Insulation: Air vs. fibreglass, styrofoam, etc.

[HeyBub]

Ah, of course you're right. I'm in a metropolitan area - lot's of cars, busses, trucks, heavy industry, and chickens driving Cadillacs to Washington, D.C.

[Jim Elbrecht]

Where? Why? I would think it would lower the insurance. Lasts longer, more durable, and more resistant to fire.

Jim

[Michael A. Terrell]

Florida. High winds. Harder to put out a house fire, since you can't walk on it and cut holes to ventilate the building.

[gregz]

Vacuum is best, with two reflective sides. I never figured out argon a heavy gas. I think air with humidity is worse than dry air. Syrofoam is a little bit worse than some other foams. I could never figure that out. Fiberglass tends to be a little worse than cellulose because cellulose stops air flow better. I also think extra thin fiberglass is better. Corning used to make very fine, no itch. Insulating the roof will shorten the life of shingles, they get hotter. Maybe if every building had reflective or plants, the world would cool down.

Maybe someday I'll install reflective sheeting on the house like I did on garage. First I need to cut openings in the wooden sofet where they installed perforated aluminum over the wood. Well they did drill a couple holes in the wood, here and there. Bought the house like that.

Greg

[krw]

I was living in NY (~70mi North of NYC) at the time. There aren't any white roofs down here but I suspect it's because of the red clay.

[DD_BobK]

:

[Larry Fishel]

Neither fiberglass nor Styrofoam are "solid". They are basically air with a tiny bit of structure to keep that air from moving (much)...

Even if your attic were hermetically sealed, the air in it would be moving all over the place (though too slowly to notice).

[Jim Elbrecht]

Wind makes sense- we rarely have roof-ripping winds in my neck of the woods-- But our firemen have ladders & don't walk around on our rather steep rooves. [roofs?- suit yourself, I looked it up because rooves looked weird-- both work]

Jim

[RicodJour]

There are a number of metal roofing systems that have been approved by Dade County for high winds, and if it's good enough for Dade, it's good enough for your application.

Impact resistance might be an insurance consideration for hail damage, but there are metal roofing systems that have that...errr...covered as well:

I know of no fire rating criteria that requires a building envelope component to be easily compromised to improve firefighting access. If you could provide a citation, that would be grand.

R

[trader4]

Exactly. And they rely on the fact that stationary air is in fact a poor conductor of heat.

[Ed Huntress]

Another way to explain it is that there are two variables that control how effective trapped air can be as an insulator. One is preventing mixing and convection -- movement of the air, for practical purposes. Therefore, small pockets are more efficient than big ones. The other is the relative conductivity and relative volume of the entrapping medium: Plastics usually are better than glass, which is better than metal, etc.

So fine, closed-cell foam is an extremely efficient insulator. Fiberglass batts are relatively less so, but still are quite good because a dense network of it is very good at preventing internal movement. Nanogel, which is a type of aerogel, is one of the best that use air as the insulating medium. (Most other aerogels have another gas in the pores.)

[DD_BobK]

Rico-

Good point about the "thermal bridging" ....

OP-

I'd be inclined to take down the ceiling, remove the existing insulation and replace it with rigid foam between the existing joists. If the joists are 2x6's you could get >R-30.

Depending on your locale this might be enough or you could do another inch or two below the joists and go even higher.

Vapor barrier issue could be addressed at the bottom surface of the insulation or the surface of the draywall.

cheers Bob

[Higgs Boson]

Why particularly do you suspect the people you are indicting (without evidence)? Please defend your thesis with actual data.

HB

[James Waldby]

I contend that as a practical matter, "small pockets are more efficient than big ones" does not remain true as cell size shrinks to zero. Instead (for any given medium and filler gas) efficiency improves as size decreases to some point, after which efficiency gets worse, due to increased importance of heat conduction relative to convection as size decreases. With an ideal medium, the ratio need not change, because ideally the ratio of gas volume to medium volume can remain constant as cell size shrinks and as constant strength (or, at least, constant cross section of medium) is maintained. But as a practical matter, after cell wall thickness reaches some minimal amount, it cannot shrink further as cell size decreases.

[snip re aerogels & nanogel]

Makers of "cenospheres" are quite proud of their products -- see eg and

[Ed Huntress]

Sure. But the R value of polyurethane foam, for example, increases as the pore size diminishes to the practical minimum you can obtain and still have continuous foam.

But that practical limit diminishes further with aerogels, including nanogel, because the thickness of the entraining walls can be vanishingly small. And then you get even better R values.

[Michael A. Terrell]

My dad had a house in Lake County and couldn't get insurance because of the metal roof. They all told him to replace it with a shingle roof, or they couldn't insure him. Of course, Dade county approves of all kinds of construction that other places won't accept. I know people who left there after Andrew. Houses less than two years old were badly damaged, yet passed al hurricane rated inspections. Afterwards, he damage revealed all kinds of substandard work. Curiously, all the inspectors involved had 'retired' and couldn't be located.

[J. Clarke]

Perhaps he should have called State Farm. They list 20 pages of metal roofs for which they offer discounts. To get the discount though you have to take a cosmetic damage waiver--I guess they don't want to pay for every little dent.

House I grew up in in Nassau County had a metal roof. Never any problem getting insurance.

Dade County changed the building code radically after Andrew you know, based on lessons learned, and it is one of the toughest in the country now.

[Michael A. Terrell]

I'm sure he did, since they insured his car at the time.

They never want to pay, but there are plenty of roof types they don't want to insure. By listing '20 types' it sounds like they are decorative styles over plywood or other construction instead of a plain 'Five V' roof on perlings and rafters.

[J. Clarke]

I didn't say "20 types", I said 20 _pages_, listed by brand name and manufacturer mostly.

[Michael A. Terrell]

How do you prove who make the 'Five V' on your home?