Insulating Cold Air Return in Basement

Reply

  #1  
Old 04-24-18, 07:54 PM
N
Member
Thread Starter
Join Date: Apr 2018
Location: Canada
Posts: 7
Received 0 Votes on 0 Posts
Insulating Cold Air Return in Basement

I'm insulating between all of the joists in my basement with Safe'n'Sound (3" thick). My issue is between the joists that are being used as a duct for the cold air return from the main floor.

My thought is to move the 'Thermopan' up 3" allowing for me to stuff a bat of insulation under it. this would make the duct 6.5" deep instead of 9.5" deep.

I don't see this being an issue, but want to get other opinions incase I'm missing something obvious.

Apologies is this or similar has been posted before.
 
Attached Images   
Sponsored Links
  #2  
Old 04-24-18, 08:09 PM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Location: USA
Posts: 26,013
Received 675 Votes on 624 Posts
The volume of air the cold air return needs is usually carefully calculated and is balanced with the forced air side, so reducing the space could restrict and strain the unit... it might also weaken the velocity on the forced air side.

3" is probably reducing the volume of that cold air return by a whopping 30%.

I'd say skip the insulation there, or maybe cover the bottom of it with a piece of dense Foam board instead.
 
  #3  
Old 04-24-18, 08:23 PM
N
Member
Thread Starter
Join Date: Apr 2018
Location: Canada
Posts: 7
Received 0 Votes on 0 Posts
To be clear, the insulation is for sound muffling/control from.the basement yo the main floor.
 
  #4  
Old 04-24-18, 08:25 PM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Location: USA
Posts: 26,013
Received 675 Votes on 624 Posts
Yes, I know what safe-n-sound insulation is for. Hope you understand what reducing/restricting your cold air return will do.
 
  #5  
Old 04-24-18, 08:28 PM
N
Member
Thread Starter
Join Date: Apr 2018
Location: Canada
Posts: 7
Received 0 Votes on 0 Posts
@XSleeper I was thinking the same as you. But knowing my builder I can't see that actually being the case. Also, isn't the floor restricted by the size of the vents and not the volume of the duct itself? or do I have that wrong?

Also, I can't I can't insulate under it as is as the resilient channel will be running along the joists so anything added will throw the whole basement off.
 
  #6  
Old 04-24-18, 08:31 PM
N
Member
Thread Starter
Join Date: Apr 2018
Location: Canada
Posts: 7
Received 0 Votes on 0 Posts
Sorry, I posted that clarification about the insulation before I saw your reply. It was just for general info. Not directed at you @XSleeper.
 
  #7  
Old 04-24-18, 08:33 PM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Location: USA
Posts: 26,013
Received 675 Votes on 624 Posts
No worries. To be fair, 3" of safe-n-sound isn't going to make a "huge" difference. Leaving it off one floor joist bay will be unnoticable, imo.

The size of the duct and the grill both work together to equal volume and velocity. It's like the difference between chugging beer straight out of the bottle or forcing you to drink it through a straw. (With the ductwork being the straw)

You can always wait for others to chime in, but don't look for anyone to rubber stamp your idea and tell you what you want to hear.
 
  #8  
Old 04-25-18, 03:04 AM
S
Member
Join Date: Jan 2017
Location: USA
Posts: 541
Received 5 Votes on 5 Posts
; I would recommend that if you are going to finish the basement into living space, make sure that you add enough supply registers and return grills to heat and cool the space for comfort. . You should have almost the same number of supply and return registers as there are on any 1 floor. 1 or 2 supply registers and return grills are not enough to condition that space. To add to @XSleeper's answer and comments, years ago, when many of the returns were put in metal duct work,the normal duct size used was 3 1/4" X 14" for each return. You could determine how many return air grills are feeding into that joist space and decide how to proceed.. If that space carries more than 2 standard return grills then you should not decrease the free area by moving the panning up to allow for the addition of the insulation. If it carries 2 or less then do it .If you are not sure of the ducting ask for professional help.
 
  #9  
Old 04-25-18, 04:25 AM
airman.1994's Avatar
Member
Join Date: Dec 2005
Location: VA
Posts: 5,795
Received 8 Votes on 8 Posts
You are probably under sized on return air if that is the only return. So reducing it even more would be even worse. I guessed it was 15 x 8.
 
  #10  
Old 04-25-18, 08:25 AM
N
Member
Thread Starter
Join Date: Apr 2018
Location: Canada
Posts: 7
Received 0 Votes on 0 Posts
No, it's not the only return air. The duct shown feeds one return air grill on the main floor (there is a second one off of another run. The house was designed (or supplied anyways) 5 years ago with one return air duct in the basement located on the ceiling. I'm also installing a natural gas fireplace, so I had the contractor here yesterday to bring that cold air return down the wall so that it pulls from the floor and he blanked off the one on the ceiling. The blanked off grill was 18" x 10" and i'll be replacing it with a ~30" x 14" grill so that will increase the return air from the basement by quite a bit. The basement also came with 6 supplies.

I'd like to think that by increasing the size of the basement return significantly that reducing the duct size to that one grill on the main floor will not significantly affect things or may have a neutral effect. But i'm still nervous.
 
  #11  
Old 04-25-18, 12:54 PM
A
Member
Join Date: Mar 2010
Location: USA
Posts: 4,108
Received 81 Votes on 75 Posts
When the entire joist bay or stud bay is used as part of a return duct, chances are that no precise scientific analysis was done earlier to determine the actual cross section of duct space needed for the return air volume. You need to do that calculation or leave that joist bay untouched.

Do not leave exposed batt insulation or pour-in insulation in the stud or joist bay used as a duct. You do not want tiny fiberglass or other particles stirred up by the air flow and being distributed throughout the house.
 
  #12  
Old 04-27-18, 03:13 PM
N
Member
Thread Starter
Join Date: Apr 2018
Location: Canada
Posts: 7
Received 0 Votes on 0 Posts
Yeah, i understand that part about not using any insulation inside the duct. I want to move the duct up 3" and put the insulation between it and the resilient channel/drywall for sound purposes.
 
  #13  
Old 04-27-18, 04:25 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 59,150
Received 1,113 Votes on 1,033 Posts
There is no way to know for sure the effects of partially closing that return duct down but I wouldn't recommend it. Most HVAC systems need as much return air as possible.
 
  #14  
Old 04-27-18, 04:41 PM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Location: USA
Posts: 26,013
Received 675 Votes on 624 Posts
I don't see where you actually tell us the size of the one cold air return that trunk supposedly is venting.

The pan could also be enclosing one or several cold air returns that run up any interior wall along its path... if there is a 2nd floor.

We don't want to cause a problem by saying it's okay when you might be wrong about the one supply thing.

And like I said, if you make it smaller, it has to suck harder.
 
  #15  
Old 04-27-18, 11:46 PM
S
Group Moderator
Join Date: Oct 2004
Location: WI/MN
Posts: 19,575
Received 94 Votes on 83 Posts
As X said earlier, you're not likely going to get any of us to endorse your plan. If you really want to proceed with this, I would get your HVAC person out to sign off.
 
  #16  
Old 04-30-18, 06:15 AM
N
Member
Thread Starter
Join Date: Apr 2018
Location: Canada
Posts: 7
Received 0 Votes on 0 Posts
Thanks for the feedback everyone. Seems pretty clear that i should leave well enough alone and hope it doesn't allow too much sound to travel.
 
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: