Hi, I am in the middle of a remodel project on my home and I am at the point where I need to get the insulation done in the next week or two. I am in climate zone 5 (Michigan) and I am looking at a couple options. I have read extensively on these options and I figured I would ask some others for their opinions, as I have formed my own. My house is ~1600sq ft., I have a 4/12 hip roof, 2x6 rafter on 16" centers, 2x6 ceiling joists @ 8' ceilings. The house will be heated with hydronic floors in half of the house and hydronic ceilings in the other half. Right now there is no air coniditioning but when I do install it, I want to put the air handler in the attic to save on the already low height ceilings in the basement. Right now my attic is vented via the drip edge and vents on the roof. I am thinking about doing closed cell foam on the bottom side of the roof sheathing and increasing the conditioned envelope of my house as opposssed to just spraying 15" of a cellulose on top of the ceilings. As you all know, the cost difference between the two is substantial but I figured if I plan on putting an air handler in the attic in the future, it might be the best option long term. What are your thoughts?

You don't want to fill up the rafter bays completely with foam. The roof won't breathe, and therefore won't dry.
Also, it'll reduce the lifespan of your roofing.

You'll need to install baffles in there to leave an airspace next to the underside of the sheathing.

Here's a couple options.

https://www.amazon.com/Insulation-Baffle-16-Rafter-Bays/dp/B07MZ8VF49

https://www.amazon.com/AccuVent-Retardant-Cathedral-Ceiling-ACBP18426/dp/B01G2AO7MS

You can then fill the rest of the bay.

Yes, if you want to put your HVAC stuff up in the attic, or any venting, simply insulating the rafter bays isn't going to be enough. You really want to bring the attic into conditioned space.

Note: You STILL want to insulate the rafter bays in the attic though. Use whatever you're going to use there. Loose fill, batt, sprayfoam.
Just make sure to fully seal any and all penetrations into the ceiling.

Also, you do NOT have to go with just spray foam.
You can do a flash & batt setup. Basically sprayfoam just enough to air seal the bay (you still need the baffles). Then batt insulation after.
ALSO, you don't have to stop at the underside of the rafter bay!
You can install panelized insulation, foam, rockwool, etc.

Think about it.

Foam the bays. Strap the rafters. Install rockwool panels to the strapping.

or

Flash & Batt the bays. Strap the rafters. Install rockwool panels to the strapping.

I understand the theory behind putting baffles on the bottom of the roof sheathing but that would require me to have a ridge vent for the baffles to move air from the vented drip edge to the ridge vent, which I do not have. The sheathing would only need to breathe if the shingles leaked. I would like to avoid furring out the current rafter in order to accept a thicker layer of wool or fiberglass batts.

Is filling the rafter bays with foam frowned upon? It seems like if your shingles arent leaking, you wouldnt have a problem? If i were to have a leak, would end up having to possible replace some rafter and roof sheathing down the road.

If so, close up the vents in the drip edge and soffit.

Just make sure you actually ventilate the attic as part of the conditioned space.
And realize that, as I said, you're basically cutting the lifespan of your roofing materials.

A leaking roof isn’t the reason to provide ventilation. Building assemblies ALWAYS need to have a drying path because water can ALWAYS form in them via vapor diffusion and condensation. So the goal is to minimize water getting into them and to provide a drying path for when it does. There isn’t a drying path through the roof building assembly exterior side, so the only option is to have one through the interior side. You might find this and the associated instructions to be informative:

https://www.borstengineeringconstruction.com/Building_Assembly_Moisture_Analysis_Calculator.html

Cutting a ridge vent along the main ridge beam is not a problem, can be done pretty easily. Adding baffles is a pain but can also be done but I have one question. This is a hip roof, not gabled. So at the ends of the house, the rafter bays dont make it all the way to the ridge beam, so they would not be able to make it to the cut for the vented ridge. What exactly is done about these areas of roof sheating if I wanted to create a closed envelope effectively?

The soffit vent baffles are just to create an air channel past the ceiling insulation which is typically blown in cellulose or fiberglass. The soffit vent air just needs to be able to move upward through the attic to the ridge vent so it doesn’t matter if you have a full hip roof, which is my personal preference by the way. You should target at least 2 times more soffit vent area than ridge vent area so as to not depressurize the attic resulting in higher infiltration heat losses.

https://inspectapedia.com/ventilation/Roof_Venting_Ratios.php

https://www.greenbuildingadvisor.com/article/lstibureks-rules-for-venting-roofs

In my climate zone, there is only maybe 2-3 months of air conditioning use. It would take a very long time for ROI to kick in with a fully foamed roof deck. Is my best option just to run the baffles up a few feet from the top plates and blow in 15-16 inches of cellulose? I would still put the air handler up there but use the insulated ducts and just eat the inefficiency for the few months the cooling required.

My daughter lives in Charlotte and her air handler for her second floor is in her attic and it doesn't seem to make that much of a difference. The difference will be made in the ceiling insulation. Perimeter baffles and celly will do the trick.

I am planning on building a ICF home with a metal roof near STL MO.
Was thinking of taking aluminum screen and fastening to the bottom of the top cord of the roof truss then spraying closed cell foam to it.. as well as the gable ends. With a ridge vent . was going to allow my fresh air supply to be pulled through this space to somewhat acclimate before drawing it into the basement for fresh air..
Not sure if this the right thread? Thoughts? Suggestions?

From memory, if you insulate a roof in this fashion, it extends the cost to insulate by 1/3 and also the surface area to loose heat expands by 1/3 because of the roof peak. So the rest of the life of the structure the roof will be loosing 1/3 more energy for climate then needed, even if you insulate really good.

I would also research the fresh air need. I think lately it has been said that it is not needed as much as people used to think. My house is .79 ACH and I have a HRV that I only run when someone uses the bathroom. It pulls the air out of the bathrooms and adds it back in through the HVAC ducting. There are motion sensors in each bathroom

I thought about adding a CO2 sensor that would turn on the HRV if the readings get high but never did it. I think a controller to do that is just over a $100.

If you are wanting the attic for storage for conditioned space I think I would still insulate the floor and slightly do the roof. Just my thoughts. I am sure others know more then I do.

Good Luck.

I would look at mini-split ductless AC. Since you are only moving refrigerant in tubes instead of conditioned air in ducts, the install is a lot less intrusive beside the wall units. That would let you keep your conventional attic insulation and save money.