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Best way to build a cathedral roof for a garage?

wattsup | Posted in Energy Efficiency and Durability on

Hello to all;

I am in the 6A zone just north of Montreal, Quebec. I am in the process of building a garage separate from my home. It will be an L shaped garage with main section being 24′ by 20′ and the L part at 7′ x 11′.

I am seriously considering a Cathedral roof and would like to know from any experts here if you can please look over the following design I have made.

Basically, I would make the trusses with 2 by 8 or 2 by 10 lumber. On the trusses I would add 2″ of closed cell foam panels and on top of that an aluminum type radiant barrier layer. Both the foam panels and radiant barrier would be held in place with 2 by 2 lengths of lumber that are nailed (or screwed) through all three into the trusses. Then on top of the 2 by 2s I would add standard 7/16″ OSB sheathing, then the outer edge membrane, roofing paper and tar shingles.

The trusses would also be sprayed with closed cell foam to the complete depth before a vapour barrier is installed on the inside of the garage ceiling.

My thoughts on this design is that it takes into account all the criteria required for a good cathedral ceiling.

The 2″ foam panels would provide insulation and eliminate thermal bridging of the trusses. The aluminum radiant barrier sheet would lay on top for the 2″ foam panels and keep summer heat inside the 2″ ventilation chutes that would also provide air to dry any water infiltration from the shingles, while keeping in winter heating inside the garage.

The other Alternative 2 I am showing is just an idea on maybe using the ventilation chutes as the air source to a heat exchange system for winter usage since the cold winter air entering the soffits should increase in temperate as it travels through the ventilation chutes even in winter since the roof would be covered with a certain amount of snow. During the summer this same air is very hot and may be used to heat water.

Hope the diagram is understandable.

I understand that a cathedral roof will want to push against the outer walls of the garage and I have already taken note of that to add the appropriate beams to counter any such event.

Thanks in advance.


PS: My Garage will have a well insulated footing foundation, under slab, each side of 12″ high cement wall on the slab plus 2by6 framing walls will also include 2″ exterior foam and sprayed foam between studs. I am looking to make a very efficient garage and if all goes well, I will start to modify my poorly insulated home next year.

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  1. GBA Editor
    Martin Holladay | | #1

    1. You cannot place a layer of rigid foam directly on top of your roof trusses, because the foam is squishy. The foam cannot support the weight of the snow on your roof without compressing. You also need to tie the roof trusses together with OSB or plywood for structural reasons. So, the first layer that needs to be installed on top of your roof sheathing is OSB or plywood.

    Second: You can buy foil-faced polyiso foam, or foil-faced EPS foam, so you don't need to install a separate radiant barrier (if you want a radiant barrier). Just buy foil-faced foam. Remember: the radiant barrier won't do any good unless the foil is facing an air space.

    Third: You don't need to install a polyethylene vapor barrier. Closed-cell spray polyurethane foam is already a vapor barrier.

    Fourth: There are many ways to achieve your goals that are less complicated and expensive than the roof assembly you have drawn.

    For more information on this topic, see How to Build an Insulated Cathedral Ceiling.

  2. wattsup | | #2

    Dear Mr. Holladay,

    Thank you for your response to my post. I would like to answer your comments as per their numbers.

    1) a) According to my investigations, there are different types of insulation panels such the FOAMULAR C-300, Extruded Polystyrene Rigid Insulations that has a rating of 30 PSI, which should be more then enough to support a 40lb per square foot load anticipated from snow fall when only supported by the 1.5" width of the trusses.

    b) As well, I can always insert metal spacers through the foam at every 6 inches or so that will align with the trusses and meet the 2 by 2 ventilation spacers. So in one operation with proper length nails or wood screws, I could secure the 2 by 2 ventilation spacers over the aluminized radiant barrier (ARB) and through the 2" rigid insulation that has internal metal (or other supports). This would provide a solid base to then nail the 7/16" OSB sheathing (shown in above diagram) onto the 2 by 2s and thus preserve the structural integrity of the trusses. Then just continue finishing the roof over the OSB using standard methods.

    c) The other option is to install OSB sheathing directly to the trusses then follow the rest of the diagram with 2" rigid insulation, ARB and so on. Also, in one of your referenced documents they mention using 2 by 4s (instead of my 2 by 2s vent spacers) so this would increase the surface area on the rigid insulation to more then required plus the first OSB on the trusses would assure structural integrity as you had mentioned.

    M preference for putting the rigid insulation on the trusses first is to produce a continuous insulation where the sprayed foam between the trusses would be in direct contact with the rigid insulation over the trusses and not have an OSB in between.

    2) Yes, The ARB would be facing the air space created by the ventilation chute (VC) so summer radiant heat would be held in the VC while it is being vented away.

    3) Agreed

    4) I am not really concerned about expense. At the stage of building the roofing any overall differences in cost for labor or for an extra layer of OSB or other materials would be frivolous compared to the long term benefits. The main criteria for me is to maximize the insulation factor of the total garage. The Cathedral roof would enable me to build a mezzanine for storage purposes.

    The rigid foam mentioned above is used under concrete slabs so it should be able to withstand roof pressures very well, plus if I use an inserted support system that keeps the weight off the rigid foam, this should be more then enough and suitable to reduce or eliminate thermal bridging through the trusses.

    Thank you again for your kind response.


  3. GBA Editor
    Martin Holladay | | #3

    It sounds like you know what you want to do. Good luck.

  4. wattsup | | #4

    Hello again. Well, I built my garage and have put up a video of the complete process.

    The insulation in the roof is not how I had planned above simply because I started the garage build rather late in the summer and by the time I got to the cathedral roof it was really starting to get cold and I just did not have enough time to work out the double sheathing layers to produce a dedicated air ventilation channel. So I did the next best thing and built a 1 inch air channel below the sheathing.

    I made two videos of the complete build using a time-lapse camera. The main video is 1 hour long but I made a short version of only 20 minutes that will give you a good idea on the build, It is located here;

    Hope you enjoy the video.

    Leon Stepanian

  5. wattsup | | #5

    Hmmmmm. I forgot to post a photo of the garage so here it is.

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