Covering Ducts in Attic

Reply

  #1  
Old 11-05-02, 01:05 PM
artschwartz
Visiting Guest
Posts: n/a
Covering Ducts in Attic

I have a ranch house built in 1968. The attic has blown insulation in it that fills the area between the joists. I was contemplating adding insulation in rolls across the joists to raise the R value in my attic.

Central A/C was added to my house after it was originally built. Flexible aluminum ducting supplies the A/C. The Flexible ducting is in my attic. Should I cover it with the new layer of insulations. Should I just put the insulation around it and not cover it. Since it is flexible ducting it is also possible that I can raise it and put the new layer of insulation under the ducting. What is the optimum method?
 
Sponsored Links
  #2  
Old 11-16-02, 04:50 AM
A
Member
Join Date: Aug 2000
Posts: 160
Received 0 Votes on 0 Posts
There is a rule of diminishing return on dollars invested. The cost of the additional insulation may not save you a lot of energy dollars. Topping up the value will have some benifit, but doubling R values does not translate into doubling energy savings Approx. 70% of heat loss is stopped in the first three inches of insulation. Check your local building department for the attic insulation requirements for your zone. Having said that, covering your A/C ducting with insulation is an excellent idea. This will increase your units efficientcy & reduce its operating cost.
 
  #3  
Old 11-23-02, 09:17 AM
rbisys
Visiting Guest
Posts: n/a
Greetings,

DO NOT cover the aluminum with bulk insulation. The aluminum is an insulator and if you have bulk insulation touching it, it will become a conductor.

Adding more insulation to the existing will probably not save you any money and your a/c cost could go up because you're adding more heat retaining mass.

If you install a radiant barrier (RB) over the existing insulation you can save up to 30% on a/c and about 12% on heating.

Your situation requires more detailed info(both ways) than what I can provide here.

If you want to pursue this contact me on e-mail and I will give you my phone number.

Thank you for considering my opinion.
 
  #4  
Old 11-24-02, 04:14 AM
A
Member
Join Date: Aug 2000
Posts: 160
Received 0 Votes on 0 Posts
"The aluminum is an insulator and if you have bulk insulation touching it, it becomes a conductor" ????? Helloooooo.
 
  #5  
Old 11-24-02, 06:52 PM
M
Member
Join Date: Jan 2002
Location: Illinios
Posts: 258
Received 0 Votes on 0 Posts
Allen I was about to say the same thing. Rbisys metals including aluminium are excellent conductors of heat. If you want to try an experiment hold both an aluminum bat and a wood bat over the burner of your stove and see which one gets gets too hot to hold first
 
  #6  
Old 11-25-02, 06:21 AM
54regcab's Avatar
Member
Join Date: Jul 2002
Location: Oklahoma City
Posts: 661
Received 0 Votes on 0 Posts
Arrow Radiant vs convection

Aluminum REFLECTS heat, this is how it "insulates"
Look at the car windshield protectors.
Do the same test on a wooden bat and a shiny alumnum one with the heat coming from ABOVE (such as a raidiant heat lamp)
But that's interesting that 3" of insulation stops 70% of loss, I have about 4-5" in my attic and I might just leave as-is instead of adding more
I am considering the radiant barrier for next summer but don't know if I will live here long enough to recover the purchase in energy savings.
 
  #7  
Old 11-25-02, 07:17 PM
A
Member
Join Date: Aug 2000
Posts: 160
Received 0 Votes on 0 Posts
I agree with you that aluminum reflects some of the radiated heat, but an exposed aluminum duct will conduct far more heat into the duct increasing the temperature of the conditioned air it is carrying. A Radient barrier without an air space between it & the duct will act as a conductor. I do agree that a properly installed radient barrier in conjunction with batt insulation has its benefits. However laying an RB directly atop the insulation is not recommended. Dust will soon cover the barrier reducing its reflective properties. Proper installation of the RB is on the bottom of the rafters, shiny side down. with an air space between it and the roof sheathing. Use of required batt insulation on the ceiling for the respective temperature zone is also needed. Continuous vented soffits and a continuous ridge vent are also key elements of reducing attic temperatures. My advice to artscwartz stands. Cover the duct with batt insulation to reduce heat transfer from the attic into the duct. As far as rbisys's claim of a 30% A/C energy savings, I believe those numbers are highly over rated.
 
  #8  
Old 11-27-02, 10:59 AM
rbisys
Visiting Guest
Posts: n/a
Greetings,

RB refect more than some. Try 95%. FG and cellulose about 8% or less.

The dust factor is highly over rated. The highest reduction I have seen is about 20%. That really dosen't mean much whem you consider the down side doesn't accumulate dust and maintains its 95% eff. On a two layer material over the existing the dust would mean nothing. I have installing over the existing for many years without any problems. BUT, you have to know how to do it properly. I have seen up to 30% with the two layer method. Some home owners have reported more. Most web reported installations are of RB to bottom of rafter. I do not use that method unless unable to use other method. Those installation show a 22 1/2% reduction. The main thing is that you see a up to 50% or more reduction in the run time of the a/c and the home is more comfortable because the ceiling temp is dropped and there is less energy being radiated to the occupants.

In new homes a three layer material in the ceil'g and two layers in the walls will out perform any FG or cellulose installation I've come across. One of the ways that the competition tries to kill a RB sale is to claim it has to be with another material, or that it's not as efficient in winter as summer. NOT SO. Just look at what they use in the arttic and on space ships.

By the way the attic temp in a RB house on a 95 deg. day will be about 110 degs with a ridge vent system. Take that FG. Also ridge vents are for removing moisture not heat. Heat rises which means it shouldn't get into the house. And it wouldn't, if it wasn't for that other physical law, radiation.

As far as conduction in the duct, I think you mean transfer by convective air movement. The foil still emits only 5%. By laying bulk insulation on the duct you increase the amount of energy available to be transfered from the storage of the energy in the bulk insulation.

I hope this helps you understand better the physics involved here.

Thank you for conmsidering my opinion.
 
Reply
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: