Adding attic insulation


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Old 12-10-12, 12:40 PM
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Adding attic insulation

Brand new to the forum. I have been a DIY'er for years. I'm adding insulation to the attic in a house I had bought recently. It's a 1700 sq. ft.ranch. The attic currently has r19 batt with 1/2in. plywood covering it for a floor. I would like to add r30. Can I lay this over the plywood or do you think I have to take it all out? Thanks.
 
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Old 12-10-12, 12:52 PM
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If you add the insulation over the plywood, you will loose the room for walking and/or storing of items. If that is not a problem, you can add another layer of R11 unfaced batt insulation over the plywood which would give you 9.5" of insulation. I assume the installed R19 has a paper/foil facing on it already and is facing down (towards your living space)?
 
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Old 12-10-12, 01:00 PM
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Hi always and welcome to the forum.

Your question is a common one and it is good you asked. Most homes were never well air sealed from the house to the attic. Having an air barrier (the plywood) on top of the insulation runs the risk that warm moist air leaking up through the insulation may find a cold surface onto which it can deposit its moisture. We have had threads where someone found moisture beneath similar flooring. Whether that is happening in your case we do not know, but the correct approach would be to remove the plywood, work around the existing insulation to air seal, add as much insulation as desired, and elevate a walkway over the top of everything if access is necessary. Just a walkway and not the entire attic floor.

How big of a project did you want to get into?

If you are not experiencing moisture now, or in the coldest months, then adding more insulation over the top can only help, ie that plywood would not get as cold. How it will perform in the summer with air conditioning I cannot say, if you run AC.

Bud
 
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Old 12-10-12, 01:03 PM
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What about blown in insulation? It will help stop up any gaps in the current insulation. The big box stores generally let you borrow the hopper/blower for free if you buy enough bags.

No matter what type of insulation you use - make sure the soffit vents will still have air flow to the ridge or gable.

btw - welcome to the forums!
 
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Old 12-10-12, 01:20 PM
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Adding attic insulation

I'm not really going to use the attic for storage. Just the section over the garage. That can stay at r19. I am planning on air sealing the attic so I guess I'll be taking up the plywood anyway. I only need a walkway down the middle for access to the hvac. The builder built the house for himself so I guess he had a lot of plywood let over from other jobs. I wasn't sure how much the plywoood would affect the insulation or if it created another vapor barrier. I didn't think of moisture underneath the wood. I did pull one 4x8 up and no signs of wetness. There are 16 non ic recessed cans under the plywood. I'll be changing them out for airtights.
 
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Old 12-18-12, 11:01 AM
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Condensation in attic

The water vapor you produce in your home is programed by nature to head for the nearest cold surface to condense.
This may be your windows, inside your walls or in the attic.
You can stop it entering the attic by blocking all holes between your warm rooms and the attic, making sure they are water vapor tight. You can also paint your ceilings with either gloss or laytex paint, these are water vapor proof.

If you have drywall ceilings that are not painted as above and fibreglass insulation, both of these let water vapor through into the attic, once there it needs an escape route to the sky.
 
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Old 12-18-12, 04:23 PM
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There are 16 non ic recessed cans under the plywood. I'll be changing them out for airtights.
Your new lights need to be IC (Insulation Compatible) as well as air tight. Two different standards.

Originally Posted by Perry525
You can stop it entering the attic by blocking all holes between your warm rooms and the attic, making sure they are water vapor tight. You can also paint your ceilings with either gloss or laytex paint, these are water vapor proof.
There's actually paint-on vapor barrier available. But I would only consider using these if there wasn't a VB under the existing insulation. No two vapor barriers for any one surface is the rule. Air sealing, to prevent the physical movement of air, is an excellent and often overlooked improvement. But only one vapor barrier.

It's a 1700 sq. ft.ranch.
What is the shape of your roof? How is it vented now?
 
 

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