insulating only part of attic

Thinking about adding insulation to the attic, which is floored over. The easiest thing would be to put insulation on top of the flooring. However, I need part of the space up there for storage. If, say, I put insulation all around except for an area in the middle, would that eliminate most of the gains of insulation? In other words, if I leave 20% of the space uninsulated, do I cut heat loss by 80% (of what I would by insulating all over)? -- H
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What you say is true to an extent since heat loss is a function of the area times the R-value, it's perfectly fine to inculate part of the attic better. However, a better way to do it would be to put in joists going cross the direction of the existing ones, then insulate that whole area, then put the floor on top of that, effectively raising the floor.
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Who's "you"? :-)

No. Heat loss is proportional to ared DIVIDED BY R-value.

Better to inculate all of it.
Nick
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On 9 Oct 2006 13:02:23 -0700, "Heathcliff"

Right now the 20% of the floor that you're going to not-cover looses 20 units of heat, and the 80% that you are going to cover looses 80 units of heat. If you double the insulation on the 80%, you halve the heat loss there, so you'll be loosing 20 + 80/2 = 60 units of heat. Still worth doing, but:
Put foam insulation covered by plywood over the part you want weight/traffic on, and use cheaper insulation (of your choice) everywhere else.
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Goedjn wrote:

I like the foam boards covered by plywood idea. That would have some advantages over raising the floor with cross-joists, which would reduce clearance (at a premium) and would be awkward at the access point (access by stairs). How thick does the plywood have to be to stand up to light foot traffic, when on top of the foam board? Does it matter if the foam board is the expanded kind or the extruded kind (i.e. is one kind stronger)? thanks, -- H
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Heathcliff wrote:

I did a similar thing. I used 2 3/4" sheets of foam board, one on top of the other, in a 2' wide path going down the center of the attic at the highest point. On top of that, I put 1/2"+ plywood that is used for sheathing (not OSB or the like). The truss/joists are at 2' centers. It is a bit springy, however, for storing boxes, etc. it works quite good. I only have about 3 1/2' of height.
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Heathcliff wrote:

It will help but it would not be 80%, well it would not be unless you have found a 100% effective insulation. If so I really want to talk to you. :-)
It would in fact be something less than 80% of what it would be if fully insulated. That is still good.
--
Joseph Meehan

Dia duit
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On 9 Oct 2006 13:02:23 -0700, "Heathcliff"

The floor itself offers some insulation. The insulation property is a factor of material, coverage area, and thickness. You could add a thick underlayment and carpet to the floor. Often you can find used carpeting and underlayment for free.
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Heathcliff wrote:

Something to think about. Part of you house is probably warmer than others. Often people want their livingrooms warmer than their spare bedrooms, etc. If that's the case, it might help to insulate over the warmer areas if you're only going to insulate part of it. Also, I would stop at wall lines to provide insulation over an entire room, not stop in the middle of a room.
I'm sure someone can produce tons of formulas saying it doesn't matter, but I would be that it does and do it that way.
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since household junk expands into every available cavity of the home and forces its way into a second garage, nip it in the bud right now with a garage sale. then take the money and remodel the fully insulated new attic space.
Heathcliff wrote:

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If I cover 80% of the holes in the bottom of a sieve, does that mean it will hold 20% of it's water?
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