Best way to insulate 100 year old unfinished attic ceiling?

[Anonymous]

Anonymous wrote:Thank you so much for this very helpful discussion! We really get that it is all about moisture thanks to your comments. OP here to clairfy:

1. Our goal is to finish the attic into living space.

2. It is correct - we have just the air handler unit in the attic and the compressor is outside. This is how it was when we bought our home.

3. Our current thinking is to wall the air handler unit into the conditioned space we are making. And use a smart membrane product like intello or membrain to prevent moisuture from reaching cold surfaces in the walls and rafters. Combined with an unfaced batt insulation.

What do you all think?

A smart membrane is good, but it is in no way sufficient to protect the roof from condensation. You need either ventilation between the insulation and sheathing, spray foam, or insulation on the exterior.

[Anonymous]

Thanks! This is very helpful.

[Anonymous]

Anonymous wrote:

Anonymous wrote:
We have a HVAC unit that provides AC to our upstairs located in the attic.

old insulation that looked to be like finely shredded paper. There were a lot of moths in there and moth larvae on the exposed rafters. The contractor removed the insulation. We think it might have been original to the house as there were generally very few updates throughout.

Is blown in cellulose the way to go?

First of all, having an AC unit in the attic is horrible for moisture disaster issues, though that's a whole other project and big money to fix.

So the insulation that looks like shredded paper is called "cellulose" insulation, and it is made of, wait for it, shredded paper. Very cheap and poor quality insulation, but "green" aka a scam mostly. You will spend more in utility bills from its lower rating so it is not green at all.

Cellulose insulation also is a prime mold growth medium and attracts insects, as you have found out.

Fiberglass is what you seek. Blown in is cheaper and easier to do, batting is better and more costly, doing both is best.

Agree with everything but the conclusion (Rockwool beats fiberglass in every way) and would add don’t fall for borate treated cellulose either.

[Anonymous]

Don’t blow in anything. It’s as permanent and messy as it sounds.

Rockwool is more expensive but quite frankly you can install it with a butterknife and patience.

[Anonymous]

Anonymous wrote:

Anonymous wrote: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

Op here, what would you suggest doing in this situation?

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

[Anonymous]

Anonymous wrote:

Anonymous wrote:

Anonymous wrote: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

Op here, what would you suggest doing in this situation?

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

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.

[Anonymous]

And any roofer worthy of the name is going to know the code and know how to tell if a roof is vented.

Insulation guys seem to be a mixed bag, an awful lot of guys in that business barely know which way to point the foam gun.

[Anonymous]

The only place to point that foam gun is away from my house.

https://www.theguardian.com/money/article/2024/aug/12/they-encouraged-us-to-insulate-our-home-now-its-unmortgageable

[Anonymous]

The advice above is terrible for the average house in DC. Close cell foam the underside of your roof and watch it rot.

[Anonymous]

Anonymous wrote:The only place to point that foam gun is away from my house.

https://www.theguardian.com/money/article/2024/aug/12/they-encouraged-us-to-insulate-our-home-now-its-unmortgageable

That article is about open-cell foam, which is permeable and needs to be vented. There is some contention in the building science community that a thick enough layer of open-cell foam becomes impermeable, but there's not much agreement as to how thick is "thick enough." There have been failures in the US of open-cell foam applied directly to the underside of roofs as well, it's a risky approach.

Closed-cell is fine.

[Anonymous]

Anonymous wrote:The advice above is terrible for the average house in DC. Close cell foam the underside of your roof and watch it rot.

OK, what's your advice for a code-compliant way to insulate the underside of a roof?

[Anonymous]

Anonymous wrote:

Anonymous wrote:The only place to point that foam gun is away from my house.

https://www.theguardian.com/money/article/2024/aug/12/they-encouraged-us-to-insulate-our-home-now-its-unmortgageable

That article is about open-cell foam, which is permeable and needs to be vented. There is some contention in the building science community that a thick enough layer of open-cell foam becomes impermeable, but there's not much agreement as to how thick is "thick enough." There have been failures in the US of open-cell foam applied directly to the underside of roofs as well, it's a risky approach.

Closed-cell is fine.

Article on the risks of open-cell foam:
https://www.greenbuildingadvisor.com/article/open-cell-spray-foam-and-damp-roof-sheathing

"If you want to lower the risk of damp OSB, choose closed-cell spray foam, not open-cell spray foam, to insulate the underside of the roof sheathing."

[Anonymous]

“That” article is. But any spray foam in an older house in the attic roof assembly is now uninsurable and unmortgageable in countries that are ahead of us on building code enforcement like the UK.

Spray foam is a toxic soggy moldy mess. Also off gasses. Yet people pay for this to destroy their homes.

https://www.snugg.com/blog/common-spray-foam-insulation-problems-and-what-to-do-about-it

[Anonymous]

Anonymous wrote:“That” article is. But any spray foam in an older house in the attic roof assembly is now uninsurable and unmortgageable in countries that are ahead of us on building code enforcement like the UK.

Spray foam is a toxic soggy moldy mess. Also off gasses. Yet people pay for this to destroy their homes.

https://www.snugg.com/blog/common-spray-foam-insulation-problems-and-what-to-do-about-it

UK is ahead of us in building code enforcement? That's laughable.

I used to live in England. Their housing stock is dismal compared to the US.

[Anonymous]

With the sloped ceiling how much useable space are you gaining?

I live in 2 story semi detached 1909 house in a southern city. Have been up in my attic in the summer and cant imagine the amount of AC required to make it tolerable!

You should really consult a roofer experienced with older homes.

Lived in a 1940s house in Chevy Chase where someone tried to insulate the attic. Caused mold under the insulation the roof. They did not leave enough venting. We had it all pulled out, remediated and roofer added ventilation. Lucky for us all found in inspection so lowered the sale price! Took it back to an unconditioned space. The ceiling was so low most of it was unusable anyway.