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SIP ENVELOPES WITH STEEL ON THE INSIDE

9/1/2014

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I was recently asked about my early experiences with SIPs and how I came to be such a SIP enthusiast.  I could easily recall when the lightbulb went on in my head even though it was almost 20 years ago.  I had gone to visit a SIP manufacturer and part of the tour included a visit to a project of about fifty homes.  The houses were in various stages of construction so the entire process was visible with some homes getting their final coat of paint and some weren’t quite through being framed.  I was excited at the prospect seeing the project as I was such a novice with respect to this material.  I knew that if I could get a good look at all the details of construction while in their various phases of completion it would be like getting an ultra-dense full college course in SIP design and construction.

The first building we stepped into was about in the middle of the project and represented a building that had been completely framed, with its SIP envelope finished as well as all the interior framing. By this time in my career, not only had I been in hundreds of stick-framed structures, but I had framed quite a few myself.  Seeing 2 x 4s was certainly nothing new to me.  But here I was seeing something that didn’t seem quite right.

I realized I was looking at some of the worst framing I had ever seen!  I don’t really know that I remember that the studs were out of plumb, I didn’t check anything with a level, but things just looked really bad.  I began to understand what I was seeing and experiencing.  The framing material was completely unlike what I was used to.  In my region the standard material is douglas fir, No.2 or better. No one would ever think of using anything else.  Spruce, hemlock, southern pine, No.3 or 4, the framing police would come and point and shout, “Shame!, Shame!”  Actually, the local inspector from the building department would be sure to make some disparaging remarks and perhaps even fail the framing.

But here I was seeing what I never expected.  The 3 & 4 grade hemlock seemed so, well…shabby, especially when seen against the crisp SIPs that had all been computer-controlled cut.  What usually would be unseen because it was so ubiquitous and so “expected,” so naturally reflexive, was now jarringly striking.  It was frosting on the cake that the quality of the workmanship matched the material.  This kind of “poetic justice” was certainly understandable.  Who could take pride in their workmanship when you had been uncaringly given such junk to work with?  It couldn’t be more obvious that this cost-cutting action was not inspiring the crew to give up their best.  The factory-true composite floor joists and exposed glu-lam roof beams couldn’t make up for the glaring deficiencies of the framing.

This is when the Eureka Moment came.  Why go through all the trouble to eliminate framing material from the envelope construction –- because it was energy inefficient and environmentally dreadful – and then go fill up that envelope with the same stick framing that was rejected as a material of choice for the exterior?

Why indeed.

When our office first took up with SIPs we simultaneously changed our default framing material to light gage steel.  We have not looked back nor have we ever been sorry.  Certainly light gage steel framing has its practical advantages over wood, particularly the fast-growth stuff we find in our lumberyards these days, but it also has a smaller environmental footprint.  We enjoy its structural superiority for floor framing, the fact that steel studs never warp, split, check, or shrink and the feature that everything is screwed – not nailed – together.  It must be noted that steel is far more compatible with SIPs, if the test is resistance to cracks at gypsum wallboard joints.

Steel framing in the commercial sector has evolved to incorporate the drywall installation.  In some cases the drywall is installed on one side to help stiffen and true up the framing, then the mechanical trades throw their stuff in  -- with the aid of all the pre-punched holes that eliminate all their drilling – and then the final skin is installed.  This works because the drywall installers are the same crew that installs the framing.  In our old wooden world, the framers are a separate crew from the drywall guys and the drywall guys are always complaining about the poor quality of the framing and the missing blocking.  If you can get a commercial “combo” crew to install both for you that means one less trade to coordinate and worry about.

Separating the inside from the outside also allows for the economical and efficient use of a specialized SIP crew.  The entire SIP envelope may be installed –- Balloon Style – without the necessity to interrupt the process by framing the first floor walls, second floor, and second floor walls.  After the SIP envelope is completed a steel framing/drywall crew can move in and complete the framing.

A summary of the features of this approach is as follows:

1.    A SIP exterior envelope gives at least twice the thermal performance of a steel exterior frame or a wood stick exterior frame.
2.    Balloon framing speeds erection and allows for efficient use of a specialized SIP assembly crew, which might have been “imported” just for this task.
3.    Light gage steel framing components never warp, split, check, shrink, twist, rot or support mold and mildew.
4.    Light gage steel framing components are pre-punched for ease of installation of in-the-wall mechanical components.
5.    Light gage steel framing components are most compatible with SIPs that are very dimensionally stable.
6.    The same crew that installs the framing installs the drywall thus eliminating the coordination of a whole trade.
7.    Light gage steel framing components have the sheet materials screwed to them, not nailed, thus insuring against the dreaded nail-pop repair call back.
8.    We are not filling up our envelopes with materials cut from either prime trees from dwindling forests or plantation trees which yield inferior quality framing lumber as it is prone to excessive movement with humidity changes.
9.    If we live on the East Coast we are avoiding shipping our wood framing material 3,000 miles across the country from the Pacific Northwest.
10. I can’t think of a last advantage to make an even “10,” but you get the idea.

