Living in the Charlotte area with an unfinished bonus room above the uninsulated garage.

It has 2x6 rafters and the requirement is R-38 for insulation.
Furring the rafters to accommodate the additional thickness of batt insulation will eat away at the room width, bringing a 22’ x 12’ room down to 22’ x 10’ as a 5’ knee wall is required.

Looking at spray foam as it seems this is the best method for insulation of bonus rooms.
Neighbor has a bonus room with batt insulation and the room is noticeably hotter in the summer and colder in the winter.

The spray foam would go directly on the roofing and directly on the drywall garage ceiling (from above) leaving no air gap for ventilation between roofing and insulation, and leaving an air gap beneath floor and spray foam insulation.

I am told air gap below floor does not matter as the whole area would be encapsulated.
Sounds reasonable.

I am told spray foam can go directly on roofing and an air gap for ventilation is not required.
This I am struggling with having watched a YouTube video where it was claimed shingles were coming loose due to spray foam insulation.

Any experience or advice with insulating a bonus room above the garage for a house in the Charlotte, NC area would be appreciated.

The spray foam that a professional sprays on has amazing R value for it's thickness and does an incredible job of sealing any air gaps. That is the direction I'd go. Yes, it is sprayed directly on the wood of the underneath of roof decking or walls,

I can’t answer your question specifically but FWIW I saw this video a couple days ago. Essential Craftsman has been building this house for over a year now and his latest video is insulation. I found it interesting and informative. The last 3rd of the video is more of the attic, which has knee walls. The 11:30-ish time mark may be helpful.
Hope this might be helpful to you.

https://youtu.be/wQisIav-278

Good luck to you.

An added benefit of spray foam in addition to the Rvalue. it acts somewhat as an adhesive to stick the plywood decking to the rafters. In fact there is a specific adhesive made for that purpose that spray foam contractors apply with the same equipment. You might ask the contractor about it while they are at it, definitely a plus in hurricane country.

quote:

Originally posted by OKCGene:
Essential Craftsman has been building this house for over a year now and his latest video is insulation. I found it interesting and informative.

I'm a big Essential Craftsman fan, and have learned a ton from his videos over the past few years.

Watching their spec house come together in particular has been fascinating.

I’m about 2 1/2 hours north of you and have finished out many light attic storage areas over garages.

When you mention spray foam insulation I hope you’re referring to closed cell insulation as opposed to open cell. Closed cell has an R Value of 5/inch whereas open cell is about 2.5/inch, the same as fiberglass insulation.

If you have a 5 ft. Knee wall on the sides you might want to look into sheathing the “exterior” side of it and then spraying the interior with foam reducing the volume of the space needed to heat and cool.

One thing to be very careful in these build outs is what are you doing for HVAC? If you can keep everything in the Thermal Envelop of the enclosure you will be much happier.

We have 2 heat pumps, one for downstairs and one for upstairs.
This would not have it’s own HVAC, it would be on the same system as the rest of the upstairs.
We just replaced the upstairs heat pump, upsizing it with the plan to enclose the bonus room.
It will have an inlet near the exterior wall and a return on the interior wall.

I believe code calls for R-38, however with a small space cathedral ceiling area under 500 SqFt such as this, it will allow for R-30.

I like the idea of having ventilation between roofing and insulation running up to ridge vent.
Now this looks interesting.

Here in South Florida, when they spray foam closed cell to the roof deck, they seal the attic and there are no gable vents or roof vents or any of that.

What happens if you need to run wires etc. after spray foam? Plan ahead.

I have used spray foam for both my house and more recently the shop I built. But in both cases I did sprayed from up in the attic down covering the ceiling drywall and the complete bottom chord of the truss.
In your case I would not use foam against the roof sheathing but do it like your video leaving a vented air gap to prolong the life of the shingles, and sheathing itself.
Curious why you couldn't use blue board instead of that messy, dusty panels in the video? Blue board is R5 per inch and easy to cut with no mess and dust.

https://www.dupont.com/product...rand-utilityfit.html

quote:

Originally posted by Windwolf:
What happens if you need to run wires etc. after spray foam? Plan ahead.

Run the wires in conduit would be a solution for that. Easy to pull new ones.

Yes, already plan on running conduit.
Even for garage door opener.
Not going to rewire just run conduit to the middle of the floor and put a face plate on in ceiling of garage.
Near soffit vents I will run conduit to corners of the house.
Conduit goes down regardless of insulation selected.

Shingles degrade when the space below them get overheated, the result of inadequate airflow. When foam is applied directly to the roof under deck, there is no air space to heat. Therefore, the shingles do not suffer any degradation.

I’d think that leaving an air channel might actually be more of a liability. If the channel gets blocked in any way, then you would indeed have superheated air just below the roof deck.

I had a house spray foamed back in 2016. It’s the one I rebuilt after the fire. The utility bills dropped unbelievably, and the spray foam seems to have tightened up (that’s the best I can describe it) the whole 1888 structure.

Good information sigcrazy7.
Waiting for an estimate on the Spray Foam, hopefully early this week.

With rafters now I have 5.5” from bottom of roof deck to edge of rafters.

2” thick Rmax Thermasheath is R-13.1

1.5” air gap, plus 2 x 2” R-13.1 foam board = 5.5” even with rafters.
Then horizontal run of 2” R-13.1 gives me insulation over rafters and R-39.3 without furring rafters.

Spray foam seams of the foam board and that might be a good alternative to Spray Foam.