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Old 02-09-2010, 11:30 AM   #1
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adding attic insulation to an old house

I just bought an 1867 farm house with lath and plaster ceilings. The attic has rough plank floors installed with approximatel 3-4 inches of blown fiberglass in the trapped space between the second floor ceiling and attic floor. ther is no vapor barrier installed. No soffet vents but a ridge vent. The attic space is used for storage. After a 500$ gas bill, I would like to add insulation. I was thinking of placing furring strips down,unbacked foam insulation boards between the furring strips and a laying a second floor of plywood to add R-value. Is this plan advisable? should I use a vapor barrier? Other options? Any thoughts would be appreciated


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Old 02-09-2010, 12:50 PM   #2
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Adding that much additional weight and movement while making the install to say nothing of the hammering and screwing on top of what was intended to be nothing more than ceiling joists only, should be looked at very closely in a house that old. You could bring down the plaster ceilings if you aren't careful.

How about blow in more insulation and forget about stressing the structure?


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Old 02-09-2010, 12:56 PM   #3
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Building Science

Take a look at the Building Science Corp article on attic venting:

I would suggest you add soffit vents and interior vent baffles at the eaves, to keep your new soffit vents open, and then blow at least a foot of loose fill insulation in there. Save the plywood for building a box or "well" around the attic entry point.

Also, before you add the insulation, take time to seal any holes around fixtures, wires, pipes, ducts, etc., where air may be flowing freely from your living space into the attic.

Good luck,


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Old 02-14-2010, 09:54 AM   #4
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Since you have an Old House. You will need methods that are Meant to be complementary to an old house without compromising it's construction (ie: trying to retro in new materials that don't play well with the old materials)

Order up a couple of back issues of "Edifice: Old Home Magazine"
Issue #19 regarding proper roof/attic insulation and soffit venting. It will blow you away how warm an old house can be "Without" blown in insulation.
Also get a copy of Issue #7 regarding all the ways of using caulking in balance with the house construction to make an old house non-draughty and warm. No need to start filling the walls with insulation at all, and if you will more than likely get a serious rot issue very fast.

These old house are NOT modern houses...and should be treated as such.

Good information that you're not going to find at Home Dep*t

Last edited by Skuce; 02-14-2010 at 10:00 AM.
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Old 02-14-2010, 11:45 AM   #5
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to fix this right you first need to remove the existing insulation. Save it and use it after you fix the pressure boundary. For an attic to be done right you need to bring the pressure boundary in line with the thermal boundary. after removing the old insulation seal every hole to the lower ceiling that you can find. using spray foam seal the holes where electrical or plumbing or ducts penetrate this plain. Check the wall cavities .... these should be sealed also. You want no air from the lower floors to find its way into the attic. Build a box around the attic opening with 2" foam and tape magnetic strips to the floor and the box to allow it to seal completely. After it sealed you can put your own insulation back in place and add more.
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Old 02-14-2010, 04:57 PM   #6
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Don't install plywood or OSB sheathing on the ceiling joists, with furring or not. They are vapor retarders (0.7 perms), semi-impermeable-Class II vapor retarder. Right now the moisture can escape through the gaps in the boards, with ply, you will have mold because the lath and plaster has a perm rating of 11 (vapor permeable- greater than 10 perms:

Be safe, Gary
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Old 02-14-2010, 04:59 PM   #7
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OR... if you need the flooring for storage you can install the plywood as you propose but more the insulation into the slope of the roof rafters.
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Old 02-15-2010, 07:48 AM   #8
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Thanks for the link, Skuce.


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