F/G Insulation Cold Weather Performance

All:
Somewhere I’ve read/heard that as temps get colder, the performance of fiberglass as an insulator decreases. My question is WHY? Read the merits of foam & dense packed cellulose, so I’m a “believer” and don’t need to be “sold’ on them. Just wondering if the decrease in performance due to cold can be discussed in “layman’s terms”?
With this said and because of convenience, recently placed fiberglass batts in a small ceiling area (~120 sq. ft.). Wanted to go with loose fill, but the cost of rental & mess was prohibitive.
Perhaps some energy consultants out there can explain?
Rookie
Replies
fiberglass allows air to flow freely thru it.
FG, by its nature then, does not HELP seal up a living space.
Air flow is a very big factor in heat retention in living spaces
(R value is only one part of the equation, some say a small part of it, compared to air flow (leaks) and convection (heat transfer thru air movement).
Air leaks into and out of stud bay cavities will be a bigger problem with FG insulation than with other types that tend to restrict free air flow.
Even in a perfectly sealed stud bay cavity, convection currents can flow around inside a FG filled cavity (because the FG does little to prevent the air flow inside the cavity). The convection currents will transfer the heat from the warm inside wall surfaces to the cold outside wall surfaces. Without this air movement, the two surfaces would transfer less heat energy to each other.
This condition becomes more of a factor in cold weather because the difference in temperature will be greater. The greater the temperature difference, the stronger the convection currents will be. Also, with larger thermal difference, the rate of heat transfer thru the materials will increase. This is the place where R value really helps. A greater R value sheething will have less thermal transfer at a given temerature differential, but this depends on how well all the joints are sealed against air leaks!
The other insulation types tend to help prevent air flow. Cellulose has a natural tendency to restrict free air movement, as does an 'open cell' spray foams (like Icynene). Closed cell foams stop all air movement. These all are easy to pack into all the little nooks and crannies in a housing envelope to help your effort to seal up the place against leaks. Foam board also stops all air movement, but its installation is very dependent on also spray foam sealing up all of the joints to get the full benefit (or else air leaks can exist at every joint).
FG allows the air to just flow right thru, so its installation is also VERY dependent on preventing any moisture laden air from flowing thru it and condensing out (like when one side of the wall is cool air and the other side is warm and moist air - if those two are allowed to come into contact the moisture can condense out under the right circumstances (bad).
Hope this is helpful.
Norm
Edited 1/14/2004 12:52:53 PM ET by Norm
Good stuff Norm.
Thanks and a hat tip to ya.
Norm & Paul:
Thanks! I "get it" now ... warmer air cells in the FG induced to flow outward (to the colder surface) and then quickly being replaced by colder air. Rookie
Rookie, Fiberglass insulation works by trapping small pockets of air among its fibers.But, it also allows air to move through it. Directions on fg remind us to fluff up the batt, or don't compress blown in. It works like ducks down. It needs its air pockets. When temperatures drop enough, the little air pockets cool and the warmer air moves to the cooler area. Thus convection currents begin, which begin to drain the heat holding ability of the fg.
On a more technical basis,at very cold temps,the temperature differential between what we want to insulate, and the ambient air temp, becomes so great that convective movement begins within the fg insulation. If you mashed it flat to prevent air movement, you would have a glass mat, not very effective R-value.
Foam, on an exaggerated basis, is more like little ping pong balls that keep their little air pockets captive. That's as layman as I can put it.
Paul
Good posts by Norm and Paul.
For what it's worth, here's an excerpt from the cells folk who were trying to get the R-value testing porcedure changed. They felt it was biased towards FG by overstating the settling nature of cells when compared to FG in the testing procedure:
"Fiberglass dry-applied loose-fill insulation was installed at R-19 in the Large Scale Climate Simulator at Oak Ridge National Laboratory. The R-value of the fiberglass insulation was measured at various attic cold air temperatures. With the metering chamber temperature held at a constant 70oF, measurable convective heat loss began at an attic temperature of 30oF. A 40% to 50% loss of R-value occurred at cold temperatures. An identical test was run in the Large Scale Climate Simulator at Oak Ridge using dry-applied loose-fill cellulose insulation, also installed at R-19. The measured R-value of the cellulose insulation actually increased from R-18 at 40oF to R-20.3 at -18oF."
That was loose fill. I'm going to guess that a high-density batt will do somewhat better, though not eliminate it.
Pete