation to come into more contact with other parts of the
solid material and therefore conduct heat (or cold) more easily. I forgot
though that you were intending to use foam. Most foam won't compress much so
that isn't so much of a factor. Another advantage is that most of the foams are
so-called closed cell. This means that pretty well each bubble is a complete
independent bubble and air cannot readily pass from one bubble through another.
This makes more effective insulation and it also produces more vapor resistant
barrier so you should have less vapor into the attic space and therefore
reduced need for ventilation.
I suspect you will want more insulation than just enough to fill the 2 by 4
bays between the ceiling joists though. You might want to install a series of
peers to the depth of the intended insulation then lay joists on them to
support the plywood so you can get 8 or 10 or even more foam over the ceilings.
you might also suspend the platform from the rafters.
----- Original Message -----
From: Spiro
To: blindhandyman@yahoogroups.com
Sent: Wednesday, October 21, 2009 4:09 PM
Subject: Re: [BlindHandyMan] additional about spray insilation question.
thanks for your reply.
I am thinking about a quarter of the space at most; say 5 or 6 feet to
each side of the access, and the same or a little more along the beams.
144 is about a seventh of the total to be accurate.
I'd like to put a 2x around the perimeter.
I would like to ask about your statement that compressing the fiberglass
will lessen it's insulating properties. Is that due to there being less
air as a heating cooling medium; or did I misunderstand?
Is it then fair to assume that the styro would not be wise regarding these
vapors to save with handling all that fiberglass?
On Tue, 20 Oct 2009, Dale Leavens wrote:
> That probably depends on just how cold it gets in that attic space and how
much ventilation there is above the plywood. The plywood won't be air tight so
there will be at least some migration of moisture up through it which should be
carried away by even modest ventilation. If the attic space though gets really
cold then you may find moisture is condensing on the under or warm side of that
plywood which will wet the insulation and if enough collects may begin wetting
the ceiling or allowing mold or mildew. It will also depend on just how much of
the area you cover.If just a four foot strip down the middle it is unlikely to
be enough to cause a problem but if the entire space then it could well be.n
>
> Another concern is that the plywood does not compress the insulation as
this will reduce the efficiency. Often people will put 6 inches of insulation
between the ceiling joists particularly if it is fiber glass bats or they will
run a second layer of thicker insulation across the top of the joists.
>
> Is it your intension to create a floored space to allow for storage? It it
is just to get around up there I have made sort of duck boards to put down over
ceiling joists to crawl or walk along on and which I can pick up when not in
use.
>
> Usually the ceiling is the most effective place to put insulation and
usually more is better though there is a diminishing return curve.
>
>
> ----- Original Message -----
> From: Spiro
> To: blindhandyman@yahoogroups.com
> Sent: Tuesday, October 20, 2009 12:58 PM
> Subject: Re: [BlindHandyMan] additional about spray insilation question.
>
>
> considering this evaporation Dale
> is it then unwise to tack down something like 3/8 ply down over the
> insulation between the two by's?
> thanks
> I've obviously not done anything yet as it was way too hot up there, and
> so dusty I couldn't breath.
>
> On Mon, 14 Sep 2009, Dale Leavens wrote:
>
> > A certain amount depends on the climate where you are applying the
insulation.
> >
> > The reason you ventilate the roof space is to keep it cool in hot weather
and to evaporate moisture which leaks up into the attic space in heating
weather.
> >
> > Usually you wouldn't want to be ventilating a part of the living space
allowing heat to escape in heating weather or cooling to escape during cooling
season.
> >
> > If you insulate under the roof deck you will retain all of the radiant
heat from the sun right there. The insulation wont allow any of that heat to
dissipate. That may help to keep the living space cooler to some point at least
but the shingles or what ever else the roof is made of will become outrageously
hot in the sun.
> >
> > Think of the roof as weather protection only.
> >
> > Well, that is the usual situation.
