I am turning a former workshop and storage building into a house. I am in a  hot summer/cold winter climate in interior coastal northern Ca. that can experience many winter  nights in the 20s plus a few in the teens some years. Summer can go as high as 115. I will heat with wood in the winter with a back up Rinnai wall heater for the upper level master suite. Summer nights are usually in the 50s so religious opening and closing of windows will keep the interior daytime temperatures reasonable without air conditioning. A minimum of R30 was required by my California Title 24 analysis.

    The basic building  shape, excluding a new addition, was two shed roofs, one over a single story and another over a two story with clerestory windows from the two story section overlooking the one story section’s roof. The rafters are 2×6 16″ oc and the roofing material is Metal Sales PBR panel. The roof pitches are 2/12 and 3/12 with the former pitched south and the latter pitched north. This has given me two cathedral ceilings to insulate unless  I wanted to do a major fur down which was unacceptable because of the loss of headroom on the low side of the rooms and would have been tricky to vent where the lower level roof died under the upper levels wall with windows. For reasons of economy I am doing most of the work myself and as spray foam is not commonly used in this area and would have required a subcontractor’s help  I didn’t consider it as an option. 

  I  fully filled the bays with poly iso rigid to give me r35.75 on the ceiling of the two story section and covered the underside with kraft paper for an added vapor barrier. It was a lot of work as well as expensive. After much online research I have decided to use a hybrid system on the lower section. 

   I am putting 2″ of poly iso against the underside of the roof deck in the rafter bays then 3 1/2″ of kraft faced fiberglass roll below that to fill the cavity to r25.9 and then sheathing the underside of the rafters with 1″ poly iso rigid which will be covered with 1/2″ drywall attached with 2 1/2″ screws.  That would give me about r32.4 but using the CEC (California energy code) appendix JA4 Table 4.2.2 – U-factors of Wood Framed Rafter Roofs, my assembly should have a U-factor of about .032 which is about halfway between r30 and r38 without any insulation bridging the rafters on the underside, which means in the end about r34, justly slightly less than my fully filled bays of r35.75.

  I was initially concerned about the possibility of condensation in the fiberglass layer. My reading indicated to me that as long as there was a thick layer of poly iso against the roof deck and an effective vapor barrier on the inside this should not be a problem. The foil facing with aluminum taped seams and around all room edges on the bottom will provide that vapor barrier and the poly iso on the rafter undersides decreases the bridging effect of the rafters.

   I should add that the style trend of my project is modern (think Dwell magazine) and my building department is allowing me to run mc cable on the surface from the wall switches  to supply my ceiling light fixtures, meaning there will be no electrical in the ceiling to introduce unwanted airflow and with it moisture. 

    As a footnote I will add my experience cutting poly iso for this type of installation. I used a handsaw to cut the poly iso rigid for the first section which was very messy and left little crumbles on the edge to fall on you as it was pressed into place. For the lower ceiling I am using FHB Editor at large Chuck Miller’s method,  a 6″ drywall knife with sharpened edges. I never considered a table saw. The drywall knife is quick and generates little dust or fragments. You may be able to pick those up at the table saw using a dust collector but it seems to me that the edge would still be a little rough. Cutting to close tolerances is easy including dealing with rafter bay width irregularities. Unless there are iregularities I cut against a 4′ drywall T-square. The four foot sections are easy to handle and I run aluminum tape over the seams once installed. I start each piece with one long edge fully in and the other part way and using a large square head wooden carpenters mallet combined with a longer scrap of 2×6  I tap them in snug. They almost always fit well. I have rarely had to fill any gaps along the edge or remove and recut because of an oversize issue.

 Using  a fine file, in a few minutes I put an edge on the drywall knife about like a cheap kitchen knife. I touch up for a few seconds when the cutting action seems to slow. I have tried sharpening further with a diamond file but it seems unnecessary. Cutting the board is almost as easy as cutting drywall with a utility knife. Part way through gives a clean snap and then the reverse side foil is easily sliced with the sheet standing on edge. I do  sharpen all three edges so I can do a plunge cut as well as a slice cut. This comes in handy at the ends of the cathedral where I put a vertical piece to fill the bay top to bottom against the blocking. I lay a framing square riding on the rafter lower edges and plunge in parallel to the roof angle.