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Raising Attic Floor for more Insulation? Bob Sisson 12-02-2006
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Posted by Bob Sisson on December 2, 2006, 8:02 pm
The house we are moving into has a Nice, Large attic with a
plywood/pressboard floor over 2x6's and 2x8's depending on the spans.

The insulation is blown in Fiberglass, but only 6-8" max.

The attic was DESIGNED for loads by the original owner/builder as I have
seen the blueprints.

What I want to do is take up the floor, put down something that will raise
the floor 6-8" and put in that much new insulation (no vapor barrier of
course).

I don't plan to do the entire attic this way, just a 10-12' "path" down the
middle, the rest can just have more insulation put on top of what's there...
but to be able to put stuff in the attic AND insulate UNDER it....

As this will have MINIMAL load on it, what would be the best material to
use?

2x6's or 2x8's would obviously work, but seem like overkill, and costly in
material.

If my Time is not worth anything, Can I build a "truss" that looks like a
comb that sits on every joist and raises a 2x4 up 6-8"s?

There USED to be product like Attic-Dek that would keep the insulation
thick, and still be easy to use...

Ideas and thoughts are welcome...

Bob



Posted by marson on December 2, 2006, 9:02 pm

Bob Sisson wrote:
> The house we are moving into has a Nice, Large attic with a
> plywood/pressboard floor over 2x6's and 2x8's depending on the spans.
>
> The insulation is blown in Fiberglass, but only 6-8" max.
>
> The attic was DESIGNED for loads by the original owner/builder as I have
> seen the blueprints.
>
> What I want to do is take up the floor, put down something that will raise
> the floor 6-8" and put in that much new insulation (no vapor barrier of
> course).
>
> I don't plan to do the entire attic this way, just a 10-12' "path" down the
> middle, the rest can just have more insulation put on top of what's there...
> but to be able to put stuff in the attic AND insulate UNDER it....
>
> As this will have MINIMAL load on it, what would be the best material to
> use?
>
> 2x6's or 2x8's would obviously work, but seem like overkill, and costly in
> material.
>
> If my Time is not worth anything, Can I build a "truss" that looks like a
> comb that sits on every joist and raises a 2x4 up 6-8"s?
>
> There USED to be product like Attic-Dek that would keep the insulation
> thick, and still be easy to use...
>
> Ideas and thoughts are welcome...
>
> Bob

I guess that would work. you would have to have some plywood gussets
or something in place of truss plates. and i bet a lot of your "comb
teeth" would not contact the underlying joist.

how about this...run 2x6's 48" oc standing up on edge running
perpendicular to the existing ceiling joists. then run 2x4's on top of
them parallel with the ceiling joists, stacked over them to facilitate
insulation installation.


Posted by Bobk207 on December 3, 2006, 1:15 am

Bob Sisson wrote:
> The house we are moving into has a Nice, Large attic with a
> plywood/pressboard floor over 2x6's and 2x8's depending on the spans.
>
> The insulation is blown in Fiberglass, but only 6-8" max.
>
> The attic was DESIGNED for loads by the original owner/builder as I have
> seen the blueprints.
>
> What I want to do is take up the floor, put down something that will raise
> the floor 6-8" and put in that much new insulation (no vapor barrier of
> course).
>
> I don't plan to do the entire attic this way, just a 10-12' "path" down the
> middle, the rest can just have more insulation put on top of what's there...
> but to be able to put stuff in the attic AND insulate UNDER it....
>
> As this will have MINIMAL load on it, what would be the best material to
> use?
>
> 2x6's or 2x8's would obviously work, but seem like overkill, and costly in
> material.
>
> If my Time is not worth anything, Can I build a "truss" that looks like a
> comb that sits on every joist and raises a 2x4 up 6-8"s?
>
> There USED to be product like Attic-Dek that would keep the insulation
> thick, and still be easy to use...
>
> Ideas and thoughts are welcome...
>
> Bob

IMO the easiest way to redo.....

