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Community and Q&A

Inadequate insulation under attic floor boards

Ryan Welch | Posted in General Questions on

Hi –
Will adding batts of insulation over attic flooring be effective?  I noticed that there is only R13 and some R19 batts under the floor and some gaps around old non-IC can lights (that are difficult to access).  The flooring is used for storage and access to our HVAC so pulling all of the nailed down boards would be difficult to accomplish.   Also 3 of the rooms that are the most difficult to cool (temps vary throughout the house), are also the furthest from the AC and have the longest duct runs.  Is there a solution for increasing airflow to those rooms?  I had heard the blower fan could be increased and I was also told to change the duct sizing and add additional reflective insulation to the ducts.  Thanks for your help!

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  1. Expert Member
    Dana Dorsett | | #1

    How deep are the joists?

    A popular and not too expensive retrofit solution that still allows for storage is to install a set of joists of sufficient depth perpendicular to the existing joists, filling it with cellulose, and installing new decking (which could be re-using the floor boards) above that.

    Before insulating it's important to air seal the ceiling, and put air tight boxes with sufficient clearance over any non-IC light fixtures, etc.. Air sealing may require temporarily removing the existing batts, but it's fine to re-use them, blowing the new insulation over the top. When the batts are compressed a bit by the weight their R/inch goes up as their contribution to the total depth goes down- it's still doing the right thing.

    To hit R49 with loose-fill cellullose takes an initial blow of ~15". If the joists are milled 2 x 10s (9.25") adding a set of 2 x 6 joists (5.5") perpendicular to them gets it to the right range. If they're 2 x 8 (7.25") a second set of 2 x 8s will do it. With 2 x 12s (11.25" ) adding 2x4s will do.

    By setting them perpendicular to the existing joists the thermal bridging is much reduced, so even if it turns out to be R3 shy of a true R49 it will still be pretty good.

    Are the ducts in the attic, above the insulation?

    Flex, or hard-piped?

    Air sealing every duct seam & joint with duct mastic, and sealing the seams of the air handler with aluminum HVAC tape, as well as caulking the duct boots to the ceiling/wall/floor maximizes the available air flow. If there isn't a return duct or guaranteed return path for every doored-off room that has a supply register it may be necessary to cut in jump-ducts for a return path. Often this can be implemented by using stud bays of partition walls, with a grille near the floor on one side, near the top on the other, so that even with the door closed the static air pressure across the door/wall is under 0.02" water column. (Energy Star duct systems must be less than 0.012" .) As long as the free area of the return grilles are at least as big as the supply registers it will usually be fine, but oversizing doesn't hurt.

  2. Ryan Welch | | #2

    Thank You, Dana, this is helpful.

    I notice you mention using Cellulose in most of your posts. Do you believe it is better than blown fiberglass? I currently have batts and blown fiberglass. 2 insulation companies in my area (I live in the Southeast) have said that they do not use cellulose and only offer fiberglass because it's "much better".

    Also, is it OK to put cellulose over fiberglass?

    Are the ducts in the attic, above the insulation? they are unfortunately in the attic above the insulation. They had started to come apart so I taped them with foil tape and I plan on putting mastic along the tape lines to reinforce the seal. I assume the extreme temps have caused them to separate.

    Flex, or hard-piped? Rectangular sheet metal coming off the unit and the 6" flex ducts for the branches. (they are in the attic, should the sheet metal be wrapped with additional foil based insulation? The inside of the sheet metal contains about 1/2 inch of fiberglass)

  3. Expert Member
    Dana Dorsett | | #3

    Cellulose is greener (recycled material), more air-retardent than blown fiberglass (comparable to dense-packed fiberglass), and is opaque to infrared radiation from hot roof decks, whereas fiberglass is somewhat translucent.

    It's fine to install cellulose on top of fiberglass, and in fact that enhances the performance of the fiberglass by blocking convective loss of performance when the attic is really cold, and loss of performance to IR radiation when the roof deck is really hot. It takes about 3" of cellulose to restore a fiberglass layer to it's labeled performance levels.

    It's unlikely that extreme temperatures have degraded the ducts. Condensation on cold AC ducts can cause corrosion, high static pressures from undersized ducts (= oversized air handler) can cause ducts to flex and wear at the seams joints. Measuring the static pressures across the system is worth doing- high static pressure on ducts cause more noise and lower flow and cutting into system efficiency too. (Having the ducts in the attic above the insulation cuts into system efficiency a lot as well.)

    Unless the ducts are pretty shiny the long term adhesion of tapes is a bit iffy- mastic sticks to anything, and has a better shot at lasting. Fiber reinforced mastic can handle a lot more flexing than unreinforced mastic, but either is better than tape. Tapes work best on shiny-clean galvanized ducts or clean painted air handler seams.

    A half-inch of fiberglass inside the duct is worth about R2. Current code min for ducts located outside of conditioned space is R8. Burying the duct in 3" of cellulose would be R10+ . While it's risky from condensation accumulation perspective to bury uninsulated air conditioning ducts in any of the humid "A" climate zones, the R2 inside the duct mitigates against that risk by a lot. Whether you need more insulation on the ducts to avoid condensation depends a lot on your climate zone and the air conditioning load. Where is this house?

    More here:

  4. Ryan Welch | | #4

    The house is in Birmingham, AL. The ducts I'm having issues with are flex ducts that are separating where the outer foil and insulation are. The ducts themselves seem to be OK, so I used HVAC Duct Tape and painted over that with Mastic. For some reason they are all coming apart, so I assumed it was from the extreme attic conditions in the summer months. Should I replace them all even though the interior liner is fine?

    I will bury them in cellulose per your suggestion once I have repaired/replaced the damaged ones.

    The sheet metal rectangular ducts are definitely very old and NOT shiny, but I did go around and apply mastic at all of the seams. I found some R8 fiberglass wrap with foil backing at a box store. (I've heard the cheaper/thinner "bubble" type shiny wrap has no R value) Would wrapping them significantly help with reducing the output temps? I noticed that the bare sheet metal on the exterior was well above 100 degrees, so I can only imagine how warm it is inside with the mere R2 insulation from 25 years ago.

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