Building Science Corp. recommends a Class II vapour (retarder) barrier in your area. You can use kraft paper followed by mineral wool. Or, probably simpler, use kraft paper-faced fiber glass batts topped by mineral wool.

As for the foam, I use a low-expansion polyurethane foam--mostly because I can never judge just how much regular foam to use. Great Stuff is a popular brand.

Why would you want to put rock wool in your house? It is one of the foulest materials I have ever encountered. I see a lot of it removed during renovations and I won't go near the site while it is being handled.

Why is rock wool such a terrible product? It has an excellent fire class rating vs fiberglass or foam, it does not compress with time and the R value is the same as fiberglass. I have never worked with it; is it more difficult to work with than with fiberglass? I was planning on using the batts. Gee, I did not realize that Great Stuff breaks down/shrinks over time!!! I have thought about foam sheathing boards but it seems like alot more work to fit, cut and caulk. It was a foam installer that suggested that foam sealing the top of the wall plates would be very beneficial even if I didn't get to increase the rest of the insulation at the same time. Is there any other class II vapor barrier other than kraft paper/fg?

Just saying the name makes me want to gag and cough violently. I take a respirator (not a dust mask) when I visit an existing structure in case I run into it. Even a short exposure to it can ruin the rest of your day. If you actually disturb it you will need to change your clothes and take a shower immediately. In my experience, the only substance more aggravating is tear gas.

Fiberglass can be irritating during extended exposure but no way near as much as rockwool. Perhaps some modern versions of it are less irritating but I will not go close enough to them to find out. Let me know how it works out. Excuse me, I'm going to cough now.

For a vapor retarder I would paint the ceilings below with alkyd paint and make sure there are no unsealed penetrations. I can't think of a way to add an effective vapor retarder from the attic side.

I'm OK now so I wanted to add that in the real world only a bathroom with a shower and no exhaust fan would need a ceiling vapor retarder south of Canada IMHO. I would repaint the bathroom ceilings, blow something into the attic (while you are in the Islands) and be done with it.

I went to a home remodeling show last month and spent a long time talking with the rep from the local university extension office. He said that the newest research has shown that in "old houses" (over 100 years) it is best to NOT use a vapor barrier when insulating. I personally have not looked into it much, but his reasoning sure seemed sound. He said that old houses are never air tight and the vapor barrier can sometimes cause water condensation problems since it cannot evaporate from areas where it previously could.

I specifically was asking him about insulating walls, which I know can be problematic with water rot on sil plates due to trapped water vapor. he also recommended including thin PVC tubes inside wall and attic spaces where insulation is added to ensure good airflow.

Anyway - Just a thought

"Chiming in" here just because I wasn't even aware that rockwool came in bats. Maybe that would be better. If you don't ever have to move it around or deal with it, loose rock wool is fine insulation. We were just glad our 1913 house had ANY insulation in the walls and 1/2 story joist spaces and under the rafters. But when we demolished the finished portions of the upstairs in order to rennovate the living space and replace all of the electrical, we removed all the loose rock wool. We DIY demo and I will NEVER forget how dirty that 50+ year loose rock wool was! I hauled 90+ heavy lawn/leaf size garbage bags of the stuff to the dumpster and it was a filthy job. At the end of every work day I looked like a coal miner! It is still in our wall cavities and I'm grateful for the insulation existing there, but I hope we never have to disturb or deal with it again. I'm grateful for the fiberglass and foam we have upstairs now and would never go back to loose rock wool. YMMV.

including thin PVC tubes inside wall and attic spaces where insulation is added to ensure good airflow.

That's ridiculous. The last thing you want in insulation is airflow. This character should go back to school! (Or maybe you didn't hear right?)

I'll stick with the advice of the leading experts in the field.

macv said I would repaint the bathroom ceilings, blow something into the attic

Might be okay. Two coats alone, I recall, brings down the permeability to 1-2.

I've usedROXUL batts. No more irritating than fg. Maybe it's the vaguely feces colour that inexplicably puts some people off. A poor contrast with the cotton candy pink that the fg marketing geniuses came up with.

I think the guy with the old house air leak philosophy is basically saying it is important that the old walls be allowed to dry to the inside and the outside like they always have but that only works for sure in mild climates or if there is no insulation in a cold climate.

With insulation installed in a cold climate, the interior side of the exterior sheathing becomes cold enough for moisture to condense against it before it can pass through to the very dry (in winter) outside. If an interior vapor retarder is omitted, the exterior sheathing must be highly permeable (no, I'm not going to say breathes) and that may be the case with old style horizontal boards or beaver/buffalo wood fiber board.

Omitting the vapor retarder can sometimes work if all the other elements are right but I would add a coat of vapor retarding paint on the interior to slow down the passage of water vapor in the winter in rooms that have a lot of moisture. Think of water vapor as trying to equalize itself just like air pressure only incredibly slower.

For me, the irony of all this is that interior air in old houses in New England is so dry, chairs are constantly falling apart so the idea that moisture would condense in a wall cavity is pretty far fetched except near a shower, a sink, a dishwasher, etc., or if the HVAC system has a really good humidifier.

Roxul batts was what I was considering. I was hoping it would be better to work with than fiberglass ins. It sounds like awlful stuff to work with except it never compresses/loses shape and maintains its R value. Boy, this really gets complicated. What to do?

Why not just use loose insulation sprayed in, as macv suggests?

I did not realize that Great Stuff breaks down/shrinks over time!!!

Nor did I. Perhaps because I can find no impartial scientific studies demonstrating that polyurethane foam does so. On the contrary.

As always, I stand to be corrected.

I don't think you need a vapor barrier in the attic.. You use them in walls on the warm in winter side of the walls so condensation from the inside doesn't travel into your insulation and get trapped between 2 walls making the insulation useless.

In the attic any moisture(if any gets there which is doubtful) will evaporate because there is no wall or barrier on the attic side. In fact there should be no moisture if you have adequite venting in the roof and soffits and the insulation is not jammed over the soffit vents.

Our house is 40 years old. We have no vapor barrier and 6 inches of fiberglass batt insulation above the ceiling and have never had a problem. We live in northern Ontario where we often have temperatures of -30 C or -22F

Worthy, I do not see where the abstract on polyurethane foam mentions shrinkage? Am I missing something? It seems to discuss thermal performance over time.

Roxul is less difficult to work with than fiberglass. A vapour barrier is not required in a ceiling in a home in Mass.

Bringing back an oldie! But why not? The questions are the same.

From the linked study:

Over the same period, the surface of the concrete on the inside of the insulation showed no evidence of water penetration through the SPF layer

If it had been shrinking or breaking down over time, I'm presuming water would have penetrated to the concrete wall.

roxul is a wool product that saying many people are bothered by wool and some are not it is wonderful to work with and have in your home, we used a metal serrated knife no problems cutting. different friends helped put it up it bothered some not others.......the sound qualities are amazing......

Roxul is not wool; its "mineral wool" made by melting iron-ore slag at high heat and spinning it into fibers that are held together with a phenolic resin. The modern batts are less damaging to your lungs than the old loose attic insulation but it is important to wear a high grade disposable mask not just a dust mask.