Insulating unfinished attic/3rd floor - staging for Geothermal

thanosSeptember 3, 2008

I'm so confused here, I really hope someone can help me figure this out. I've been doing so much research and found so much conflicting information I can't make a decision.

I have a 130 year old victorian house. I will be switching from oil to geothermal soon but first I wanted to insulate the unfinished 3rd floor. At the same time, I'd like to finish the space and make it usable.. I can afford to insulate using tradition fiberglass batts and completely finish the space.. but I'm not sure if I would be better off with some kind of foam, icynene.

I've read good and bad regarding icynene.. I don't know if it is worth the extra expense or delaying the finishing of the space. I don't think I could afford to do the entire project at this time if I were to go with spray foam, but I don't want to make a hasty decision I will later regret.

I am not as concerned with R value as I am with moisture and how it will hold up over time.

PLEASE someone help me get past this mental roadblock!

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Start with the design conditions and work forward. How deep are the rafters and what is their slope, what is the roofing material, what is the sheathing and what R-value do you need to achieve?

    Bookmark   September 4, 2008 at 1:55PM
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When you speak of insulating in order use the attic as occupied living space, what parts of the walls, ceiling were you thinking of insulating?

Insulating ceilings directly under roofs, as you may know, is problematic and requires special care. You can't just slap insulating material up under the roof sheathing.

The attic/third floor insulation that is commonly recommended as the first step to weatherize a house is usually installed in the floor of the attic, blocking heat from escaping through the ceilings of the floor below. This is usually cited as the first, most energy efficent task that is done. Faced fiberglass batts between the joists are pretty easy to use here, but some people use blown-in cellulose within the joist acvities. If you have extra-deep joist cavities you can lay additional bats of fiberglass above the first ones.

But if your plan is to insulate the side walls (if any) of the attic and the roof then you may want to consider foam, either as sheets or sprayed, and the special ribbed panels made by Owens Corning (among others) meant for installations in catherdral ceilings. These allow "contact" insulation against the roof while still maintaining an air channel above the insulation. What you should be looking for in this case is info about insulating so-called cathedral ceilings.

A good first step, meaning fairly cheap, DIY, reversible and immediately effective on its own would be simply insulating the attic floor between the joists with fiberglass bats. Then as your heating plans firm up and your plans for using the third floor evolve you'll have something that you could remove and reuse (depending on what you have to do the fit the fiberglass batts in), and certainly someting that doesn't foreclose any future options, or interfere with wiring changes, new vents, etc.

You asked about vapor barriers; although it would be ideal to have a vapor barrier install just above the ceiling interior finish in the second floor, it is often impossible. You can use careful fitting of faced insulation to help you in this case, and you can prime your ceilings with vapor retardant paint. From what I have read vapor barriers in ceilings are not as critical as vapor barriers in walls. I imagine this is because heat rises and even with insulation you will have some, perhaps adequate, vapor dispersion through the fiberglass and on out into the unheated attic. And hopefully, adequate (or at least no worse than you have now) dispersion through attic vents to the outside.

Some of the issues of vapor barriers are related to how extreme your winters are. The colder they are, the more vapor issues intrude.

FWIW, I am in northern NY (very cold) and I have faced fiberglass laid in parts of my attic with no other VB and nothing untoward has happened after nearly 20 years. Main issue is meticulously blocking entrance to the joist cavities so that mice and squirrels don't nest in it. Degrades it and makes it a bit stinky.

Are you aware of the Preservation Briefs series of handbooks regarding care and maintenance of old buildings? I'll attach a link, there is one directed at weatherization and energy conservation issues that you may find very useful.



Here is a link that might be useful: National Park Service Preservation Briefs Series - A wide variety of topics useful to old house owners

    Bookmark   September 4, 2008 at 2:17PM
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Thanks guys! Molly, that was really great information.

I want to make sure I use the right type of insulation. I've had many people recommend icynene. Do you have an opinion on icynene? Is it really THAT much better than fiberglass or other options?

    Bookmark   September 5, 2008 at 9:24PM
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The first issue is ventilation of the insulation in the rafters and any knee walls or floors under roof eaves (so it can dry out when your roof leaks).

Are there air vents in the roof eaves? Are there ridge vents? How was the original attic vented? Will there be a space above the future ceiling for venting? If not, the cost of adding these vents to an old house must be added to the cost of porous insulation like fiberglass. The vents would not be required for polyurethane foam. No vapor retarder would be required in any event.

Icynene is a trade name rather than a generic name for open-celled polyurethane foam. Closed cell-polyurethane foam has a higher R value but is more expensive.

Get a good insulation contractor and a carpenter to price fiberglass insulation and installation of new vents and also a price for open-celled foam. Don't make any assumptions based on internet information, get real prices from real installers.

    Bookmark   September 5, 2008 at 10:46PM
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