Attic Insulation Over Old Attic Floor - Insulation - DIY Chatroom Home Improvement Forum

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Old 10-17-2011, 05:02 PM   #1
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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.

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Old 10-18-2011, 03:43 PM   #2
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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.
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Old 10-18-2011, 03:53 PM   #3
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I'd use blown in or unfaced insulation right over the flooring. It will keep you from storing things on the beefy joists.
"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."
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Old 10-19-2011, 12:55 AM   #4
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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.
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Old 10-19-2011, 05:04 PM   #5
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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.
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Old 11-17-2011, 07:04 PM   #6
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Thanks for the advice everyone! Had to delay the actual insulation installation to finish replacing some hidden knob & tube wiring.
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Old 11-18-2011, 07:21 AM   #7
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You must, must, must deal with that first.

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