GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Attic insulation

onthewater1 | Posted in GBA Pro Help on

Hi, My air conditioner doesn’t keep the house cool enough in the summer. I live in SW Florida and the house was built in the 80’s. The existing blown in insulation has matted down. I’m about to get a new roof. I’m hoping that if I seal the attic cracks plus add inslulation it may solve my problem. I’ve come across many options and not sure which/what to do, they include which insulation should I use, fiberglass or foam on the floor, should I put a couple of inches rigid foam insulation between the roof and the plywood and insulation on the inside ceiling of the roof between the rafters? Should I cool the attic or replace the air conditioner? Help, there’s so many options.

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. GBA Editor
    Martin Holladay | | #1

    Every house needs a good air barrier -- so if there are cracks that need sealing, then air sealing work comes first.

    If you are installing insulation on the attic floor, the first choice is usually cellulose. Aim for R-38 to R-49 of insulation.

    If you have ducts in an unconditioned attic, you may want to consider moving the insulation from the attic floor to the sloped roofline. This work is expensive, but it brings your ducts inside your home's thermal envelope.

    For more information, see these three articles:

    How to Insulate an Attic Floor

    Creating a Conditioned Attic

    My House is Too Hot

  2. Expert Member
    Dana Dorsett | | #2

    Even if the ducts are going to remain in the attic making sure that the ducts & air handler are well sealed goes a long way toward getting the nameplate performance out of the AC unit. Air leakage from ducts or air handlers in the attic, outside the thermal and pressure boundary of the conditioned space cause a HUGE parasitic load from air-handler driven outdoor air infiltration.

    Duct mastic on all seams and connections (including the register boots) matters, as does sealing the register boots to the walls or ceilings. Sometimes the latter is most easily done from the room side, with can foam or caulk, taped over with housewrap tape to improve long-term air tightness. This guy's register-boot sealing to the ceiling was made more difficult by the texturing of the ceiling paint, but is still WAY better than nothing:

    Flex duct that isn't properly supported and not stretched tight can dramatically reduce the total air flow, impeding performance, and can be re-commissioned at low cost, even as a DIY.

    Unbalanced duct designs are commonplace, and can usually be improved at low cost. Unbalanced ducts lead to pressure differences between rooms, that use "the great outdoors" as part of the return path. A sub $100 box store 2-port manonometer that measures with a resolution of 0.01 water inches is good enough for finding and fixing the big offenders (and for measuring your duct impedances, if you get into playing "junior HVAC tech".) An Energy Star duct system would have no more than 3 pascals (=0.012" water) room to room pressure differences under all air handler speeds, all conditions (doors open, closed, etc) Most 1980s vintage duct systems will have at least one or two rooms with inadequate return path and more than 0.3" pressure difference when the door is closed. With some partition wall jump ducts or other measures it's usually possible to bring that down to the point where it's at the bottom level of what a cheap manometer can measure.

    Re-commissioning the duct system has a bit of learning curve, but it's not rocket science. A cheap dual-port manometer can measure the total supply to return plenum pressure, the pressure drop across the air filters, etc, and you can map the whole duct system to find the weak spots where flow is excessively impeded. For a primer on the subject, let this guy walk you through it:

  3. onthewater1 | | #3

    Thank you so much for your time and input.
    So if I insulate the sloping underside of the roof should I remove the old matted down blown in insulation on the floor? Also if I insulate the underside slope should I still use some rigid foam insulation between the shingles/metal and the plywood? Since the attic will be in the envelope will be cooler but still warm right, I don't want to ac the attic do I?

  4. GBA Editor
    Martin Holladay | | #4

    Q. "So if I insulate the sloping underside of the roof, should I remove the old matted down blown-in insulation on the floor?"

    A. Assuming you are doing a thorough job of creating an unvented conditioned attic, with adequate R-value at the roof plane, you can remove the insulation on the attic floor. That said, it probably isn't strictly necessary to do so.

    Q. "If I insulate the underside slope, should I still use some rigid foam insulation between the shingles/metal and the plywood?"

    A. If you are insulating the roof slope, you have to follow a method described in this article: How to Build an Insulated Cathedral Ceiling.

    There are several possible approaches. You can add a continuous layer of rigid foam above the existing roof sheathing, with or without additional fluffy insulation under the roof sheathing. Or you can add closed-cell spray foam on the underside of the roof sheathing, with or without additional fluffy insulation on the interior side of the cured spray foam.

    Q. "Since the attic will be in the envelope, it will be cooler but still warm, right? I don't want to A.C. the attic, do I?"

    A. In most cases, you don't need to provide cool air to the attic. The exception would be for a conditioned attic with open-cell spray foam on the underside of the roof sheathing. This type of attic is subject to moisture problems, and in many cases such atics need deliberate cooling. The best way to avoid this problem is to avoid the use of open-cell spray foam. For more information, see High Humidity in Unvented Conditioned Attics.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |