I just bought 4 pallets of roofing 4" Polyiso fiberglass faced from Insulationdepot.com. ........................................................................................................................................................................................................ Seems like a good price at $.75 ft2. R24 2#/ft3 ........................................................................................................................................................................................................ Question about screw attach to 2x6 studs, as the steel screws conduct heat so well, I'm thinking of using a 1.25" wood bit to countersink the heads & washers about 2" into the foam & fill holes with great stuff spray foam. ........................................................................................................................................................................................................ Will this help with heat conduction & support foam weight + Tuff2 surface?............................................................................................................................................................................................ As I don't get new paragraph here? I used............to make this somewhat readable...........................................................................................................................................................................................

see: http://www.greenbuildingtalk.com/Forums/tabid/53/aff/21/aft/80232/afv/topic/Default.aspx and http://rightenvironments.com/low_energy_buildings_class/101109%20point%20thermal%20bridges%20v2.pdf slide 14 "Anchors – can be ok, e.g. buried under insulation, sometimes no anchor at all." slide 16 "@ 10 sf per anchor: adds 0.0011 btu/hrsf°F to the final U-value. = 4% of the heat loss budget for an R-40 wall = 6% of budget for an R-60 wall"

On this forum, I must use firefox browser to get paragraphs to show up.

It was discussed here, but it's very deceiving. As soon as there is any covering insulation (like the studs that the screws go into), the % of whole wall numbers go way way down (to the point where you really don't need to worry about it).