Fiberglass will not seal or stop air movement from leaks.

It takes only the slightest temperature difference to move air through the fiberglass.

One way of finding the leaks in older jobs is to look for the spots the fiberglass is dirty from air flow.

Spot foaming before blowing can seal up the infiltration paths.

farmgirlinky

Your "experienced local man" seems to be suggesting a conditioned attic. But you can't create that with fiberglass!

See link for the foaming and other details that will work on a conditioned attic.

OTOH, you can also get good results with a vented attic. In that case, you have to seal and then insulate the top floor ceiling. Cellulose is your best bet there. And in that case you must be sure the soffits are clear of insulation and there are ridge or roof vents.

Here is a link that might be useful: Deep energy retrofit

farmgirlinky,

There is nothing about experienced local man's commentary (as reported by you) that would lead me to believe he knows what he is talking about.

Perhaps there are some transcription errors between what he told you and what he meant, however, everything he said is incorrect.

worth and brickeye are both 100% correct.

Foam is actually the material of choice when it comes to converting an attic into conditioned space, however, that is not necessary to get the home efficient and converting a previously vented attic to an unvented (i.e. conditioned zone) attic can be a recipe for disaster if not done properly.

Vented attic assemblies work just fine and can be done for a fraction of the investment when compared to converting to an vented attic to conditioned space in most cases.

Thank you worthy, brickeye, and windowsonwashington -- I'm calling in another consultant, because I agree that something doesn't add up -- could be my ignorance as a homeowner, could be a consultant's disinclination to keep up with the latest information on insulating old houses, we'll see if another company can clarify the issues. Our attic as it stands is definitely vented, so: do I want to convert it to a conditioned space, or do I keep it as a vented space and pursue a vented attic assembly? What would the latter entail?
Lynn

How to proceed depends on what your end goal is. A vented attic is the norm and the cheapest to achieve. However, that means that attic itself is going to be cold in winter and hot in summer. The attic is on the "outside" of your conditioned home and you seal and insulate between the living space and the attic. The attic itself will have outside air flowing through it and will keep the roof cool in the winter.

If you are trying to turn this into livable space, then you will need to create a barrier between it and the "outside" space and then seal and insulate accordingly.

You should not mix these 2 approaches.

No we're not trying to convert the attic into livable space, it really is a vented attic above the third floor (half-story with unfinished "luggage room" and dormers and knee walls) with no need or potential for it to be anything else. The house is stucco with a wood frame, no insulation in the walls, rock wool on the attic floor. The attic is accessible through a trap door in the third-floor ceiling. Drafty house. Big ice dams in winter, on a slate shingle roof, but no leaks last year -- the roof was extensively rebuilt with ice and water shield ten years ago. What approach makes sense for this situation?
Lynn

The basic plan it to draw an imaginary line between the "inside" conditioned areas and the "outside" areas. Any place that those 2 meet, you want to seal up any air movement and then add a layer of insulation. (If you have plywood or similar on the floor, you might have to take that up temporarily to seal and add the insulation. )

For venting, you want vents at the lowest point and vents at the highest point and nothing obstructing air from flowing between the two. That way, cool air is pulled in from the bottom and hot air vents through the top. That keeps the attic cooler in summer and keeps heat from building up and melting snow in winter.

Thanks, billl, it is just this talk about sealing and managing air flow that was missing when I was going over the plan with the insulation guy we consulted. I began to feel queasy about his plan. In fairness to him, maybe the routinely do a fantastic job of sealing up areas of infiltration and he didn't feel the need to mention it. So we'll see what next week brings when I talk to another person. Any opinion about foam vs fiberglass in a 100 yo stucco wood-framed house with a vented attic?
Lynn

If you are just insulating the floor, the cheaper insulation are just as good as foam. Foam is used most effectively as an air sealant. Foam is also good for insulating surfaces that will be more difficult with fiberglass and cellulose (i.e. rafters, under subfloors, etc).

Bill is right on. Make sure that the air barrier (envelope) and insulation layer line up perfectly.

Ice damning is more an issue of heat/air loss than it is an issue with the roof.

Knee walls need to make sure they are sealed up, insulated, and proper venting from soffits to the ridge.

The preferred attic insulation in your climate is blown cellulose. Research at the Oak Ridge National Laboratory showed fg insulation losing as much as half its nominal R Value in cold weather as a result of air currents within the insulation. Furthermore, blown cellulose forms an excellent air barrier, helping to fill up the little leaking nooks and crannies in the attic.

Look for an insulation company that can provide both foam and cellulose.

+1

Cellulose is the blown in material of choice.

a. It actually performs at its intended R-Value as compared to fiberglass
b. It does not have near the exposure risks of fiberglass
c. It can be an air barrier when blown under dense pack (3.5lbs) conditions.
d. It does not allow for convective looping inside the material like fiberglass does.

Speaking from inside the big wet sponge that Irene wrapped around southern Connecticut: what about moisture problems with blown cellulose? Are we going to rot our heretofore wonderfully sturdy wood-framed stucco house?
Lynn

Only if the roof is leaking.

"what about moisture problems with blown cellulose?"

Blown cellulose tends to absorb and release humidity better then fiberglass stabilizing the humidity.

Fiberglass cannot absorb humidity, and in cold enough conditions allows frost to form in the insulation.

Cellulose can absorb and hold moisture without forming frost, then release it without forming water.

Cellulose is preferred for older homes that may not have an actual vapor barrier.

Only if the roof is leaking.