insulation of air ducts in attic 1

[samv]

Hello everyone,

If I were to run air ducts in an attic and do not want condensation to form on them, what guidelines or standards
should I follow for insulation thickness?

Thanks
Samv

 

[fredb]

Hello

Assuming you will use some sort of foil faced insulation, and also that the air in the duct is 55F, then:

@ 70% attic RH, an R value of 2 is good to 100F ambient
@ 80% attic RH, an R value of 4.5 is good to 100F ambient
@ 90% attic RH, an R value of 11.5 is good to 100F ambient

 

[stevenw]

Most codes require R-8 insulation in an attic space.

 

[samv]

Thanks fredb and stevenw -

may I ask where these numbers were taken?

samv

 

[fredb]

Hello

go to:

http://www.naima.org,

then look in the insulation section.

 

[lilliput1]

The duct insulation surface temperature must be above the dewpoint temperature of the space.

 

[samv]

I'd like to hear your opinions on which standards are usually have higher standards (excuse the redundance!)

Is it ASHRAE, International Energy Conservation Code, or local building codes... ?

liliput1 - I'd be interested in hearing your prefered method of calculating the surface temp of the duct insulation if you're willing to share it?

 

[gnordo]

A few thoughts on attic ductwork.

Prior to any insulation effort, inspect all joints/access doors,etc, for air leakage. Any conditioned air LOSSES TO THE ATTIC GO ONLY TOWARDS COOLING THE this unoccupid space.
Mark all leaks, then seal as necessary to approach 0 cfm leakage prior to insulation effort.

Further, what is the likely attic temperature? Even moderately well ventilated attics rountinely run temperatures well above 100 degree F. I routinely see well ventilated attic temperatures in well in excess of 120 degrees F.. 120 degrees F. is likely at the low end of expected temperatures.

Conditions that adversely raise the attic temperature are
1. Color of roof shingles, with black probably being about 4+/- times [ a bit of a guess] worse than very light colored tiles.

2.Geographical Latitude of the building; the highr the sun is, the more solar radiation the roof will receive.

3. Roof shading- a well shaded roof is a big assist. I still would expect temperatures to likely be at least 110 degree F.


So unless I had field data that proved otherwise, I would select the insulation R value on the basis of 140 degrees +F.


Also, it is well worth it to consider improvements on the attic ventilation.

For passive means, be sure there are vents both low in thew attic as well as high. The low ones should be spread out the perimeter, while the the upper one may be located as high as possible near the apex.

Personally, i prefer a small attic vent fan, operating on a Thermostat located low in the attic, say 2 foot above the ceiling below, while the exhaust fan should be located as high as possible. Typically, I set the thermostat to activate at 115 degrees, and presuming a 5 degree fixed thermostat differential, it will turn off at 110 degrees.

The reason I say to locate the Thermostat low, is that there normally can be a rather significant stratification temperature gradiant from attic floor to attic roof. I have measured attic gradients such as 120 F. low to 170+/- F. high.

As an aside, the highest temperature I have ever measured in an attic was mid summer/noon/ in New Orleans, La. where I measured 185 degrees near the attic roof. The attic was a poorly ventilated with a black slate roof.

The better the ceiling insulation, the higher the thermostat can be economically set.

The design dew point of the attic will be the same as WORST CASE Outdoor Air Dew Point.

Enough idle thoughts for now......Gnordo