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Insulating Scissors Trusses

rrausch's picture

This summer I'm going to be building my house.  It will be the first house I've ever owned, and I'm mighty proud.  I've done a lot of finish carpentry and misc. building (barns, outbuildings) most of my life.  But I'm not a Contractor. 

I'm going to be building simple scissors trusses for this house.  The upper chords will be 2X8 and the lower ones 2X6.  The spans will be 12 feet, 19 feet and 17 feet. I've built trusses like this before, on my barn that spanned 23.5 feet.  Those trusses were built and installed over 20 years ago.  I used Southern Yellow Pine and am going to use the same wood for my house.  My plan is to secure drywall to the lower 2X6 chords for the ceilings.

I'm thinking of installing baffles to the underside of the roof to provide air flow from the soffits to the ridge vent, and then insulating between the upper 2X8's with fiberglas batts.  But I'm a little worried I won't be able to get thick enough batts in there to insulate well enough.  So I'm thinking another option would be to also use batts between the lower 2X6 chord members.  Or another option might be to go to 2X10's on the upper chord.  Or another option might be to use nailers on the 2X8 upper chord to get thicker batts in there.  But if I'm going to go to the trouble of installing nailers on those 2X8's, I might as well just go to 2X10's.

This is in the Ozarks of Southern Missouri, out in the country, where there are no building permits needed and no Guv'mint Inspectors to come nosing around.  We like it that way.  You can build anything you're man enough to build, as long as you're willing to accept the consequences.  Therefore my plan is to overbuild a good, strong house.  Your questions, advice and comments all needed. Thanks!

Insulating scissor truss (post #205957, reply #1 of 15)

If you are in the design stage, I would suggest making energy trusses with a raised heel to accomodate deeper insulation.  This would provide savings in using smaller dimensional lumber for your trusses.  Blown in insulation, either borate-treated cellulose or fiberglass probably provides the most cost-effective energy efficient insulation. It is easier to get full coverage without having to worry about detailing a tight fit with batts.  Blowing insulation with rented equipment should be a one-day, two-man job.  Here in Northern MN, I retrofitted blown-in cellulose over batt insulation that was very inadequate.  It seals in around the truss chords.  I now have a R-50 attic over the vaulted celiing in part of my house.  The improvements in comfort and reduced heating and cooling costs are dramatic.

Bob

Duluth, MN

On the end of Lake Superior

What is the pitch of your (post #205957, reply #2 of 15)

What is the pitch of your upper chords and lower chords?  If they are parallel, what is the distance between the top of the top chord and the bottom of the lower chord? 

If you are going to the effort to use scissors trusses, you will have plenty of space within them for insulation.  I don’t believe you need to limit the insulation to being only placed between the top or bottom chords. 

I am working on superinsulated house designs that use parallel chord scissors trusses with 7:12 pitch, and 24” on center.   I will use 30”of fiberglass batts starting from the bottom, and about 6” of airspace above the insulation.  The typical recommended air space is 1-2 inches, but more is better for the cold-side ventilation airflow. 

I will also fill the triangular openings between the truss struts and two chords with 1.5” extruded polystyrene board cut to fit.  That way, the insulation bay has continuous sides in one plane, so the batts fit snugly at their sides when they are not directly between the chords.

I would not use 2 X 10s for the upper chords in order to try to get enough insulation space for the conventional scissors trusses without their triangular spaces filled.  Even 2 X 8s seems too wide for the structural purpose.  But I understand you are using that width to get depth for the insulation.  The manufactured trusses that I will use have 2 X 4s for both chords.        

Of course, that far south you (post #205957, reply #3 of 15)

Of course, that far south you can get away with a lot, in terms of poor insulation, but if you want proper insulation for good heating/cooling economy you should not use a standard scissor truss design.  As suggested, adding an "energy heel" is what you need to do, if the basic scissor style is what you want.

But the question is whether you can effectively and cheaply do this, as a DIYer (especially as over-designed as you'd have to do it), or whether you wouldn't be better off simply buying the trusses.  An energy heel makes the design much more complicated and harder to DIY.

Here is one example of a raised heel scissor:

A simpler design, with less insulation space, involves simply placing a length of 2x between the two chord ends and parallel to one of them, so it's width holds them apart at the heel.  You kinda get the idea in this image:http://resourcecenter.pnl.gov/cocoon/morf/ResourceCenter/dbimages/full/1400.jpg


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Venting scissor trusses or (post #205957, reply #4 of 15)

Venting scissor trusses or valted ceilings is never the ideal way to go - it just never works well  - never ever have I seen any decent insulation.  Having said that I do like the "energy heel" type of scissor truss since it can be built with enough room for decent blown in insulation.

