Ductwork and Insulation

[Joe Mascitti]

I plan on adding more blown-in insulation to my attic. I have made great strides in improving attic temp over the past year (95* outside temp, 118* attic temp in sun) and want to see if I can now help reduce the heat gain to the ductwork, which the majority is on the attic floor. If I want to bury the ductwork, how much insulation should cover it to be beneficial? 2 inches, 3 inches?? etc...

Thanks

Joe

[dan sw fl]

I plan on adding more blown-in insulation to my attic. I have made great strides in improving attic temp over the past year (95* outside temp, 118* attic temp in sun) and want to see if I can now help reduce the heat gain to the ductwork, which the majority is on the attic floor. If I want to bury the ductwork, how much insulation should cover it to be beneficial? 2 inches, 3 inches?? etc...

Thanks, Joe

What is the current R-value of the duct insulation?

Is there a significant measured temperature gain from the air handler outlet to the diffuser (i.e vent, room outlet,... ) ?

Overall perspective:
If there is a 2'F difference and the duct insulation value is "doubled" with addition of attic insulation
one might save about 5% of your daytime cooling co$t attributable to the duct itself.

A slight savings in reduction of overall heat gain due to increase in ceiling r-value would also be realized.

[dash]

Covering the ducts with insulation,can cause them to sweat,in humid climates.

Florida code requires ducts suspended bove the insulation.

[Joe Mascitti]

I think the duct insulation is R6 or R8...

Temp spread is good...

I was planning on blowing in another 6 inches to help with the heat transfer...started thinking about the ductwork...would blow in more if it would help the ductwork...

[energy_rater_La]

as dash said, in hot humid climates burying ducts in insulation causes ducts to
condensate. code dictates that ducts be strapped above insulation.

radiant barriers work well in hot climates.

air sealing between attic and house will allow insulation to preform as spec'd
air moving through insulation lessens its R-value.

duct sealing (mastics not duct tapes) will also be of benefit by
reducing amount of conditioned air escaping into attic

what is your current insulation type and R-value..(or inches of installed product)?

best of luck.

[Joe Mascitti]

I have about 12-15 inches of insulation already and the ductwork is fairly new, wrapped well. About 60% is run on the floor, but I do not see anyway around it. I am happy with the results, but always looking to improve. If it will cause condensation problems, then they do not need to be buried...

I have such a high roof line and so much bracing that foil RB is not really a good option...maybe the spray on, but from what I've read here...that does not yield the greatest results...

Can Foil RB be used under the tar paper on the top on the roof? reflective side down..say on a re-roof job

[10bender]

Have you given any thought to a gable mounted exhaust fan? It operates with a t-stat to draw outside air through the attic to cool it and reduce heat gain. Then you would not need to bury your ducts.

[energy_rater_La]

gable ends work fine without mechanical aid.
these fans create negative pressures in attics
making duct and house leakage problems worse.
do search for many many discussions about pav's on
this site. (power attic ventilatior's)

[Joe Mascitti]

PAV's are a no...no...

When it's 95* outside my attic is 118* so I think it is ventilated enough...I have 3 gable ends and full ridge vents on all roof ridges...