Insulation: Air vs. fibreglass, styrofoam, etc.

[Existential Angst]

Awl --

Recently, on one of these How They Do It ditties, they featured but more Dubai profligacy, this time indoor skiing in the middle of the effing desert.

The key to the insulation, they explained, was a huge air gap, asserting that air -- caveat: trapped non-moving air -- was among the best insulators.

First, is this true? Before I insulated my roof, that air would become blisteringly hot, and it didn't seem to be moving much. That attic seemed like a pretty good air gap to me, and it didn't seem to be doing much insulating.

Second, I seem to remember one strategy where push/pull fans were used with air gaps in a roof-type situation, to keep air flowing, to reduce the heating transfer, like what accumulated in my attic -- ie, the exact opposite of static air.

Now mebbe air behaves differently in conduction vs. *radiant* heat from roof-type situations that is making the attic so hot, not hot air itself -- if the two can be distinguished wrt air.

iirc, the Dubai ditty used reflection, insulation, AND that big air gap.....

But I've read about this insulating property of air before, so I'm wondering how it might be employed in a house. It would seem that if air itself was so good, solid insulation wouldn't be so high a priority

I wonder what mooslims think about Dubai.....

[Robert Roland]

The key here is convection. Even though the air is enclosed in an airtight room, the air will circulate inside the room so that the temperature becomes fairly evenly distributed. The point of insulation material is to prevent this convection, so that the air stays still.

The insulation material must also have low heat conduction, which is why it is made from rock or glass, and not copper or aluminium.

Some scuba divers replace the air in their drysuit with argon to stay a little bit warmer. It only makes a small difference, though.

[rangerssuck]

You've got a bunch of apples and oranges mixed up in your fruit salad. In your attic, you have a roof which is being heated by the sun. Then you have convection, which mixes up the attic air and brings it in contact with the lower floor's ceiling. The object of insulation material is to keep the air still, to (virtually) eliminate the convection.

a properly vented attic space should have a considerably lower temperature. Air is drawn into the soffit vents and expelled through the ridge vent. Even better is a powered ventilator or, the push-pull fans you mentioned. In the ventilated space, the sun-heated air is replaced with cooler ambient outside air. This has nothing to do with the insulation value of the air, just its temperature.

Besides the house cooling issues, it is generally a good thing for the logevity of the roofing materials to keep the inside attic temperature as close as possible to the outside temperature.

A reflective roof would make a huge difference. I don't understand the fascination with black (or othr dark color) roofing shingles.

[rangerssuck]

Now, having said all that, I have a related question:

My attic floor is insulated, but rather poorly. Adding more insulation in the attic would not be easy. I am getting ready to repair the ceilings in two bedrooms. The plaster is badly cracked and there are definitely some broken keys (plaster / wood lath). The standard parctice is to install furring strips and sheetrock over the plaster. What I'm thinking about is adding a couple of inches of ridgid foam insulation between the plaster and sheetrock.

I'd use screws and plaster washers to secure the existing ceiling to the joists. Then use screws with fender washers to secure the insulation, and then screw the sheetrock through the wholew mess to the joists with, say, 3 1/2" sheetrock screws. I CAN afford to give up a couple of inches of ceiling height.

Is this plan at all sane?

[PrecisionmachinisT]

Some people don't live in a cooling-dominated climate.

[Jim Wilkins]

"Typically, most sealed units achieve maximum insulating values using a gas space of between 5/8 to 3/4=94 (16=9619 mm) when measured at the centre of the IGU."

AFAIK thicker air spaces permit heat transfer by convection.

jsw

[stans4]

:

Yup, my dad did the same at our old house. We had high ceilings, though, newer crackerbox construction wouldn't have enough room. The way we did it was to put up some 2x furring strips on edge, then fill in with foam between. Sheetrock over the works. He did some rewiring at the same time, dropping some romex in was easy. Had knob and tube before with a little BX mixed in. We made up a hot-wire cutter with an old train transformer and some nichrome, worked well enough to rip foam strips and cut to length.

As far as the other poster's comments on shingle color, we'd reroofed with a mostly white pattern, made a huge difference in that walk-in attic heat radiated into the livingroom below. Must have cut the temp in the attic by at least 20 degrees. Later on, they stuck a big fan in the attic window for the summers, that really made a difference.

Stan

[Existential Angst]

You've got a bunch of apples and oranges mixed up in your fruit salad. In your attic, you have a roof which is being heated by the sun. Then you have convection, which mixes up the attic air and brings it in contact with the lower floor's ceiling. The object of insulation material is to keep the air still, to (virtually) eliminate the convection. ====================================================

If I saw/understood correctly, the air gap space in this Dubai skiing thing was the size of an airplane hangar. =======

a properly vented attic space should have a considerably lower temperature. Air is drawn into the soffit vents and expelled through the ridge vent. Even better is a powered ventilator or, the push-pull fans you mentioned. In the ventilated space, the sun-heated air is replaced with cooler ambient outside air. This has nothing to do with the insulation value of the air, just its temperature.

Besides the house cooling issues, it is generally a good thing for the logevity of the roofing materials to keep the inside attic temperature as close as possible to the outside temperature. =======================================================

Yeah, I've heard that too. I've got a slate roof, I wonder if insulated the rafter space was the best move.

