Attic insulation in 90 year old house


Old 01-04-05, 06:48 AM
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Attic insulation in 90 year old house

New member looking for advice. My apologies in advance if I use the wrong terminology.

About a year or so ago I bought a 90 year old semi-detached row house in Washington, DC. There is old insulation in the attic under the roof line, but I am not sure what it is. Cardboard sheets are stapled between the rafters, with a layer of loose cottony stuff resting on the cardboard (I guess blown in). Over the years, a few of the sheets have fallen down. The floors are beautiful original heart pine that could be refinished later. Currently, I am only using the attic for storage, but I might finish it sometime up the road. Heating bills last year were high, but that might be from the totally uninsulated walls (apparently there is no easy way to insulate them) and too many drafts in the windows and doors (getting new storms for both). Electric bills for the window AC units in the summer were fine.

The rafters are 2 x 6 construction, mostly 24" on center. However, a few of them are narrower, and one is wider by a few inches. There is a slate mansard roof at the front of the house, with a gently sloping roof (10-15 degrees maybe) to the back. The only ventilation is a window in a gable (?) in the mansard roof. I have not had any moisture problems so far.

I had an insulation contractor come in, who told me that the best option would be to remove the old stuff and blow in insulation below the attic floor. But to do so, the contractor would have to drill 3 rows of 3" holes 16" apart in the hardwood floors. Not only would that ruin the appearance of the floors, but I fear it would damage them structurally.

I am considering either leaving the current stuff in or replacing it with rolls/batts between the rafters. I know that I would have to put in foam spacers to prevent the fiberglass from being in contact with the underside of the roof. From what I can tell, I could get R-13 or R-19 up there.

One final bit of information - the slate mansard roof is shot, and is getting replaced in the next couple of weeks. That could give me an opportunity to get some sort of ventilation installed (although I don't know if that is possible).

A few questions:
1. Is keeping the old stuff or replacing it with rolls/batts generally a good idea?
2. Will there be significant heating bill savings if I get rolls/batts?
3. What kind of moisture/ventilation issues do I need to worry about if I get rolls/batts?
4. If there is no ventilation other than the window, where will the (presumably moist) air that gets collected in the foam spacers go to? Does it just seep through the roof? Is that what is what is going on now?
5. If get rolls/batts, do I need some sort of vapor barrier or is that a bad idea in a house in which there currently are no moisture problems?

Sorry this is so long, but this is all new to me. Thank you again for any help.
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Old 01-04-05, 03:32 PM
Join Date: Nov 2001
Location: USA
Posts: 1,873
The slate roof can be described as anything except air tight. That is the reason you did not have a moisture problem with the rock(mineral) wool insulation. Once you fix the mansard roof to today's standards and if you leave the insulation there, you will end up with a moisture problem.

The insulating contractor is correct, the insulation should go under the attic floor. Having it under your roof does several things, one that might interest you is that it was probably one of the causes for the drafts in the house. It known as the stack effect.

Recommend having another contractor evaluate situation and express your concern about the floor. There are ways to get around damaging the floor.
Old 01-06-05, 04:40 AM
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Thanks for the advice. Two follow up questions.

First, it is only the small mansard roof in front that is slate and being replaced. Most of the roof (and all of it that is under the mineral wool insulation) is modern and was replaced 7-8 years ago. Are you saying that the slate mansard roof in front is providing so much ventilation that I will have a moisture problem after it is replaced?

Second, does it make sense to have the insulation under the attic floor if I plan to finish the attic space? I haven't made that decision yet, but it at least a possibility.

Thanks again.
Old 01-06-05, 11:00 AM
Join Date: Nov 2001
Location: USA
Posts: 1,873
Yes the slate roof right now is providing the ventilation needed for the roof. Once you replace it, you are going to have to install some vents.

I would still recommend the insulation under the attic floor. If and when you decide that you want to finish the attic, the insulation in the attic floor will not be a factor.

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