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beffreyj3 10-17-2011 05:02 PM

Attic insulation over old attic floor
Here's the situation:
Previous homeowner converted an attached garage to a bedroom long ago (
house is 1927, garage conversion I'm thinking circa 1940-50's). Needless to say, we need to add more insulation. Going up through the ceiling layers, there is ceiling plaster, 2x4 joists packed with 4" of old fiberglass insulation, and 3/4" x 5" floorboards running perpendicular to the 2x4 joists. You can imagine it being a storage area above the garage.

(There is also knob-and-tube electrical running through the old insulation, but that's already in process of being modernized).

Question: is it ok to just add insulation over the floorboards, or should I remove the floorboards first and add new insulation? Floorboards are going to be a big painful job to remove.

Initial fear was that the floorboards would act a moisture catch when sandwiched between the old and new insulation.

I'm leaning towards leaving the floorboards and adding new insulation on top. I'm thinking floorboards 1) won't act as a moisture catching layer, and 2) are probably something of a structural component at this point, especially with the 2x4 joists.

I'm in Northern Ohio where temps swing from 90+ in summer to 0 in winter.


Maintenance 6 10-18-2011 03:43 PM

As long as there isn't any old flooring material or old tar paper laying on the wood flooring, it won't act as a significant vapor barrier.

Ron6519 10-18-2011 03:53 PM

I'd use blown in or unfaced insulation right over the flooring. It will keep you from storing things on the beefy joists.

jklingel 10-19-2011 12:55 AM

wood is in no way a vapor barrier. it's perm rating is somewhere around 30/inch. blow in cellulose and have fun. just make sure you air seal like a madman before you do any insulating.

Wildie 10-19-2011 05:04 PM

I once owned a house with a walkin attic that had insulation between the joists and had a tongue and groove floor on top.
I laid R20 fibre glass bats over top of the floor and it worked quite effectively.
You have to make sure that leakage cannot occur where the joists meet the rafters.

beffreyj3 11-17-2011 07:04 PM

Thanks for the advice everyone! Had to delay the actual insulation installation to finish replacing some hidden knob & tube wiring.

Windows on Wash 11-18-2011 07:21 AM


You must, must, must deal with that first.

You home will be warm with knob and tube for all the wrong reasons....:furious:

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