Remove the insulation in the area you want to have the "10-12' "path"
down the
middle" & use the removed insulation in the areas of the attic

replace removed insulatin with foam board or spray in foam ~R7 per
inch

you get the higher R value you want w/o any modifications to the
structure....only down side is foam is more $'s than fiberglas :(

cheers
Bob


Posted by marson on December 3, 2006, 9:58 am

> Remove the insulation in the area you want to have the "10-12' "path"
> down the
> middle" & use the removed insulation in the areas of the attic
>
> replace removed insulatin with foam board or spray in foam ~R7 per
> inch

cutting and fitting rigid foam between joists is laborious and doesn't
work that well because of the inevitible gaps (plan on getting a few
cases of Great Stuff). Also expensive--5 1/2" would run you about
$1.75 a square foot. Also, I have to find out where bob is getting R7
per inch foam. in these parts, our rigid extruded polystyrene is R5
per inch. (polyiso is R 6 per inch, but its a lot more expensive and
doesn't work well if you have to cut it to fit between joists) plus,
in a 2x6 cavity, you will be unlikely to fit 5 1/2" of rigid. I think
5" or R 25 is the max you will fit in practice. considering that you
can get an R21 fiberglass batt in the same space, thats a small gain
for a lot of trouble and expense..

spray foam is an option, if you can afford it and don't mind the hassle
of tracking down a contractor. where i live, $850 is the minimum
setup charge for a contractor to blow in icynene. also, you would have
to over fill the joist cavities and saw off the excess with a saw. you
will have to mask off everything you don't want stuck full of foam.
spray foam makes a MAJOR mess.

so it's hard for me to see that foam is the "easiest way". fiberglass
has its uses.
>
> you get the higher R value you want w/o any modifications to the
> structure....only down side is foam is more $'s than fiberglas :(
>
> cheers
> Bob


Posted by Bobk207 on December 3, 2006, 11:09 am

marson wrote:
> > Remove the insulation in the area you want to have the "10-12' "path"
> > down the
> > middle" & use the removed insulation in the areas of the attic
> >
> > replace removed insulatin with foam board or spray in foam ~R7 per
> > inch
>
> cutting and fitting rigid foam between joists is laborious and doesn't
> work that well because of the inevitible gaps (plan on getting a few
> cases of Great Stuff). Also expensive--5 1/2" would run you about
> $1.75 a square foot. Also, I have to find out where bob is getting R7
> per inch foam. in these parts, our rigid extruded polystyrene is R5
> per inch. (polyiso is R 6 per inch, but its a lot more expensive and
> doesn't work well if you have to cut it to fit between joists) plus,
> in a 2x6 cavity, you will be unlikely to fit 5 1/2" of rigid. I think
> 5" or R 25 is the max you will fit in practice. considering that you
> can get an R21 fiberglass batt in the same space, thats a small gain
> for a lot of trouble and expense..
>
> spray foam is an option, if you can afford it and don't mind the hassle
> of tracking down a contractor. where i live, $850 is the minimum
> setup charge for a contractor to blow in icynene. also, you would have
> to over fill the joist cavities and saw off the excess with a saw. you
> will have to mask off everything you don't want stuck full of foam.
> spray foam makes a MAJOR mess.
>
> so it's hard for me to see that foam is the "easiest way". fiberglass
> has its uses.
> >
> > you get the higher R value you want w/o any modifications to the
> > structure....only down side is foam is more $'s than fiberglas :(
> >
> > cheers
> > Bob


I'm getting R7 off the mfr's packaging / info but I guess like most
stuff the mfr is a little "optomistic"?

http://www.dow.com/styrofoam/na/iso/tuff_r.htm

looks like "new" iso value is 6.5

http://www.sizes.com/units/rvalue.htm

fiberglas is ~3.3 which is about 1/2 of iso

the best he can do with fiberglass is a 2x6 cavity is R18 / 19 & I got
the feeling he was after more

also how well does fiberglass R value holdup up over time

when I said easiest, I meant for the small area he had in mind.....no
structrual mods

I agree the spray is a mess & the iso cutting / fitting is a bitch but
again it seemed like he was doing a few 100 sq ft

and maybe the loss of few inches of head room is meaningful (or not)

cheers
Bob


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