You may want to check out my previous companion article on balloon framing at the end of this piece.  We have completed many structures that employ these techniques and these advantages are substantiated by our experience.  As I have said many other times, SIPs can be seen as stick substitutions but they yield much higher value if their true nature is more fully exploited.  We continue to hope that the architectural/design/engineering community will build on the experiences of the SIP community to date and evolve additional techniques and applications that extend the value of SIPs even further.

Originally written:  July 2003
Originally written:  July 2003
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BOLDLY GO WITH SIPS WHERE FEW HAVE GONE BEFORE

9/1/2014

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Originally written:  July 2003

By now there have been many articles in the popular "shelter" magazines about SIPs.  Articles have also appeared in the newspapers, SIPs have been seen on TV (that makes 'em real!), and even the architectural press has yielded up some inch-columns on our favorite "new" material.  Stories about "green" construction almost always mention SIPs.  We are beginning to finally gain acceptance within the building community insofar as "name recognition."  People now have heard of SIPs and have a basic idea of what they are and how they work.  It is the exception rather than the rule that a reference to Structural Insulated Panels is met with a blank stare.

Additional evidence for this is that there is growth in the industry and an increasing confidence about creating "stick translations." - taking that builder's set of conventional construction drawings and converting the building into SIP construction with the help of a proper set of "cut" drawings.  Most of the people responsible for creating the "cut" drawings utilized by the factory for pre-cutting the panels and the field crew for site assembly have had little difficulties - if any - performing the small amount of engineering required for "panelizing" a conventional stick structure. These buildings have been going up without any problems for decades and the process has lulled the design community into thinking that this is what is do be done with SIPs…and not much else.  But the real fun is in fully exploiting the full capabilities of SIPs and doing things that sticks could never economically do.  My list of SIP applications that take panels well beyond where most stick structures even think of going is as follows:

  1. Cathedral ceilings
  2. Lintels
  3. Columns
  4. Floors Over Unconditioned Space
  5. Curved Roofs or Walls
  6. Box Beams & Cantilevers
  7. Point Load Distribution
  8. Shear Diaphragms
  9. Seismic & Wind-loading Resistance

We'll take these in order, briefly, as each really deserves it's own chapter in a proper book.

1.    Cathedral Ceilings
This is where I suggest that anyone wanting to stick his toe into SIP waters begin because it is the most cost-effective application out there.  The new codes require a minimum roof R-value of 38.  In order to achieve this with sticks you need to utilize a minimum of 2 x 12 rafters, regardless of load/span requirements, in order to fit in 12" nominally thick batt insulation.  One must than put down lathing for a self-venting wood shingle roof or jump to 14" deep composite members to allow for venting over the insulation and under the plywood roof sheathing deck.  The cost of these can be pretty steep.  If we use SIPs instead, we may use a 6 inch SIP with a urethane core, an 8 inch SIP with XPS core, or a 10 inch SIP with an EPS core.  See the following chart:
Picture
Because venting is not required, we can really save quite a bit.  To maximize savings with this application, lower your plate height down perhaps as far as 5 feet off the floor.  Where before, with conventional construction, you may have had a boring 8 foot high flat ceiling with trusses or sticks giving you an unconditioned attic, now you may bring the whole roof down thus decreasing the total volume of conditioned space and introducing architecturally interesting shape and height.

2.    Lintels
When SIPs are used as lintels they may be used as engineered box beams which are capable of carrying enormous loads.  In most houses for most openings conventional lintels become totally obsolete.  This saves layout time and expensive material.

3.    Columns
Many times we are seeing columns on a stick-framed job that are built up of many sticks…sometimes as many as 6 or 8 members between windows or at corners.  This may burn up quite some board feet, especially if these are 2x8s or greater.  SIPs may do very well as columns in place of all these sticks and -- depending upon the wall design -- may come as just part of the wall between windows or doors and still handle considerable loads.

