[Greenbuilding] Foam installed between framing during framing

[Eli Talking]

I am wondering if it would be reasonable to install accurately precut eps foam between joist, rafters, or top chord of truss during the framing to achieve a cheap relatively tight installation when compared to fiberglass with the air seal being achieve outside the framing with continuous 6” eps foam managed for air tightness.  More detailed description is below.  

From: John Straube 
Sent: Wednesday, November 28, 2012 1:13 PM 

Experience has taught those who have built a few of these externally-insulated buildings that they can be easier to build too.  This means one has to learn how to handle the remaining challenges
1. how to find low cost long screws that screw in quickly and easily
2. the furring needed to allow for easy cladding fastening
3. the details at window and door openings required to mee
Point 3 is still evolving and improving.

My comments:
I am designing a house mounting eps on exterior of framing.  I found a source of eps that is a lower price than fiberglass/R.  By making it on the outside, I avoid the conflict with wires and plumbing commonly installed in the framing and achieve continuity.   Also, the framing is inside the thermal envelope and therefore near the conditioned temp.  I am addressing the three issues John brought up.  

I found a source of long screws. 
http://www.loghomestore.com/1748-headloc-panel-fasteners-9-to-18-inch.php
Prices seem reasonable.  Far less than GKR screws that I love.  However, the screws are a significant cost.  My original intent was to mount 12”eps over the roof sheathing.  However, using a spacing of 2’oc on furring strips that are also at 2’oc, it results in 4 screws/4sf, or 1 screw/sf.  On the walls, my 6”foam is setting on the foundation with no significant loading except for wind suction.  My site is windy.  For this I need am planning a 9.5” screw at 
$.50/screw in bulk.  However, the thicker roof would need 16” screw for $1.22/screw.  That is a significant sf upcharge.  The 12”eps is only $1.56/sf.  I am aspiring to keeping the cost down to create more access for high performance.  As a result of this cost, I made the recommendation that we insulate the ceiling of continuous 4” eps  mounted below cheap plane jane truss to achieve the continuous thick air barrier with cellulose toped off to achieve higher R value.  I believe this is the most cost affective way to top off the thermal envelope.  However, my clients want to develop the attic. Therefore, we are back to wrapping the insulation above the roof sheathing.  However, because of the cost of the fasteners, I am looking to skin it with only 6” foam to achieve thick air barrier and put some insulation in between attic truss top chord to increase the R value.  Fiberglass would be the easiest.  Because of the above roof eps, it probably would be ok, even though it is air pervious.  I would follow www.buildingscience.com ‘s recommendation for proportion of air pervious to non air pervious to insure inner most foam temp remains above dew point in winter with 50% relative humidity in house.   

However, I am wondering if you could precisely cut eps foam to the exact 22.5” spacing between truss top chord mounted at 24”oc.  I see this as being installed during the framing with a friction fit.  Since the top 6” insulation achieves the air barrier requirement, I would not think additional sealing would be needed.  Even without sealing, the nature of eps would be much tighter than fiberglass.  It also turns out it is less expensive.  This could also be done at the band joist.  Installing during the framing allows the foam to be used as a spacer to create a tight fit that would be difficult to achieve after framing.  However, I have not seen it done this way.  Therefore, what are the risk that I am not seeing?  Or is this a reasonable idea to increase R value at a low cost?

List input is always appreciated.  

Eli
www.conservationarchitect.net 


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