[Joe]

:

It will work OK if you can put up with wavy sheetrock. Probably wouldn't be that noticeable. This assumes the plaster is unstable and therefore the furring strips may not be quite level. Having BTDT, I did like most everyone else does and gat rid of the old cracked plaster and lath. On most jobs the old joists were nearly all true, so the drywall crew were quickly done and the odds and ends I did turned out well. Your plan may be rather labor intensive, so choose carefully.

Joe

[trader4]

It actually is employed in your house if you have fiberglass insulation. The fiberglass essentially traps air within the fiberglass so that it cannot move. Air that does not move will not transfer heat very effectively. That's why if you crush the insulation down and compress it, you greatly reduce the R value.

How that applies to the Dubai ski facility, I'm not clear on. I agree with your reasoning that if it's just a closed air space similar to an attic, it won't work, because the air will move via convection.

[J. Clarke]

Yep.

The attic would have a convection cell going unless there was something in it that prevented convection.

Fiberglass, styrofoam, down, etc, all trap air--that's where their insulating value comes from.

If it's just recirculating the air in the attic it isn't going to do much, if it's moving outside air in and inside air out then it's providing cooling--generally the air temperature will be less than the surface temperature.

Convection is different from both conduction and radiation. Air heats by convection.

The "solid" insulation is to keep the air from moving.

[HeyBub]

Drive around your neighborhood or town. Do you see ANY light-colored roofs?

The reason is cosmetic. A white roof will show the dirt/smog/particulates that settle out of the air, making the roof look like a mud pie.

Heck, I've got to power-wash my brick veneer ever two or three years to get the mung off of it. But as bad as that is, pity poor Rome.

The city has pressure-washing crews that travel about the city cleaning statues and marble artworks from the stuff settling on them from the air (and I don't mean pigeons).

[dss]

:

I'm thinking of doing something similar on my ceiling. In 60% of my house I've got a cathedral ceiling with 2 by 8 rafters packed full of fiberglass insulation. I'd like to add some ventilation and increase the R-factor. I'm thinking of removing the ceiling and the fiberglass insulation, and then putting six inches of the pink insulation board between the rafters. That would leave about an inch and a quarter of space on top for air to go out a ridge vent. Then I'd add two inches of foam board on the bottom face of the rafters with 1 by 3 boards screwed perpendicular to the rafters to provide a base for the drywall. I'll probably put in some radiant foil insulation as well. This would take me from R-22 to R-40 something and provide some much needed roof ventilation. (I'm in Minnesota.) I would lose about 3 inches of ceiling height and that might be a code problem at the short walls.

In your case you might also want to put up boards to attach the drywall. You might get too much flex going through the two inches of foam. I saw something online about that when I was researching the problem. The other issue is you'll be sandwiching the current plaster ceiling between two moisture barriers (the existing one and the new foam board one). However, I don't think that would be a big deal. You could drill a few hundred holes in the current ceiling if you were worried about that.

Good luck with the project.

dss

[hrhofmann]

:

YES!

[RicodJour]

The object of some insulation, such as fiberglass batt, does for a large part utilize trapped air to achieve it's overall R value. Rigid insulation...not so much. But I agree with your point as far as trapped air adding to the insulation value. Basically it's taking another step down on the heat transfer hierarchy of efficiency - taking the step down from convection to radiation through the trapped air.

I think the dark roof thing has a lot to do with tradition and what people are used to seeing. It also has to do with minimizing the objectionable appearance of roof discoloration. A darker roof usually 'wears' better.

Perfectly. Normally I'd be tempted to pull the old ceiling plaster and lath, and start fresh, but with the insulation up there, and the fact that it's the middle of the summer...I like your plan better. ;) Exactly how poorly insulated is your attic floor and where is the house?

The rigid insulation at the ceiling level is a superior solution overall. If your ceiling joists or rafters are on 16" centers, that's getting near 15% of the overall ceiling area being taken up by joists. Depending on your insulation, those joists might be a thermal short circuit.

3 1/2" drywall screws won't work with the existing plaster and 2" of rigid insulation and the new drywall ceiling. Even 4" screws are a little short for my tastes in that situation. R

[krw]

I see them. In fact, I used to have one (shingled the 2-1/2 car garage myself). I guess it depends on where you live, but worse than mung from the air is mold. It *really* shows up on light shingles.

[Gz]

You insulated your roof. Ok. What I did in my garage was install perforated reflective material to the roof joists. This does create an extra few inches of air space. The dominant force in the summer is the suns heat. When there is a large differential temperate, radiance becomes dominant. I used to go in my garage and you could feel the heat from the roof. Now, nothing. And of course it's a lot cooler through the day. Eventually the temperature of the air becomes equal to the outside through the day, but forget the baking effect.

Greg

[Artemus]

No, it's not true as others have already responded. Those shows are riddled with errors, which, I suspect come from liberal arts majors writing/editing the scripts which the narrator just reads. Accuracy in science or engineering doesn't seem to be high on their list of important aspects of the show. Art

[Terry]

You'll want to check the numbers if using asphalt shingles. We are considering a re-shingle job, and found that "white" shingles aren't hugely better than black in reflectivity (they are slighly better, though). A shiny metal roof might be very different, I dunno.

The manufacturer may give reflectivity values for various shingles.

-- Best -- Terry

[Michael A. Terrell]

Some places are difficult to get insurance if you have a metal roof on your home.