“Building down” a cathedral ceiling
Castorcanadensis
| Posted in Energy, Heating & Insulation on
I’m looking at fixing a design problem in the house we’re building. As built, the cathedral ceiling is 2×10 to be insulated with R-38 fiberglass.
I’d like to add more insulation before we button it up. I’ve seen references to “building down” the ceiling. Any suggestions from the professionals on how this is done? 2x4s at 90 degrees to the ceiling joists, held on with hurricane clips? A grid of steel 2×4 studs and tracks?
My wife and I have already discussed the option of putting in collar ties and a proper attic space. Not happening.
Replies
depends on your climate... and the structure
why are you talking about collar ties ?
if you need them, you need them
if not then the "build down" is a function of the type of insulation and the r-value you want / need
Hopefully, you have used a structural ridge or taken other measures to prevent the walls from pushing out with cathedral construction. Adding more to the rafter structure means the structural ridge has to be designed to take additional weight. One option is to use a second set of rafters, similar to a scissors truss. The inside cathedral will be at a lower pitch than the structural rafters, allowing additional insulation space and still maintaining the peaked ceiling inside.
This technique can also add strength for a cathedral without collar ties if it is built like scissors trusses. One problem is at the top of the outside walls. If you haven't planned ahead, it's difficult to gain enough extra depth at this point for insulation. However, the space gets deeper quickly and can provide as much space as you desire, depending on the difference in pitches.
Another option is to use foam board under the drywall. Some apply the drywall over 1" foam using longer screws. Others may use 2" foam and run strapping over that for the drywall. You gain R5 per inch of foam and it gives you a homogenous surface and thermal break over the framing members. SIPS on the roof deck can be a possibility and foam in the rafters may offer greater R value than fiberglass.
Insulation is part of a system. Fiberglass in cold climates with non metal roofing usually requires good ventilation to keep the heat loss from contributing to ice dams. Foamed in rafters in a hot climate may cause a hot roof and lead to early degradation of asphalt shingles. It helps to say where you live and what roofing you are using.
Beat it to fit / Paint it to match
Thanks for the replies, all. I see I left out some important information in the original question:1) South end of Vancouver Island - wet, mild climate.
2) Log house, roof supported by structural ridge pole with knee braces, signed off by an engineer.
3) Roofing is cedar shakes, installed on 2x4 purlins on the 2x10 roof joists. We've improved venting with ridge and soffit vents that weren't originally included. We'll probably go with metal roofing once this roof needs replacing. Something about living on the forest interface and those Kelowna wildfires is making us nervous.
4) R38 fiberglass batts were originally spec'd.Putting 2" of foam board under the fiberglass sounds interesting. This would be a simple way to hide a mistake the builder made cutting the ridge bevel without fiddling around with scribed trim like I thought I'd have to do.
Given that proportionally the log walls and windows are going to be your weakest link, I don't think it is worth putting more than an inch of foam on the underside of the roof joists.
Are you sure you can use R 38 batts and still have the 3" airspace required by code for ventilation?
It must be hard to even think about insulation with the weather we are having on the island this week :)
>>>My wife and I have already discussed the option of putting in collar ties and a proper attic space. Not happening.
You might mean ceiling joists; collar ties are a different thing.
I once did what hammer referred to, foam under drywall. I used 1" polyisocyanurate board with foil facing as a vapor barrier. Worked well.
Scott.
What about just using high density foam instead of the FG.
William the Geezer, the sequel to Billy the Kid - Shoe
I did something similar. Had 2x12 rafters and added a 2x4 along the bottom edge (making approx. a 2x16), Held on with 6 inch pole barn spikes and construction adhesive (although I'm not sure that the adhesive is necessary). I was pre-drilling and hammering them in until I discovered the palm nailer drives them with no trouble at all.
take a look at my thread, i "built down" a cathedral ceiling,
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new ridge under the old. pair of SYP 2x12's for a 7 ft span
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new rafters, sistered to the old, measure the HAP and cut the tail
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you can see the HAP and then the cut out for the plate, i then put a stud underneath the tail in the wall to give full bearing. this was a kitchen and i was given just enough room to place my upper cabinets under this rafter setup. i was able to fully insulate the ceiling.
Woods favorite carpenter
http://forums.taunton.com/n/find/findRedir.asp?webtag=tp-breaktime&mg=A374F7F8-1455-4CA2-A6BD-BE1A831F000C
think this is the thread hopefully it helps some Woods favorite carpenter