Insulating floor of vented attic in 19th century former farmhouse
I own a mid-nineteenth century farmhouse in upstate NY. A prior owner had built out the third floor attic space. Those improvements (sheet rock and fiberglass insulation) have been removed, because of roof leak issues damaging the sheet rock and mold growth on the sheetrock. The third floor (attic) had not be used for other than storage for over a decade.
I am now making some renovations to the house and plan to return the attic back to vented, unimproved attic space. But I am struggling with how best to insulate the second floor from the vented attic.
For environmental reasons, I would like to stay away from spray foam.
The attic floor is comprised of roughly 5 inch wide boards (which I believe were milled shiplap rather than tongue and groove), with a thickness of about .75″. The joists are roughly 3″ by 4.5″.
My preference is to remove the attic floor boards, clean out any material between the joists, pack in cellulose insulation, and rebuild the floor.
The attic space above will be vented with a ridge vent and eave vents (that will be installed this spring when the space is reroofed).
Questions:
1. Ought there to be a vapor barrier under the cellulose and above the 2nd floor ceiling?
2. With or without a vapor barrier, 4 inches of cellulose has a lower R-value than I would like. I want to keep the attic usable for storage. I has been suggested to me that I could get a higher R-value but adding to inches of insulating board (blue board?) to the attic floor, and cover the usable floor with .5 inch plywood. I’ve been told I would need a vapor barrier below the cellulose to avoid condensation on the insulating board. Do I need a vapor barrier in any event?
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Replies
Hi Garrett,
If you don't use rigid foam in your attic, you probably don't need to add any vapor retarder to the ceiling, the painted drywall should be fine. Most moisture problems in an attic like yours (besides roof leaks) will be from air leaking from the house into the attic. So, you should do an impeccable job air sealing. This article will be helpful: How to Insulate an Attic Floor.
If you decide to install rigid foam above the rafters, then you have some additional concern. The bottom of the installed foam can potentially create a condensing surface and air leaks and vapor drive (but mostly air leaks) can cause a problem. You could install a vapor retarder to minimize this potential, but a better method is to install enough rigid foam that the bottom surface never reaches dew point temperatures. With only 4 1/2 inches of joist space to insulate with cellulose, you won't need too much R-value of rigid foam, but you should make sure to get it right. This article will be helpful: Combining Exterior Rigid Foam With Fluffy Insulation.
All that said, If I were you, I'd consider how much you need that storage space, because a much easier way to go, which will give your house a more efficient lid, would be to air seal the attic floor and blow in a mountain of cellulose.
If you are pulling up the floor, the best way to add R value to the attic is cross strap the existing joists there with 2x6 on edge. You can than blow in loose fill cellulose insulation, you don't need to dense pack. Once the insulation is in, re-install the floor boards over the new joists.
Make sure to air seal the attic floor before any new insulation goes in. With older construction, especially if you have any balloon framing with no top plates, this will make a bigger difference for energy use than the extra insulation.
The nice benefit of cross strapping is that is reduces the thermal bridging of the studs, it is about as effective as rigid insulation but much cheaper. This gets you a roughly R34 assembly VS R13 as is. You can bump up the 2x6 if you want even more R value.