4.    Floors Over Unconditioned Spaces
This is an application that allows one additional design possibilities where none may have existed before.  In our office (before SIPs) habitable spaces and floors over "the outside" or even a garage were to be avoided, sort of like plumbing in an outside wall.  It could be done with special consideration and expense, but best economical practice precludes this solution.  Stick floor construction generally allows for severe infiltration that makes it almost impossible to deliver the comfort and economy we expect from a well built habitable space. With SIPs being such an effective insulator, and most importantly so resistant to infiltration, this may now be considered without any worry.

5.    Curved Roofs or Walls
Yes, we know these are also possible with stick construction, but again -- at what cost?  Curved SIPs, not available from all manufacturers but from many, offer the possibility of historic "bowed" roofs, radial roofs, stair turrets, towers -- or, whatever!  Bear in mind that each different radius calls for a different jig, so the more panels from the same jig the more reasonable the final cost.

6.    Box Beams & Cantilevers
When SIPs are properly analyzed as box beams the loads they can carry as longspan beams or cantilevers are surprising.  The load/span charts created under contract to SIPA by Thomas Bible, P.E. has a section on box beams that show 25 1/2" deep and 6 1/2" thick SIPs capable of carrying 864 pounds per linear foot for a clear span of 16 feet.  This is with LVL flanges.   The same beam as an 8 foot cantilever is capable of carrying 3,456 pounds out at the end.  Eight foot high walls, understood to work as cantilevers, may carry over 6,000 pounds at the end of a 4 foot cantilever.  Needless to say, this kind of reckoning should never be seat-of-the-pants, but properly calculated by someone who knows how to do this.

7.    Point Load Distribution
The central idea behind SIP construction is that it is thin shell engineering.  Like an eggshell, point loads are dispersed over large areas of the surface; the stress then at any given point is very small.  Beams that have to carry such great loads that they are designed as steel may still, in many cases, be safely carried at panel walls with out having to post down to the foundation.  Edge blocking that distributes these high loads along a significant run of panel may be the key to having the wall SIP alone safely receive the end of a steel beam.  Again, these details should be properly designed by someone who knows how to achieve this.  Much of what I see in the SIP world is way over-engineered so as to call for too many special framing and joint reinforcing members that destroy the inherent economy of working with SIPs in the first place.

8.    Shear Diaphragms
All wall and roof planes become shear diaphragms when properly constructed with SIPs; that is, the connections between the panels are correctly dealt with.  If so done, 90% of the stresses are transmitted across the SIP joint.  If we think of a traditional gabled box, it may be understood that the two roof planes act as diaphragms that will resist the outward thrust on the top of the wall.  Th roof-to-wall connection should be engineered for this, in most cases the standard SIPA detail of sloped wall plate and panel screws through the roof at 8" o.c. will be fine.  We see that is possible, in some cases, to eliminate the usual collar ties or ridge beam usually called for as a structural resolution for this form.  Never utilize this application without checking with a qualified engineer or architect!

9.    Seismic % Wind-loading Resistance
The new national codes are very proscriptive about calling for all kinds of ties and strapping for stick construction in areas where seismic and wind loads are severe.  Ordinary SIP construction automatically takes care of most of these situations.  Assuming your sole plates are properly bolted down to the foundation, the uplift resistance may be calculated per fastener when one specifies the edge distance, the plate material, and the fastener type.  For 1" edge distance (7/16" OSB skins), no.2 or better douglas fir, and no.8 x 1 5/8" screws, we use the value of shear resistance of 100 pounds.  When you figure the fasteners on both sides of the plat, and their spacing, you may come up with a value for the wall section or per linear foot of wall.

As we can see, it may be considered a chore to run all these engineering calculations, but the rewards are great.  The new codes have also imposed calculations and documentation requirements for stick construction that had been ignored before.  If one has to "run the numbers" anyway, I think that working them through with SIPs enables you to come up with some exciting designs and structures that can't be duplicated with stick construction.  Only if you add engineered steel and exotic connections to sticks that all help to send the cost way beyond that of SIPs, will they be able to match the efficient performance of SIPs.  Go forth and try some of these applications, but do so carefully with the help of a knowledgeable engineer or architect.  Eventually we hope to see full recognition of the mechanical/structural properties of SIPs start to engage the design community and encourage the development of new structures that may better reflect the classic architectural concerns of expressing resolution of the issues of time and place.
Originally written:  July 2003
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    SIPs & more...


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    Bill Chaleff

    Registered Architect
    A.I.A., LEED AP

    East End Green Architect who has designed with 
    SIPs since 1986. 

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