Page 1 of 2 12 LastLast
Results 1 to 13 of 23
  1. #1
    Join Date
    Jan 2009
    Posts
    11

    Crawl space ductwork question

    Hello,

    We are in the process of finishing the new addition on our house. The ductwork is within the crawl space. The crawl space has only 13-14" below the steel beams that hold the floor construction. The ductwork design called for 10" duct height (supply air). The duct segments were 20x10,18x10,14x10,10x10. The return duct was 20x12, single segment. The contractor used 26 Gauge Galvanized steel and R-6 duct wrap.

    Due to tight space the duct ended up sitting on the gravel (actually on the plastic sheet that covers gravel throughout the crawl space).

    Crawl space is completely closed without any vents. It will be slightly conditioned with the supply air duct which will have a few small openings (with a mesh) to provide a small amount of conditioned air.

    Our question:

    Is there going to be any problem with the duct sitting on the ground/gravel?

    If yes, what type of problems we may end up experiencing?

    Regards,
    Bogdan

  2. #2

    Duct in crawl space

    Even though there is a few small holes in the supply duct it will probably not be enough for the area. In unconditioned spaces all metal duct should be insullated to combat condensation. With the duct being on the ground plastic or not this will still cause uneeded ware and tare. A solution might have been to use flex duct insted. It is round and can fit in tight spaces better.

  3. #3
    Join Date
    Jan 2009
    Posts
    11
    Quote Originally Posted by MArden View Post
    Even though there is a few small holes in the supply duct it will probably not be enough for the area. In unconditioned spaces all metal duct should be insullated to combat condensation. With the duct being on the ground plastic or not this will still cause uneeded ware and tare. A solution might have been to use flex duct insted. It is round and can fit in tight spaces better.
    The contractor used the R-6 duct wrap insulation which I believe would provide protection from condensation.

    Our concern is more that the duct would sit on the gravel and we are not sure if that would result in additional energy losses.

    I don't know why would there be more ware and tare? The duct would not go anywhere, it would just sit there.

    I'm not sure about flex duct because this is the main duct with a total linear length of about 40-50ft, two elbows, etc. I doubt the flex duct would work well for this length. Also, it would not be possible to use dampers (crawl space hard to access due to tight space) so the only way to really control the air flow would be with the proper duct sizing, etc.

    Total CFM will be around 1200. Duct will be supplying air to two small rooms, large open area with family room/breakfast area/kitchen, total of 1000 sqft.

    Again, the only concern we really have is that the duct would sit on the ground instead of being mounted to hang from the floor joists...

    I was wandering what was the original reason to hang ducts from the floor joists?

    Is it just to be able to crawl below it in spaces that are with more height, or some other reason? In our case, since there is no space to crawl below the duct, hanging it from the floor joists wouldn't make any difference...

    Thanx,
    Bogdan

  4. #4
    Join Date
    May 2007
    Location
    Alabama
    Posts
    909
    Quote Originally Posted by MArden View Post
    Even though there is a few small holes in the supply duct it will probably not be enough for the area. In unconditioned spaces all metal duct should be insullated to combat condensation. With the duct being on the ground plastic or not this will still cause uneeded ware and tare. A solution might have been to use flex duct insted. It is round and can fit in tight spaces better.
    Dumping conditioned air under house? Use FLEX DUCT UNDER HOUSE? Surely I'm missing something here!

  5. #5
    Join Date
    Mar 2007
    Location
    NW AR
    Posts
    2,478
    if water ever comes in contact with the insulation and the metal the insulation will deteriorate and fall off and the metal will rust through. sounds like you have gravel and a vapor barrier so water may never be an issue. by code i have to have 4 inches from the insulation to the earth.

  6. #6
    Join Date
    Mar 2007
    Location
    NW AR
    Posts
    2,478
    Quote Originally Posted by MArden View Post
    Even though there is a few small holes in the supply duct it will probably not be enough for the area. In unconditioned spaces all metal duct should be insullated to combat condensation. With the duct being on the ground plastic or not this will still cause uneeded ware and tare. A solution might have been to use flex duct insted. It is round and can fit in tight spaces better.
    square insulated ducting is hard to beat. the proper material was used IMO

  7. #7
    Join Date
    Mar 2007
    Location
    NW AR
    Posts
    2,478
    Quote Originally Posted by bogdank View Post
    .

