Office Extension Insulation – Vented Cathedral Roof and Walls
JeffDutrisac
| Posted in Energy Efficiency and Durability on
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Your cut and cobble ceiling insulation can work but it is a lot of labor for not much gained. You are better off to insulate the rafters with 2x6 R24 batts and add a layer of of R7 rigid to bring it up to code. This is only slightly thicker than what you propose but way less work and a higher effective R value assembly. They type of rigid doesn't matter here, anything that is foil faced can work, this way it is easy to tape the seams for a solid air barrier. Polyiso is a better option if you want lower profile.
The thicker interior insulation will also help a fair bit with your flush mounted ridge beam which is a pretty solid thermal bridge through the roof.
In terms of sound and thermal performance, the walls can be insulated with anything. The STC of the assembly doesn't change at all with the type of batt. The best way to keep the noise down is to never wall mount the outdoor unit, ground mount is your friend here. Also put a 5/8 layer of drywall under the paneling.
To get a solid air seal, make sure to detail your vapour barrier properly, tape the poly to the ceiling rigid with a quality tape and use use air tight device boxes (the plastic ones with the gasketed flange) for all the electrical in the wall.
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You don't need furring strips for the T&G. You can nail it directly through the foam with 2.5" nails. Does take some planning up front to mark out all the rafters but relatively easy to do.
1" polyiso is r6.2 on paper which is close enough to R7. If the inspector complains, your 3/4" T&G wood is an extra R0.9. You'll also need a code recognized interior finish under the T&G to protect the foam, which could be a thin layer of OSB, plywood or hardboard. This stackup is still under your 2" limit and can be built much more easily.
Because the foam is outside your walls behind insulation, the drywall for the walls is only needed if you are worried about sound. A well sealed layer of drywall is the cheapest and easiest soundproofing you can do. 5/8" Type X is better in this case as it weighs much more than regular 1/2" stuff.
For ground mount make sure to mount the unit high enough off the ground so it won't get blocked by snow. A small overhang over the unit is also a good idea. The corrugated drain hose that comes with most mini split kits is asking for clogs, 1/2" black poly irrigation pipe is a much better option.
I would 100% spray foam that roof with closed cell for a non-vented assembly. I did my 16x24 garage with a ridge loft and have been 100% happy, in zone 7A.
100% agree with Akos on the ground mount for your heat pump, but if sound is an issue, make sure you use high quality triple glaze units that are casement or awning (sliders, nope). You have a fair bit of glass for the size of the space. The very simplest assembly for your walls (for sound) is going to be two layers of 5/8" Type X with Green Glue between them. It is much easier to install and does not have the short circuiting issues that resilient channel does. Roksul in the walls is fine, but for the cost, size of the space, I'd spray foam the entire space if it was me. There is going to be so much flanking noise via the slab and garage, I wouldn't go too crazy with sound amelioration.
Strapping the rafters will reduce thermal bridging considerably and the air gap that is left between drywall and spray foam has some value as well. This takes care of your door/ceiling interference issue. Also, no issues with penetrations of the drywall for your LED lights as they won't need to be air sealed.
Given the nature of that slab (outside exposure on 3 sides), I'd want to make sure that if you did not integrate insulation, then you should be thinking about this for your floor. I'd also consider 24" (dig down a bit) of at least R10 XPS on the three outside exposed slab edges.
A few shots taken with a FLIR ONE thermal imager before drywall. Outside conditions, -20C, with a windchill at -24C (-11F)...brrr! The heat in the shop was on for about an hour, set at 15C (60F). These are 2x4 roof rafters with a ridge beam (not flush like yours) with 3" of spray foam. The shop is normally only heated when in use. Finished ceiling roof pic is also attached.
Smart choice to use the ridge as that space will really open up with the vault.
Jeff,
I'm not that familiar with the OBC, but I think it requires more than 1 1/2" vent space for roofs. Probably worth checking.