GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon

Community and Q&A

Installing cellulose over fiberglass

DJ Tejeski | Posted in Green Building Techniques on

What is the correct way to install blown in cellulose over batt fiberglass insulation and can you blow in cellulose over rock wool?

We are in zone 5 bucks county PA

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Sean @ SLS | | #1

    You take out the batts, install venting baffles, do your air sealing work, & then install the cellulose

    If the batts are still in good shape, donate them to a habitat restore, or person that needs them - more than likely though they are toast

  2. Torsten Hansen | | #2

    Before blowing cellulose, there are a few important steps you should take:

    - First, before even going into the attic, draw yourself a map of the ceiling of the level below the attic (i.e. the attic floor) and mark where all the penetrations are (light boxes, recessed light, bathroom fans) and also where the interior walls are located - this will help you locate these spots once you enter the attic.
    - Second, go into the attic armed with can foam and caulk, move the batts aside temporarily and seal all the many penetrations; if there is a chimney, use metal flashing and high temperature caulk to seal the gap between the framing and the masonry; install temperature resistant covers over all can lights and make sure that they also are air sealed properly (e.g.
    - Third, make sure that you have proper ventilation in the attic; install baffles if needed.
    - Fourth, build a sturdy insulation barrier around the attic hatch. It should be made of wood, air sealed and equipped with an insulated and sealed hatch.
    Now you can blow your cellulose. Follow the directions on the bags and be sure to install the proper number of bags as listed on the coverage chart.

    Or alternatively, hire the work out to a professional insulation company. It is hot, itchy and unpleasant work.

    Either way, good luck.

  3. User avatar
    Joshua Lloyd | | #3

    You can blow cellulose over any insulation you want. But there is no reason to remove old batt insulation unless it is literally falling apart. The beauty of blowing in the cellulose is it will fill in all the voids between the batts and actually make the batt insulation perform slightly better because it will compress slightly. But as Torsten said, make sure you move the batt & rock wool insulation around first and air seal the attic. It can significantly reduce the amount of air infiltration. If you pay attention to the insulation when you do the sealing work you will know which areas are leaking the most because the batt insulation will be very dirty.

    If you are going to do the work yourself, check out the Energy Star DIY guide to air sealing. It is a great resource. ( The job can be tedious depending on the head room in your attic, but it will be worth it once the job is done.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |