Cold roof system isolation

[ckimble]

Hey all,

First off, thank you for this resource. I have learned a lot reading through threads in this forum. I’m sure this will be the first of many questions I will have.

I’m in the planning stages for building out a rehearsal/recording space. This is going to use a 15’x24’ section of a detached garage/workshop. I am currently using a bedroom in an attached guest house for my band to rehearse as well as do some simple recording. It actually does a good job with sound isolation so more isolation would be desired, but not really necessary as I have some space around me but this structure is closer to my neighbors house. I’ve read through Rod Gervais book several times and have been lurking on here doing my research. I think I have a grasp on the concepts, but one area that confuses me is how to address the cold roof system isolation. I am designing off of this design in Gervais book, but am confused on how best to run the vent and seal the lower section of the rafters and ridge vent on the outer leaf.

D4286B13-FC7E-42C5-BCE5-FAE6EC5113AC.png

I whipped up this drawing to try and detail my situation. I was thinking I could put runners along each rafter on the inside and cover them with foam board. This would create the vent channel. Then spray in foam insulation on top of that. My concern is I now have a third leaf, and everything says that is a bad design decision. What is recommended for the air gap between interior and external ceiling in this situation?

3E7CB7FC-4962-401F-8387-046050E25C02.png

[Paulus87]

You build two fully decoupled, air tight, and heavy leaves and then you build your ventilated roof deck on top. The soffit vents go between the outer leaf and the roof, and your ridge vent goes on your roof ridge. Although the roof appears to be a third leaf it is a very leaky one since it has large holes in it from the vents, therefore it dies not really contribute to your overall isolation or MAM system.

If you are building a cathedral type ceiling with no loft space then you build it in the same manor as your picture but there’s no need for the spray foam since you have vents. Spray foam is used when you have a warm roof design and would do away with the need for venting altogether.

If you did a fully warm roof design with either spray foam or thick rigid foam insulation above the roof deck the you could build just two leaves with no ventilation needed.

Also, since you have a fully decoupled two leaf system there is no need for resilient channel and clips, in fact you’re better off without it.

Paul

[Paulus87]

I've drawn a diagram to help clarify how you could build a fully isolated cold roof assembly.

To save space and increase your cavity size between your inner and outer ceiling leaves you can fix some 50mm batten to your outer rafters, up tight against the bottom face of your roof deck. Then, to the bottom of this batten you can fix your outer leaf drywall layers; so you would cut the drywall to fit between the rafters inside the rafter bays, fixed to the underside of the battens you installed.

Then, continue the drywall layers down to the outer leaf top plates. Make sure it's all sealed with caulk and completely airtight.

This will provide you with a continuous 50mm ventilation path from soffit to ridge, and yet maintain isolation.

Then you would build your inner leaf walls and ceiling as normal.

If you do not have any height restriction then you could attach the outer leaf drywall layers to the bottom of the rafters, of course this would mean that you could not use the depth of the rafters as part of your cavity depth, but if you didn't have a height restriction this wouldn't matter since you can just bring the whole ceiling/roof up higher. it is a lot easier since it avoids cutting in all the drywall and fixing up battens.

Hope this helps, let me know if you have any more questions.

Paul

[ckimble]

Hope this helps, let me know if you have any more questions.

Paul

This helps a bunch.. It's much clearer to see what I need to do now.

Thank you!

Trust me, there will be more questions