Insulating an unvented cathedral ceiling in detached workshop

[confusedcarp]

Hello. I'm new here! I was told you folks might be able to help. I've searched the forums and haven't found anything quite like my situation. I'm also a beginner so my search terms may not have been complete so I apologize if this has been answered elsewhere.

I want to turn my detached workshop into a heated space for woodworking year round. It needs to be dry and warm enough to provide protection to my tools and for when I want to use wood glue. I plan on using electric heat for various reasons.

It currently has no insulation. It's basically plywood attached to studs.

Some details:

• Location: zone 4/5 marine (Pacific Northwest)
• Size: 16×11
• Roof condition: It's about 12 years old but looks like new. No leaks.
• Roof pitch: 5/12

The ceiling is open (cathedral) with zero venting. The eaves on the outside are exposed (no soffits) and there are no roof vents. The inside walls are approximately 7.5′ high and the ceiling rises to a point above that by 2 1/2 feet.

Some pics to help illustrate:







After posting the pictures, I just noticed that there are some roofing nails poking through here and there. I'm not sure if this matters but it probably does.

I have a plan for the walls (vapour barrier, rockwool, finished with plywood) but how to tackle the ceiling? I would prefer to finish it with plywood (or lightweight drywall) after it's been insulated.

So my question is can I treat the ceiling the same way as the walls but instead of vapour barrier, I put rigid foil foam directly against the sheathing? I would seal the edges with foam then attach the plywood to the studs. There would be a 2.5″ empty space between the foam and the plywood. Does this sound like a good plan? Would adding rockwool in this gap help or hinder? Or is there another way I can insulate that’s a compromise between cost and effectiveness? I'm hoping I don't have to redo the roof completely and/or convert it to a vented roof.

Any help would be most appreciated!

 

[Neggy]

are you open to putting a ceiling in and converting the space above into an attic with pull down stairs?

you need air movement on the back side of the roof sheathing

at a minimum that is gable end vents and inlet points low in the roof on both sides

 

[confusedcarp]

are you open to putting a ceiling in and converting the space above into an attic with pull down stairs?

As much as I'd like to, it doesn't leave much headroom. It'd be just over 7' of space below. Adding light fixtures would make it even more cramped. The attic is also not usable because it's so small (2 feet high in the middle).

you need air movement on the back side of the roof sheathing

Okay, so no to the foil foam against the sheathing then

at a minimum that is gable end vents and inlet points low in the roof on both sides

I'm a little confused by this. If I enclosed the attic I would need these things? Or I need these things no matter what I do?

What if I were to attach the foam board insulation to the underside of the rafters? This would leave a void between it and the sheathing.

 

[tfinniii]

You can put foam to roof sheathing ,but should be sealed for no air movement between sheets and sheathing. It will shorten shingle life somewhat.

 

[HaiKarate]

I might be tempted to use one of those DIY spray foam insulation kits in this situation.

 

[confusedcarp]

You can put foam to roof sheathing ,but should be sealed for no air movement between sheets and sheathing. It will shorten shingle life somewhat.

Ah okay, that's interesting. How much would it shorten shingle life? I'm hoping to eke out a few more years on the roof so the sticker shock of this project has time to settle.

I might be tempted to use one of those DIY spray foam insulation kits in this situation.

This might be just the solution! I didn't know these existed. Armed with a new google search term, I found a video from a guy explaining how he used it to insulate his unvented cathedral ceiling in his shed. Basically exactly what I'm looking for. Have you (or anyone else reading this for that matter) had any experience with them? I'm currently reading reviews but it does help to get insight from the pros. I'll also search the forum for it of course.

Thank you everyone so far for your help! You've gotten me farther in an afternoon than days of google searching has accomplished

ETA: I found a great thread about it with the pros and cons of doing it oneself. Here's a link for anyone who also wants to know. I'm looking at ~380 board feet to do two inches minimum. I can buy a 600 board foot kit for $700 CAD. Not sure if a local company could do it for that cheap or if they'd laugh at me for asking for such a small area. Time to do some more research!

 

[kj_mustang]

Or buy 2" thick foam board and cut them to fit in between the rafters against the roof. With a totally unvented building, you need to watch for humidity levels building up inside.