Much much better is an inch or two of sprayed or carefully applied rigid foam so the area containing the insulation is contained within the shell and any blown in insulation then doesn't have to be vented.  The best solution is all sprayed in foam but if you are planning on saving $$ by doing much of the labor yourself then carefully fitting and sealing rigid foam is the way to go cost wise.

I plan on r60 in the ceiling for all clients - insulation is the only expenditure that will pay back it's cost - the only one! 

Good building!

 

Beer was created so carpenters wouldn't rule the world.

Thanks for all your replies.  (post #205957, reply #5 of 15)

Thanks for all your replies.  This gives me a lot to think about and I appreciate the time you all took to give me your ideas and feedback.  To answer a few questions, the slope of the upper chord I was planning on being about 6 in 12, I hadn't worked out the exact slope of the lower chord.  But a heeled scissors chord sounds very interesting--more complicated, but interesting.  I would rather not buy these trusses.  I can build better ones than I could ever buy, if I can get some good Yellow Pine.  Thanks for the picture of the heeled truss. 

Question:  As far as the ceiling/roof, what depth of blown-in insulation would give me a really good r-value.  What depth of batts would give me the same r-value?

To know what's a "really (post #205957, reply #6 of 15)

To know what's a "really good" R value, calculate it out.  Figure your heat gain/loss through the roof for your hottest and coldest days for a given R, then double or halve the R and see what it is, and figure that out in $$ spent on heating/cooling.  (Also figure the heat gain/loss through the rest of the house -- no sense overdoing the roof relative to the rest.

There are a number of web sites that will help with the calcs.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

When encapsulating, do not separate insulation layers (post #205957, reply #15 of 15)

I came across this thread while researching raised heel scissor trusses... interesting conversation, and solved a problem for a project I'm working on.

IdahoDon, if you're still tracking this thread, your proposal to install a couple of inches of foam on the roof deck to encapsulate the attic (e.g., unvented) and apply the rest of the insulation on the ceiling is not wise because it  will create condensation risk. Thehe R-value of the foam isn't enough to keep the attic space above the dew point, especially with the house isolated from the attic by the ceiling insulation.

Unvented attics rely on heat transfer through uninsulated ceilings to prevent condensation. Just ask the chief engineer at Icynene, who was involved in getting the code changed to allow this configuration. Otherwise, you'd have to make the ceiling virtually air tight to prevent moisture from migrating from house into the attic. An air-tight ceiling is a worthy goal but unlikely to happen in practice.

In any case, the IRC doesn't allow what you proposed. Section R806.4.1 requires unvented attic spaces to be totally within the thermal envelope and Section R806.4.5.3 requires any air permeable insulation (batts) to be in direct contact with the foam.

http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_8_par093.htm

(Note: in the 2012 IRC, this section number shifted to R806.5)

David Butler, Optimal Building Systems

I'm not sure I see the (post #205957, reply #7 of 15)

I'm not sure I see the advantage of building your own trusses over just stick framing it.  Yes trusses do save a lot of time in construction but not if you are spending the time building them yourself.

DanH, thanks for posting the (post #205957, reply #8 of 15)

DanH, thanks for posting the pictures of the trusses.

Cussnu2, I just like buiding trusses.  I guess that's the only advantage.  Now that I'm partly retired I have the time to do this house the way I want to.

Here are some floor trusses I built for my barn in 1996.  These span 22 feet.  I used formaldehyde resin glue.  When done my brother and I tested a few of them.  (He was the code enforcement officer at a nearby town)  We put 400 pounds onto the middle and measured deflection.  None deflected more than 1/8th of an inch.

Above them you can barely make out a few of the barn roof scissors trusses.  Those are bolted together using maleable cast iron bridge washers at the joints.  Also assembled using formadehyde resin glue.

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Yes, absolutely have enough (post #205957, reply #9 of 15)

Yes, absolutely have enough truss heel to give space for the insulation layer plus some air space.  Depending on the chord pitches and the span of a scissors truss, some degree of heel may be necessary just meet the structural requirement. 

Vaulted ceilings can pose limitations and complications on the insulation design.  All of them can be overcome by reverting to scissors trusses, so it would be a shame to uses scissors trusses in a way that still compromises the insulation. 

Ventilation is a simple concept and there is no reason why it should fail to do its job.  Provide openings under the eaves to let air in, and openings along the ridge to let the air out.  The complementary detail to this ventilation is a film vapor barrier applied on the warm side of the insulation, on the bottoms of the bottom chords before the sheetrock goes on.  Part of the purpose of ventilation is to provide fresh air to scavenge out any vapor that gets past the vapor barrier.  So if the vapor barrier were executed perfectly, no ventilation would be needed to remove vapor above the insulation.  So the vapor barrier and the cold-side ventilation work together, backing each other up to keep vapor from condensing, wetting the insulation, and causing rot and mold.