> >
> > In fact my roof is insulated sort of. I put an addition onto a house
which is a story and three-quarters, that is, the walls front and back slope
and become the ceiling. I did not wish to use the same balloon style
construction for the addition and didn't much like having that top bit of the
wall only the four inches thick of the roof rafters so, while keeping the
similar design inside I built a second roof that runs over the entire first
roof and the new addition. I then insulated over the original roof and the
dummy ceiling part of the extended roof with 18 inches of glass fiber. this
also allowed me to run a continuous poly vapor barrier right over the original
roof and under the insulation and second roof. I have insulated under the
original roof as well, over the warm side of the vapor barrier. The attic space
then is part of the heated environment and the old and false roof actually
perform the function of walls. I had a hell of a time explaining to local
builders
h
> ow
> t
> > his works but believe me it works and works well.
> >
> > There is a about 8 inches of free air space under the actual roof deck.
> >
> > Probably the most important factor in energy efficiency is to have an air
tight envelope and control the ventilation, to keep conditioned air in and
unconditioned air out. You then must control ventilation to keep the air
quality satisfactory then you must add insulation to keep heat transference
from conduction and radiation under control. The direct movement of warm and
cold air though is easily the most important factor in comfort and energy
saving.
> >
> >
> >
> > ----- Original Message -----
> > From: Shane Hecker
> > To: blindhandyman@yahoogroups.com
> > Sent: Sunday, September 13, 2009 11:17 PM
> > Subject: Re: [BlindHandyMan] additional about spray insilation question.
> >
> >
> > I could see that if the vents were blocked. But if they aren't, wouldn't
they work as before? As far as the ceiling goes, I thought when you sprayed
foam under the roof deck, you turned the attic into a part of your living
space. I'm a bit confused as everything I've seen suggests you can do this.
They say if the roof is ventilated, don't block the vents, but that it still
works. But it's also said the best way to do it is to not ventilate the roof
and spray the entire underside with foam.
> >
> > Shane
> >
> > ----- Original Message -----
> > From: Dale Leavens
> > To: blindhandyman@yahoogroups.com
> > Sent: Saturday, September 12, 2009 8:29 PM
> > Subject: Re: [BlindHandyMan] additional about spray insilation question.
> >
> > He wouldn't spray the foam under the roof deck, that would negate the
point of ventilating the space and further, he would lose the air sealing
effect he would gain from covering the ceiling.
> >
> > ----- Original Message -----
> > From: Shane Hecker
> > To: blindhandyman@yahoogroups.com
> > Sent: Saturday, September 12, 2009 8:25 PM
> > Subject: Re: [BlindHandyMan] additional about spray insilation question.
> >
> > So long as you don't block the vents you will be fine. Do you have
recessed lighting? If so, be careful with those fixtures (especially if they
are older) as they can get hot. Do you plan to spray foam on the floor or under
the roof deck? Either one will work, but if you spray under the roof you will
turn your attic into a storage space, which will be only a few degrees warmer
or cooler than your living space.
> >
> > Shane
> >
> > ----- Original Message -----
> > From: Spiro
> > To: blindhandyman@yahoogroups.com
> > Sent: Saturday, September 12, 2009 4:05 PM
> > Subject: [BlindHandyMan] additional about spray insilation question.
> >
> > My attic has opening to allow blow through.
> > Could I have this spray job done to save me long hours of dragging either
> > pink or stryofoam up into that dusty oven? How will the flow of air work
> > if it gets sprayed up there?
> >
> > On Sat, 12 Sep 2009, aadorno wrote:
> >
> > > hi listers I am thinking about getting spray insilation for my house.
the way this works is that they make a hole in the wall or floor and they spray
in the insilation and after a bit it drys and expans in the space that it was
sprayed in.
> > > my question is to the list is this.
> > > is any one on the list ever had this done to there walls. also is it
safe?
> > > well that's it.
> > > thanks for your time, Angel.
> > >
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