Cut and Cobble on a Vented Cathedral Ceiling
Climate: Canada, cold, ice dams are common
I am undergoing a (budget-friendly) renovation on a 1960’s home with cathedral ceilings. I have read the cut and cobble article by Martin Holladay. I have an incredibly cheap source of 2″ and 4″ thick EPS, which is the reason I’m considering this.
Existing Build:
Roof:
-2:12 low slope leaky roof, shingles over shingles, no membrane.
-2×8 rafters 12″ O.C.
Insulation:
-Moldy and black, faced R-12 fiberglass insulation
Proposed Reno:
Roof:
– remove sheathing, screw down long 2×4’s on rafters from soffit to ridge to build-up depth for more insulation. Depth is now 2×8 +2×4 = 11 inches
– install new 1/2″ plywood and ice and water shield on the 2×4 rafter extensions
– install ridge vent based on 1/150 sq ft. venting ratio
– install durovent baffles airtight from ridge to soffit for ventilation
– install 2×6 mineral wool (R22) in the rafter cavities under the durovent
– install two 2″ layers EPS staggered or 4″ continuous (R20) under the mineral wool cut and cobble. I have a two sources of cheap EPS, but neither has enough sq ft. to do my entire ceiling.
– vapour barrier
-1/2″ drywall
The result (I hope) is an airtight R42 cathedral ceiling.
Questions:
1. Thermal bridging is an issue especially with the rafters being 12″ OC. My main concern is mold. Would this build have that problem?
2. Would I be better off installing all mineral wool insulation in the rafter cavity? I will have less R-value (R36) but it will probably be easier to install.
3. Is a vapour barrier a good idea under 4″ EPS. I have read that after 2″ it’s considered a vapour barrier but I’m not installing them over the studs, rather they are between the rafters. If I vapour barrier over the EPS, would that lead to moisture being trapped and rafter rot?
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Replies
Ramany,
I'm confused. Are you planning a cut-and-cobble approach (with EPS between the rafters), or are you planning to install mineral wool insulation between the rafters, with a continous layer of EPS underneath the rafters (on the interior side of the rafters)?
(The reason I'm confused is that one of your sentences calls the EPS "continuous.")
Cut-and-cobble is safe for vented roofs, so cut-and-cobble is OK. I just want to understand your plan a little better.
By the way, most EPS isn't R-5 per inch. It's more likely to be between R-3.6 and R-4 per inch.
Martin,
Thanks for your reply. Installing mineral wool then a 4" layer of cut and cobble EPS below it which stays between the rafters. That 4" EPS layer is made up in two methods, 2" thick + 2" thick staggered, and in other sections it will be a solid piece of 4" thick EPS. When I said continuous, I meant the joints aren't staggered. It's a Mish mash of EPS. This is dow cladmate, which is claiming R5 per inch on the website.
Ramany,
OK. The Dow CladMate is XPS (extruded polystyrene), not EPS (expanded polystytrene). That clears up one of the mysteries.
You have cleared up the other mystery by explaining what you meant by "continuous."
Q. "Thermal bridging is an issue especially with the rafters being 12 inch OC. My main concern is mold. Would this build have that problem?"
A. There is no reason that you would have any mold. As with any cathedral ceiling, you have to pay attention to the airtightness of the ceiling. If you want to reduce thermal bridging, you can install an additional layer of continuous rigid foam on the interior side of the rafters.
Q. "Would I be better off installing all mineral wool insulation in the rafter cavity? I will have less R-value (R36) but it will probably be easier to install."
A. That's probably what I would do if I were you. I would save the rigid foam to install as a continuous interior layer under the rafters.
Q. "Is a vapour barrier a good idea under 4 inches of EPS?"
A. No, you don't need an interior vapor barrier with the cut-and-cobble approach. As I wrote before, you do need an interior air barrier.