Is the attic going to have any living space?

Can you post a picture of the current attic insulation? That will help in making suggestions for options.

In general, spray foam insulation has the best R value per inch, but its also quite a bit more expensive than other options. It also makes electrical and HVAC changes difficult after the fact because you have to work around the foam, which blocks off everything. Just think of digging through 4-5" of foam to find an electrical line so you can figure out where it goes.

For a house like yours, the other consideration is whether you want to make the attic conditioned space, since its has your HVAC vents. In the past you would have just gotten the best insulation possible for the attic floor, and made sure you had good venting for the attic (soffit vents and a full ridge vent was typical). In recent years some of the green building articles have a new idea. They spray open cell foam on the underside of the roof deck, close up all attic venting, and the attic becomes conditioned space. They are making the same recommendation for crawl spaces.

You might want to read through some of the articles on this site, just get an idea of what they suggest. http://www.greenbuilding.com/

Bruce

"They spray open cell foam on the underside of the roof deck, close up
all attic venting, and the attic becomes conditioned space. They are
making the same recommendation for crawl spaces."

I don't get the rationale for this. It seems like a waste of energy to heat and cool space that nobody lives in. Or are they NOT heating and cooling them, and just using the attic and crawl space as sort-of-dead-air space?

DavidR, I did this, but I am now living in that attic space. You're probably right if there is no intention of using that area.

When they insulate an attic or crawl space and convert it to conditioned space, it is usually not heated or cooled. It is also typical to consider a basement as conditioned space, even if it is not heated or cooled. Simple answer - its usually just dead air space.

This is one official definition. "A space is conditioned if heating and/or cooling is deliberately supplied to it or is indirectly supplied through uninsulated surfaces of water or heating equipment, uninsulated ducts, or uninsulated floors, ceilings, or walls between it and another conditioned space."

Bruce

Thanks, Bruce, that makes some sense.

Bruce, my attic is not heated. Its heat comes from the floor below and that's plenty, even when it is 0°F outside (I even open the windows when it's above 32°!). I have a through-the-wall AC which I use in the summer (sometimes). Usually it's cool enough at night that I can turn it off and open the windows. I live in the NE, on Long Island.

If you look on this web page under the heading of "unvented roofs with applied spray foam" you can read the discussion. https://www.nachi.org/inspecting-spray-foam-insulation-under-roof-sheathing.htm.

I'm interested in this solution myself, since my A/C unit is installed in the attic. I don't have the same concern for heating, since I still have hot water radiators for heat.

However, my attic floor is currently under-insulated with what appears to be old rock wool, and it probably has to all come out if I want to replace it with something newer and more effective. If the roof deck insulation option would work, I want to include it in my price comparison with installing new insulation in the attic floor.

Bruce

What type of insulation is there now and how much of it?

Spray foam on underside of roof deck is common in the South. It can be a solution if plumbing and HVAC is in the attic. It's expensive to do that though - there's more sq ft to cover a roof deck than an attic floor AND spray foam is expensive. It can also be difficult to do on a retrofit because you need to be able to provide an air seal.

Also consider the type of foam (open cell vs closed cell) there's a significant R value difference between the two.

An inexpensive way to go when there is fiberglass or similar is to simply add more blow-in cellulose on top of what's there. It is fire resistant, insect and vermin resistant and you don't have to remove the fiberglass. Plumbing and electrical remains accessible.

Sealing air vents through the attic is important. A few cans of "Great Stuff" around vent boots can solve a lot of leakage. You can also get larger foam kits and foam around the boots and any other protrusions from the attic side after you clear away the existing insulation from the immediate area - then blow cellulose on top of everything.

Re: Conditioned space - it makes it less confusing if you talk about the envelope and if something is inside or outside the envelope. When you foam the roof deck, you are moving the envelope out to the deck and everything inside of that is inside the envelope.

Currently it is fiberglass batts, and it’s from 1963 and has been damaged. I haven’t personally been up there to look yet. My husband and the house inspector have been up there. Nothing urgent, but something we think should be done on our to do list.

No we will not have living space up there, joists are not sufficient for that and there is very little headroom with the hip roof and square hour style anyway.

The attic will never be “conditioned” as we do not have A/C and live in a climate in which is is not at all necessary to have A/C for comfort, and heat only runs at night for 3-4 months of the year. Our windows are open most of the day all year round. I am sure that the attic heats up a lot due to solar heat gain, though - so it does seem sensible that lining the roof with a radiant barrier to reduce solar heat gain into the attic would be more effective than insulating the floor of the attic as it is, to reduce heat gain into the house on those days that it does get uncomfortable. We have a new (one year old) energy-star roof. I understand what you are saying about the “envelope” and I assume that it would be necessary to ensure that insulation and any vapor barrier on the attic floor was removed in order to avoid trapped moisture? I know double layers of waterproofing can be risky in that way but if they are separate spaces maybe not an issue.

what about the sound insulation issue - both external sound (road noise and neighbors) and room to room? I can only guess insulation on the attic floor does more for the latter.

Given your situation, just blowing cellulose on top of what you have would be a dramatic improvement at very little cost. I've done it myself on multiple homes. You can google a map for the required R value if you don't know it already. Adding enough cellulose to cover the top of the joists a few inches will also reduce thermal bridging by the joists.

You really don't need to get any more involved than that.

As a former musician, I understand what you’re saying. But insulation does reduce the vibrations of the walls and ceiling and can help... we won’t be able to create a sound proof listening room like my husband would like

Jake, do we need to remove any damaged or contaminated insulation first?

I really don't see how insulation would reduce vibrations significantly.

Insulation inside wall and ceiling cavities does prevent noise from bouncing around in there, but the noise blocking effect of that is pretty small and not worth the effort IMO.

Acoustic foam and other soft materials will reduce noise reverberation INSIDE the room when it's attached to exposed wall surfaces (the stereotypical studio look). That can be good if you want a dead studio sound, but it doesn't reduce noise transmission significantly.

Going off topic here, if you want to give hubby a quiet room for his birthday :) without breaking the bank, what you want is something that will BLOCK noise.

First, close any gaps. Noise gets though gaps that even a mouse can't. :) Caulk every opening with acoustical sealant. Don't forget to caulk around the electrical boxes, behind receptacle and switch plates.

Add gaskets to the doors. These need to be heavy vinyl meant for the purpose, not just mass market weatherstripping. You want to close all gaps.

If that's not enough, in addition to the drapes I mentioned above, try mass loaded vinyl glued to the doors, wall, and ceiling surfaces. It's easier and cheaper than damped drywall and/or clips, or double drywall layers with offset studs. You may not like the way it looks, though.

Tmnca

In what way is it contaminated?

The general answer is "No" but waiting to hear on the contamination.

We are pretty sure the PO has rat problems in the past, so it’s possible that there are rodent feces up there. When we replaced the roof any entry points were sealed up, so there are no new/active issues.

I think you are fine to blow cellulose on top of the existing. I've never seen an attic that didn't have some mouse / rat / bird / bat / raccoon droppings in it.