Too much attic insulation.....any experiences? (house, college, costs)

[Hal Roach]

Is there such a thing as over insulating, especially in warm climates? Is blocking the attic ventilation more of a factor or does the insulation hold in heat? Seems to be differing opinions on this. One study says that R13 blocks 93% of radiant heat, and R40 only 5% more...so there are clearly diminishing returns. I would like to get more real life examples. Thanks!

[MGS4EVER]

I can tell you that a friend of mine insulated above her garage and it made an already hot garage into an oven. The builder left the space clear of insulation for a reason.

[wit-nit]

Originally Posted by MGS4EVER

I can tell you that a friend of mine insulated above her garage and it made an already hot garage into an oven. The builder left the space clear of insulation for a reason.

Interesting, we had 24" of insulation blown into the house attic including the garage and had a big drop in temp.

[Zippyman]

Well, my house had r-19 in the attic & I added 10" of cellulose, so my theoretical r-value should be r-49 or higher, if you believe the salespeople.. I have a tall attic, with big vents on the ends of the house, several feet above the top of the insulation, so I'm thinking nothing was "blocked".. We also added 10" over the garage ceiling, which had zero insulation before & had no problems.. Theoretically, we're trapping all the engine heat from the cars inside the garage, but the cars usually come home at night, not during the heat of the day, so we haven't really noticed any change at all, good or bad. The cost difference between 4" to 10" of cellulose installed was so low, that even if it didn't do much, there wasn't any reason not to do it..

I personally believe the issue isn't really heat transmission or "radiance", so much as it is air leakage - older homes here are pretty "loose" & cellulose seems to work very well with leakage (can lights, outlets & such).. Every house where I've pulled out wallboard had dirt caught along the baseboards in the fiberglass insulation.. The attics seem to "pull" conditioned air around baseboards & fixtures & into the hot space..

[Dewey59]

Numerous studies have shown that additional insulation (up to about R40 - R50) is worthwhile. However, it must be done right. The soffit vents must remain open, so that fresh air can enter the attic space, and through natural convection, force the hot air out the vents on top. Regarding garage spaces, I believe most AZ garages have low wall and ceiling vents to promote similar convection air flow. If the ceiling vent were blocked by insulation or otherwise, obviously, as Zippyman said, you could be trapping engine heat in the garage.

I was just researching all of this about a month ago, as I was considering installing an attic vent fan to help keep the attic cooler. Everything I read said that the power required to run the fan would cost more than any savings in less air conditioning costs in the home - especially if there is ample insulation in the attic. I wish I would have taken thermodynamics in college... this stuff is tough to grasp!

[snatale1]

No, more insulation means more of your cool in the house, but the attics can't be sealed up either. As long as it's done right you can keep the cool downstairs but still allow the attic to breathe correctly.