Sloped ceiling: 2×12, I-joist, or parallel chord truss?
I am designing a shed dormer with cathedral/sloped ceiling for a new house. The shed will be 22′ wide by 16′ long (span). With a 4:12 slope the slope length is 16’8″. Most of the roof is “stick built” (a large doghouse dormer on the opposite side might be sissor truss). There will be a continuous load bearing ridge beam running the width of the house supported in 3-4 locations to break up the span length(s).
The roof assembly looks like this:
Shingles
1/2″ or 5/8″ plywood
2-1/2″ ventilation space (code requirement here)
R30 blown cellulose
Roof joists (2×12, I-joist or parallel chord truss) – your input wanted here!
R10 XPS applied to face of roof joists for continuous coverage (making my R40 better than code R40).
1/2″ air-tight drywall.
(For some strange reason our code requires R28 for cathedral ceilings and flat roofs, but R40 for attics. I am aiming for “code +50%” so that’s how I arrived at R40)
I am looking for some advice as to what roof joist system to consider. 2×12’s might be too shallow to achieve R30 cellulose once you subtract the 2-1/2″ ventilation space. Parallel chord truss was my next thought but as far as I know these are only designed for bottom bearing applications and seeing as my ridge beam will be more or less flush with the sheathing I’d need top bearing or “hanging” trusses. My last thought was going with some deep I-joist, like 14″. There might be some economies here if that means deleting the XPS – don’t know. Does anyone have any experience with hanging roof sloped 14″ I-joists off a say 4×14 PSL beam?
Thanks in advance for any useful advice or insights.
John
Replies
Using I joists sloped and hung off a beam should be no issues. Standard detail for the joist manufacturer. You should be able to get a fold out of all the acceptable details for the joists. They will require rafter tails be scabbed onto them, so just be aware of that (also included in the mfg detail sheet). You probably don't 'hang' them of the beam ... you would use a joist bracket, I think or place the beam low and set them on top ... should be covered in the mfg instructions.
A parallel cord truss ... they are usually designed like any truss and if you specify top bearing, then that's what they should give you.
FYI - I think you only need 1" of vent space. You could acieve that w/ high density batts maybe in a 12" joist depth.
You can get parallel chord trusses made just about any way you want. Top chord bearing details are no problem. They can be made 1.5" wide like a roof truss or 3.5" wide like a floor truss.
2.5" of space for ventalation
2.5" of space for ventalation is astronomicly high. Are you completely sure on that, or did you misread 2.5sq" of ventalation per foot?
I'm redoing my roof and looking at a bunch of different options. In your case, I'd use TJI beams (wood I beams). Under the upper flange, stuff in a piece of 1" rigid foam, across the bottom apply insulmesh and horizontal strapping and fill with cellulose. This way you keep a waterproff layer under your roof deck - if you plan the details right you could pour water on the foam and have it run out your continuous soffit vent. You would have a matching continuos ridge vent.
Attached is a pick of what I mean, Though you would be filling the rafter bay fully with cells
2.5" ....
Thanks all for your input - probably leaning towards I-joist.
2006 British Columbia Building Code, Division B, Part 9, section 9.19.3.1:
""...no less than 63mm of space shall be provided between the top of the insulation and the underside of the roof sheathing."
I know it seems excessive but that's what the man wants.
Thanks
John
That's 2.5" above loose fill,
That's 2.5" above loose fill, blown into an typicly poorly sealed attic space. That is NOT what you are doing when putting the foam insulation at the top layer
This section applies for what you are doing:
9.19.1.3(2) “At the junction of sloped roofs and exterior walls, where preformed baffles are used to contain the insulation, the baffles shall:
provide an unobstructed space between the insulation and the underside of the roof sheathing that is: (i) not less than 25mm (1 inch) and (ii) of sufficient cross area to meet the attic or roof space venting requirements”(still 1/300 of the insulated ceiling area).
extend vertically not less than 50mm (2 inches) above the top of the insulation.
9.19.1.3(3) “Ceiling insulation shall be installed in a manner that will not restrict the free flow of air through roof vents or any portions of the attic or roof space.”
With a continous soffit and ridge vent, you don't have to worry about mold on the sheathing. The only air that is touching your roof sheathing is what is let in by the vents.
The nice thing about I joist is you can easily go with 19.2'" or 24" OC rafter spacing by sizing up the depth, which translates into less thremal bridging. This also gives you even more ventallation path as well.
I-Joists RAfters and wet conditions
Speaking of TJI for rafters. Would it be advisable to use demension lumber rafter than TJIs if it is expected that the structure might remain unroofed over the winter? I wonder how much protection TJIs would need from rain and melting snow, if no roofing material were on the roof over the winter just shealthing.
I-joists don't like getting wet. They swell up and can delaminate if they get soaked real good a few times.
where does it say loose fill?
Paul
Maybe i am not reading it right but i dont see any reference to loose fill insulation as being special (ie requiring 2.5").
Thanks for any insight
John
PS other posts about I-joists not liking the rain - i am aware of that however our summers are very dry. Thanks for the warning though.
There are many different ways to vent a roof, and almost as many ways to block those vents with loose fill insulation.
There are many ways to insulate, and almost as many ways to fail blocking moisture infiltration into that insulation.
Most houses are not properly sealed (all the penetrations though the ceiling and top plates for electrical and plumbing) between the conditioned space and the insulation space, and this allows water vapor to penetrate into the unconditioned space of the attic. Adding more insulation to a poorly sealed house will make it warmer, but does nothing to stop that vapor infiltration - thus the rule to hold back loose insulation away from the deck so it doesn't condense there... and if it does it can still easily dry out again. These homes do not have a soffit vent for every rafter bay - they have one ever 4 or 5 bays, so that extra space is needed to allow airflow around the rafter bays.
You, on the other hand, will not be building a cheap, lowest level acceptable by building code roof system. Quite the contrary. You are building a high performance roof with multiple drainage paths, Extra venatalation pathways, Extra rigidity, super insulation, and vapor sealed. You are making your attic into conditioned space, removing all those penetrations into your insulation layer. You are insulating at the roof deck - and not some measly 5.5" or 9.5" of insulation either!
Take the drawing of rigid foam in the webs of the TJI, filled will cells and covered with drywall to your building inspector, I'll bet you get a favorable review of that system. Even if they still require a super deep vent channel, that can still be accomplished with a simple modification to the foam board, making a U channel instead of a flat channel.
roof construction
I also think that the I-joist are the better solution. The parallel truss leave voids that you would have to fill up, whereas the I-joist provides a closed area which are easier to fill.
Using Roxul insulation R-20 (5 1/2" plus a layer R-14 (3 1/2) gives you 9 " insulation plus 2 1/2" airspace if that is what they require (?)
You could also strap the bottom of the joist and install R-10 XPS for added insulation, giving you R-44