 

[confusedcarp]

Or buy 2" thick foam board and cut them to fit in between the rafters against the roof. With a totally unvented building, you need to watch for humidity levels building up inside.

Thank you, good to know! I appreciate you feedback

 

[Gerald O]

Spray foam is really the only worthwhile way to insulate that roof.
You don't actually have a cathedral ceiling. The construction was not done with insulation in mind. You don't have deep enough rafters to add ventilation to the roof sheathing with enough room left for insulation.

Normally, with that kind of roof construction, if one wanted to insulate the space below, a flat ceiling would be built below the 'attic' space and the insulation would lie on top of the flat ceiling. Then vents could be added to the gable ends.

It's a bad idea to use foam board because you'll have an unvented air gap above that will trap condensation that will eventually rot the roof.
Use closed-cell spray foam applied directly to the underside of the roof sheathing at least 1" thick, but preferably thicker for your location. 2" will provide a reasonable insulating benefit while not being too expensive.
This will function as a vapor barrier and the foam will completely seal the roof preventing condensation from forming on the sheathing.

 

[Perrorojo]

Pay someone to spray foam it. I did the "Do it yourself" kit with two part foam and the sprayer, nozzles etc. Horrible experience. I could get anyone to do it for 6 months so I tried it. Should have waited.

 

[confusedcarp]

Spray foam is really the only worthwhile way to insulate that roof.
You don't actually have a cathedral ceiling. The construction was not done with insulation in mind. You don't have deep enough rafters to add ventilation to the roof sheathing with enough room left for insulation.

Normally, with that kind of roof construction, if one wanted to insulate the space below, a flat ceiling would be built below the 'attic' space and the insulation would lie on top of the flat ceiling. Then vents could be added to the gable ends.

It's a bad idea to use foam board because you'll have an unvented air gap above that will trap condensation that will eventually rot the roof.
Use closed-cell spray foam applied directly to the underside of the roof sheathing at least 1" thick, but preferably thicker for your location. 2" will provide a reasonable insulating benefit while not being too expensive.
This will function as a vapor barrier and the foam will completely seal the roof preventing condensation from forming on the sheathing.

Ty very much. I really appreciate your thoughts and information on the ceiling type. Cathedral seemed the closest description to what I have even though it's not really what it is. It's also good to know that the rafters aren't deep enough to add venting. I had been considering it but not seriously due to effort. Enclosing the attic and insulating it traditionally would lose valuable headspace in an already small shop.

At any rate, I've decided to go with spray foam as the only viable option for the reasons you've mentioned. Thanks again; I appreciate it!

Pay someone to spray foam it. I did the "Do it yourself" kit with two part foam and the sprayer, nozzles etc. Horrible experience. I could get anyone to do it for 6 months so I tried it. Should have waited.

Thanks for sharing your experience. I've been weighing the pros and cons of doing it myself but decided in the end that I'm paying someone to do it. It's a hundred dollars more than the kit I'd need. Well worth it for a professional installation with a better R value than I could hope to achieve.

 

[misterfixit]

You made a good choice. I've done a small area with the DIY kits, as well as a shop with pro rig. Pay for someone to do it right, you'll be much happier

 

[JunkBonds]

This is what you want:


And then I put in a small window shaker:

 

[stevied916]

You can put foam to roof sheathing ,but should be sealed for no air movement between sheets and sheathing. It will shorten shingle life somewhat.

There’s been recent studies that the myth that spray foam leads shortened life is not really much of a concern.

Insulated Rooflines and Shingle Temperatures - GreenBuildingAdvisor

One of the most common questions I get when I describe homes with insulated rooflines is, "What does that do to the shingles?" Some roofing companies have made noise about this topic, saying that if the shingles can't conduct heat downward into the attic, the shingle lifetime will be greatly...

www.greenbuildingadvisor.com

 

[LX-Markham]

Pay someone to spray foam it. I did the "Do it yourself" kit with two part foam and the sprayer, nozzles etc. Horrible experience. I could get anyone to do it for 6 months so I tried it. Should have waited.

I watched them spray foam my detached cathedral ceiling garage ….no way I would want to do that job myself.

 

[Outlander]

Great thread, first picture could have been my workplace! Now I know where to focus my attention if/when I decide to insulate.