On its own, the ventilation also serves to prevent summertime heat gain, which can enter the living space by radiant transfer, and it also prevents ice dams. 

I do not believe that it is essential to provide continuous baffles to separate the insulation from the ventilation air space.  There may be some limited benefit in preventing air turbulence from penetrating air-permeable insulation.  There is also some benefit in protecting the insulation layer from dust accumulation and possible water intrusion from the vent discharge features. 

The main point I would be concerned about for the air space is that it be deep enough.  The code calls for 1”.  From a practical standpoint, a design for a 1-inch air space might easily end up being zero inches in practice.  Some experts recommend an air space of 2”.  If your insulation / cavity design has soft insulation, and if it fills the cavity, then baffles are called for in order to press the insulation down an inch in order to create an air space.  The reason people are splitting hairs over this air space is that usually vaulted ceilings are framed in a way that leaves a scarcity of space for the overall insulation system.  You completely circumvent this issue by using scissors trusses.  With those, you can have all the space you need.  

Thanks KDESIGN.  I appreciate (post #205957, reply #10 of 15)

Thanks KDESIGN.  I appreciate your comments--there is a lot of insulation info there for me to think about.  The more I think about trusses, the more I like this type of truss that DonH. posted:

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Venting works great if there (post #205957, reply #11 of 15)

Venting works great if there is plenty of room for lots of insulation - which is never the case with scissor trusses or cathedaral roofs...all it does is further cut down on the available room for insulation because of the necessary air space....nope - I'm not buying the argument for a vented scissor truss being anything near as usefull as one that has at least a shallow layer of rigid foam to bring the rest of the insulation into the building envelope....it's just more efficent...period.

As for the thought that there is no use in over insulating the attic I beg to dissagree - the attic is the easiest place to get to and if you are looking at the rvalue of an entire room it's usually the best bang for the buck.....r60 over r50 costs what?  The equipment is already there...all you are doing is blowing in more bags of insulation...the incremental cost is a nobrainer for me.

The thought of building your own trusses does make me scratch my head a little since a cathedral ceiling is much more interesting to me, but to each his own. 

I hope it turns out great!

 

Beer was created so carpenters wouldn't rule the world.

VAULTED CEILING DETAILS (post #205957, reply #12 of 15)

IdahoDon wrote:

Venting works great if there is plenty of room for lots of insulation - which is never the case with scissor trusses or cathedaral roofs...all it does is further cut down on the available room for insulation because of the necessary air space....nope - I'm not buying the argument for a vented scissor truss being anything near as usefull as one that has at least a shallow layer of rigid foam to bring the rest of the insulation into the building envelope....it's just more efficent...period.

As for the thought that there is no use in over insulating the attic I beg to dissagree - the attic is the easiest place to get to and if you are looking at the rvalue of an entire room it's usually the best bang for the buck.....r60 over r50 costs what?  The equipment is already there...all you are doing is blowing in more bags of insulation...the incremental cost is a nobrainer for me.

The thought of building your own trusses does make me scratch my head a little since a cathedral ceiling is much more interesting to me, but to each his own. 

I hope it turns out great!

My understanding of what the original poster is saying is they he will build scissors trusses and will use them for a vaulted ceiling.  But rather than buy manufactured trusses, he will build his own.  I agree that insulating the attic or vaulted ceiling is very worthwhile. 

You say there is never enough room for insulation in a vaulted ceiling with scissors trusses.  I can see that problem when using rafters, but aren’t scissors trusses exactly the solution to that problem of limited space? 

I am working on a design with parallel chord scissors trusses with 30” of fiberglass and 7” of air space above it.  I don’t see where there is any limitation in insulation thickness when using scissors trusses.  But the key is to raise the truss heel as Dan has shown with an illustration above.

I don’t understand what you mean when you say to add a shallow layer of rigid foam to bring the rest of the insulation into the building envelope.  Say you are using fiberglass in scissors trusses.  Where then is the shallow layer of foam?     

Yes, I'm planning on using (post #205957, reply #13 of 15)

Yes, I'm planning on using the scissors truss to make a vaulted ceiling.  The house will have three vaulted ceilings, in the living area, guest bedroom and the master.  All other areas will have flat ceilings at or near the normal height.

Allowance for 37" of (post #205957, reply #14 of 15)

Allowance for 37" of insulation and air space is very atypical of scissor trusses, but it definitely in the right direction.

Sealing the attic with a few inches of sprayed in foam creates enough separation between hot and cold to avoid condensation, so any insulation within that space does not have to be vented. 

 

Beer was created so carpenters wouldn't rule the world.