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fiberglass insulation sizing

JustVenting Member Posts: 3
edited November 2021 in Strictly Steam
hi all,

stumbled on this site while planning a fiberglass insulation for our exposed boiler pipes in the basement (for better or worse, the asbestos wrap was removed prior to moving in this summer).

i have a question regarding fiberglass insulation: the majority of our iron pipe seems to be between two insulation sizes that my local plumbing supply store stocks. Attached are pictures of the two sizes(2-1/8 and 2-1/2) on the pipes. My question is which is preferred: the tighter fitting insulation with a gap or the loose fitting insulation?

thanks in advance


    JUGHNE Member Posts: 11,062
    edited November 2021
    Iron pipe sizes are larger outside diameter than copper pipe sizes.
    It depends upon what you ask for.

    Your 2 1/2" pipe is most likely 2" iron pipe which is 2 3/8" OD.
    The 2 1/8" pipe is most likely 1 1/2" iron pipe which is 1 7/8 OD.

    You may have a layer of paint on your pipe.

    At the supply store ask to see a 2" iron pipe nipple to compare to how the insulation is fitting. It comes out to be just right and tight.

    Often the size is printed along the overlap strip, you want IPS sizing.
  • Jamie Hall
    Jamie Hall Member Posts: 23,324
    But in a direct answer -- the tighter fit is better.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • JustVenting
    JustVenting Member Posts: 3
    thank you for clarifying, Jamie. 

    Looks like the 2-1/8 in insulation was indeed for 2” copper pipes. The 2-1/2 in insulation is for 2-1/2” iron pipe. I need 2 in insulation for 2” iron pipe. A little confusing at first, but I got it. Back to the store…
  • dopey27177
    dopey27177 Member Posts: 887
    Jughne gave you the answer if you have black pipe 21/2 inch is the proper size.

  • JustVenting
    JustVenting Member Posts: 3
    i have 2-1/2 inch pipe that leads in into 2 inch pipe. most of the risers come off the 2 inch pipe section. 
  • delcrossv
    delcrossv Member Posts: 742
    Trying to squeeze the best out of a Weil-McLain JB-5 running a 1912 1 pipe system.