Reply to Best way to insulate 100 year old unfinished attic ceiling?

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[quote=Anonymous][quote=Anonymous][quote=Anonymous][quote=Anonymous]Look up spray foam, attic and uninsurable. You could also look up unmortgageable but that’s a mouthfull.

Rockwool and you don’t need foam jockeys [/quote]

Op here, what would you suggest doing in this situation? [/quote]

Rockwool. They also have a great technical support line.

You could vent it — Home Depot sells different baffle vents.

There are some new hemp based products but not yet tested enough.

There is some, not a lot, good building science out there (mostly ignored):

https://www.energy.gov/eere/buildings/building-america-webinar-stump-building-science-chump-joe-lstiburek-text-version

https://buildingscience.com/documents/building-science-insights-newsletters/bsi-095-how-buildings-age

[/quote]

I love Joe Lstiburek to death, but neither one of those articles have much to do with the specific issue of insulating a roof. Which is a highly technical matter that is covered in great detail in the building code.

If you use a permeable insulation, there has to be a ventilated space between the insulation and the roof. It doesn't matter if the insulation is rockwool or fiberglass or cellulose batt or blown cellulose or vermiculite or open cell foam or cork or hemp or lambswool. They're all permeable, they all require ventilation. The International Residential Code is very specific about this. Your roof has to have one foot of free ventilation area for every 150 feet of attic floor space. It has to be evenly divided between the eave and the ridge, with no more than 55% of the total at either location. There has to be an unobstructed path between the eave and the ridge that is at least one inch in thickness and going the width of the roof. This is all at: 
https://codes.iccsafe.org/content/IRC2018/chapter-8-roof-ceiling-construction#IRC2018_Pt03_Ch08_SecR806

The question of whether to use rockwool or fiberglass or blown cellulose is literally nothing compared to the issue of how to ventilate your roof. If you use permeable insulation, and don't ventilate it, the roof will start to fail in five to ten years.

In new construction, it's trivial to install proper ventilation. In existing construction, it can range from easy to impossible depending on how the house was built. The usual way it's done is a soffit vent at the eave, a ridge vent at the peak and a baffle -- like this: https://www.homedepot.com/p/Owens-Corning-Raft-R-Mate-22-1-2-in-x-4-ft-Attic-Insulation-Rafter-Baffle-Proper-Vents-10-Pieces-70RM/204848302 -- to maintain the one inch clear space between the insulation and the roof.

If it's not feasible to ventilate the roof, you have two choices. One is to put the insulation on top of the sheathing and under the shingles. This is called "over-roofing," you can google it. You can use whatever insulation you can make work for that. The problem with over-roofing is it means you have to replace the existing roof. It may also change your roofline in ways that are not visually acceptable.

The other way is to use an insulation that is not permeable against the underside of the roof. The only insulation that is code-compliant applied this way is closed cell spray foam. [/quote]

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