[Greenbuilding] Foam installed between framing during framing

Eli Talking elitalking at rockbridge.net
Sat Jan 26 09:42:24 CST 2013


Thanks for all the thoughtful replies to my original post.  The result of input and further thought is that I am returning to plan “A” to mount all 12” EPS foam insulation on top of the structural roof sheathing.  This is simpler than what I proposed below.  Installing a long strip of foam during the construction between top chord rafters has a lot of potential problems.  I plan to install 2x4 furring strips at 2’oc that I will predrill for accuracy of alignment of screw.  I will also countersink with 1” deep x .5”round hole that will allow the head of the screw to be .5” above the deck, giving the screw 1 additional inch of penetration, allowing a shorter screw.  Also, the counter sink hole can be filled with can foam to block the thermal weakness from the conductive screws.  By using full 2x’s for furring strips on both roof and walls, the finish siding and corrugated metal roofing can be fastened with old fashioned nails, reducing the time and cost of installing screws for finished materials which would be needed if the alternative 3”strips of 3/4” plywood is used.  

I understand the method of installing a foam block in faming cavity slightly smaller so that a seal can be placed on one side of foam, pressed against one side of cavity, leaving a gap to be filled with can foam. I used this method during my deep energy retrofit in my home. In my case, I installed 3” foam below a cathedral ceiling and 3” foam over old plywood siding.  To create continuity, I installed foam blocks in stud cavity as described above that connected these 2 elements of the thermal envelope.  It is certainly tedious and vulnerable to execution flaws. My preferred method for making that connection is to leave the cavity to be filled with a professional sight applied insulation.  However, the situation I described, I was managing a 6” foam above the roof deck to be air tight, therefore, sealing was not so critical.  In the original concept, it would be replacing the air permeable fiberglass.  

Additional questions I will be posting in a separate message is sealing the plywood seams and using framing cavities for ERV duct chases.  

Eli 
www.conservationarchitect.net 

Another sit

From: Eli Talking 
Sent: Thursday, January 03, 2013 11:04 AM
To: jfstraube at uwaterloo.ca ; archilogic at chaffyahoo.ca ; Green Building 
Subject: [Greenbuilding] Foam installed between framing during framing


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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