What pitch of scissor truss low enough to prevent cellulose from sliding down?
mikeolder
| Posted in Green Building Techniques on
Hello folks, hope your doing well..
Ive drawn up so many plans and ideas for my bug-out acre in Iowa, but keep circling back to a simple attached garage shouse. I need raised heel scissor truss’s to get the 14′ ceiling height with 9′ sidewalls but I’m wondering, what is the maximum angle the bottom truss cord could be without having 16″ of cellulose sliding down to the eave? I could always use something like plywood dams or netting to prevent that. But is there a rule of thumb where I wouldn’t need dams?
Thanks!
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With enough adhesive in it, any slope can work. Cellulose can even be sprayed overhead.
So its a non issue.. Thanks Jon!
I'd say "check with your cellulose installer". Commonly stated is 4:12 to 5:12.
Agreed. The cellulose installer may say, "yes, we could do that but it would be cheaper and easier to use netting."
Mike,
I wrote an article on the topic: "Insulating Scissors Trusses."
Here's a quote from the article: "The consensus among builders who have expressed an opinion on this question is that the ceiling slope can be as steep as 4-in-12, but no steeper."
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Thanks Martin.. All the knowledge Ive acquired here is sometimes a curse.
My problem is Id like the recommended R60 (faced batts available only to R38) and can order raised heel scissor trusses, but what's the options for ventilated R60 without spray foam here? Can they install netting and blow through holes, or ive read work from the peak and lower the hose down the net 28 feet, because there's not much room for a worker to access scissor truss space after drywall?
Or am I stuck with R38 realistically? I really don't want the 14' sidewalls usually required for a car lift. The plan does not have scissor truss's drawn in. Thanks!
https://i.servimg.com/u/f59/14/97/22/52/img_1312.jpg
Mike,
It's definitely worth pursuing cellulose, but if you end up using batts there is nothing limiting you from using two layers and getting whatever R-value you would like.
Akos has commented on several projects he has done with scissor trusses and blown insulation. Hopefully he will chime in.
> but what's the options for ventilated R60 without spray foam here?
I'm not recommending it, but there is "above sheathing venting" with the truss (scissors or parallel) fully filled, perhaps even into the eaves.
Jon,
Isn't that approach patented ? :)
The article Martin linked and the links therein are good reading. 4:12 isn't the maximum slope--it's just the maximum slope before you need to worry about insulation sliding down. Ultimately, you can end up with something that is more like a vented cathedral ceiling for the bottom half, and then a vented attic with cellulose on the floor for the top half. You might end up with R100 in the middle and R60 at the top and bottom, but that can only help.
I like Stephen Sheehy's solution of blowing it in from ports in the roof deck.
Thanks everyone but ports in the roof deck? Like big holes cut in the plywood Charlie?
I don't like that unless your removing a entire sheet.
Ive drawn another drawing, but I'm coming to the conclusion insulating scissor trusses with fluffy stuff or anything other than with R38 battens, which maybe adequate for a garage, is very difficult and I remember last time I considered it I was stumped too. . But maybe since I have scissors in the garage side only, I can access them from the flat ceiling home side? I guess this is best asked to the installer.
Mike,
As Charlie said: I think limiting blown cellulose to pitches of 4/12 on scissor trusses came from situations where the insulation sat on the bottom chord and was not constrained above. In your situation where the cellulose will be sandwiched between the ceiling and vent channels, I'm sure you could go to 5/12 with no problems., or necessity to dense-pack.
Given that, I'm wondering whether when the pitch of your bottom and top chords are so close it wouldn't make sense to just use parallel chord trusses with a consistent depth?
I agree that accessing that from the flat-ceiling side would be the way to go. And it looks like you'd basically be packing the full space between the ceiling and a ventilation baffle, which would go all the way up. Almost more like filling a wall than blowing insulation in an attic. I think it will work well.
Thanks but Ive drawn these scale scissor trusses with 24" heels and with a span of 28' are 5' tall at the top guys.. Considering I need 16" of cellulose to achieve R60, packing these spaces would be way overkill for a garage. And I didn't plan on using baffles.. I planned on holding the cellulose away from the roof decking..
Again, unfortunately it sounds like using faced R38 fiberglass is the only realistic answer except for the wind washing.. Especially with a 28' span.
Thanks again everyone. This discussion has allowed me to visualize cellulose insulation installed in a scissor truss system, or rather how difficult it would be to install.
Malcolm mentioned doubling up fiberglass batts which is something I didn't consider. After researching the wind washing effect, this has me wondering if I should install two faced layers, surrounded or sandwiched by the paper facing.. Or in other words, the first layer installed with the paper facing up stapled to the bottom of the top 2x4 cord creating a 3 1/2" vent space with the batt hanging from the paper adhesive. Could I pull the fiberglass batt back far enough to staple the paper to the bottom of the top cord? And then the second layer stapled to the bottom of the bottom 2x4 cord.. R30 faced fiberglass battens are 9 1/2" think, so total thickness of 19" would be needed to achieve R60. And when using parallel cord trusses, would require 22 1/2" total truss height. Or I should increase to R-38 which is 10 1/4" thick to prevent a 1 1/2" dead air space when using 24" tall trusses, which are required to span the 28' on my plans.
My question is.. Is a bad idea to install the top batten upside down or face up to prevent wind washing? Would the top batten stay in place with the friction between the trusses, or would it drop down pulling away from the paper if there was a dead air space?
After searching around tonight, it looks like R49 faced battens are available. Maybe I should just make it easy and use one layer of R49, but this doesn't protect it from wind washing and my location can be very windy sometimes.
Thanks!
Mike,
RHD has done some testing to actually quantify the effects of wind-washing on various insulation types. I don't think it's worth worrying about.
https://www.rdh.com/wp-content/uploads/2017/10/Van-Straaten-Windwashing.pdf