    Our concern is more that the duct would sit on the gravel and we are not sure if that would result in additional energy losses.


    Again, the only concern we really have is that the duct would sit on the ground instead of being mounted to hang from the floor joists...

    I was wandering what was the original reason to hang ducts from the floor joists?

    Is it just to be able to crawl below it in spaces that are with more height, or some other reason? In our case, since there is no space to crawl below the duct, hanging it from the floor joists wouldn't make any difference...

    Thanx,
    Bogdan
    Its to keep it off the ground

    about the energy loss....if its laying flat on the ground then it would smash the insulation on bottom, reducing its r-value slightly

  8. #8
    Join Date
    Jan 2009
    Location
    SE Kansas
    Posts
    33
    the only problem you might have is if the duct rusts where it is touching the ground. dig the gravel away from the duct and there shouldnt be a problem

  9. #9
    Join Date
    Jan 2009
    Posts
    11
    Quote Originally Posted by ar_hvac_man View Post
    Its to keep it off the ground

    about the energy loss....if its laying flat on the ground then it would smash the insulation on bottom, reducing its r-value slightly
    Thanks for the comments!

    I was actually thinking about the insulation on the bottom. You're right. We may lose some r-value there.

    As for the water... I can just hope we would not have a problem. That was actually very good point. We are not in the flood area, we have no vents on the crawl space walls. We have 3-4" of gravel and plastic sheet. Outside of the foundation wall at the footer we have the perforated PVC pipe all around to redistribute any excess ground water evenly and the foundation was water proven from the outside.

    All gutters would go to underground PVC pipes to the dry well away from the house.

    So, we did all we could to get the water away from the house or to keep it from going in.

    Unfortunately, the contractor did not dig deep enough crawl space to allow for more height. It is just around the minimum depth allowed by the code.

    I guess he saved one layer of blocks around the perimeter...

    Thanks again for your comments!
    Bogdan

  10. #10
    Join Date
    Nov 2005
    Posts
    1,350

    DID THIS PAS INSPECTION?

    I don't want to burst your bubble, but according to code that duct cannot rest on gravel, even if there is a vapor barrier. I was also under the assumption that r-8 was required in unheated crawl spaces. I believe the code dictates this, your going to need 3" between the duct and the gravel, after insulation.
    I STARTED WITH NOTHING, AND I STILL HAVE MOST OF IT!

  11. #11
    Join Date
    May 2007
    Location
    Alabama
    Posts
    909
    Quote Originally Posted by allstar08 View Post
    I don't want to burst your bubble, but according to code that duct cannot rest on gravel, even if there is a vapor barrier. I was also under the assumption that r-8 was required in unheated crawl spaces. I believe the code dictates this, your going to need 3" between the duct and the gravel, after insulation.
    I don't know where they are at, but that's not code here. Around here, if you want R-8 you have to order it. 4.2 and 6 is all you'll find here.

  12. #12
    Join Date
    Mar 2008
    Posts
    657
    well first i would not let the duct rest on the gravel even if it was wrapped. where i'm from flex duct has to be r-8 but i don't know about supply trunks. any one have an answer for that?

  13. #13
    Join Date
    Jan 2009
    Posts
    11
    Quote Originally Posted by allstar08 View Post
    I don't want to burst your bubble, but according to code that duct cannot rest on gravel, even if there is a vapor barrier. I was also under the assumption that r-8 was required in unheated crawl spaces. I believe the code dictates this, your going to need 3" between the duct and the gravel, after insulation.
    I guess we'll find out soon if this would be OK. Our contractor and his HVAC sub-contractor claim that this would be OK (for duct to sit on the gravel). The inspection should happen this week or next week and we'll find out.

    The worst case would be that the contractor would have to take the shovel and scrape another 3-4" inches below the supply and return ducts.

    Not that we did not ask him the same question 3-4 times when he was digging the crawl space back in September...

    Code may be different state to state, county to county I guess...

    Thanks,
    Bogdan

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event