[Greenbuilding] Insulation under metal purlin

[David Wentling]

Recently, I was approached to review a renovation roofing project. They are planning to spray open cell foam under a recently replaced metal roof on an occupied, single story, masonry, commercial building. The roof was screwed directly onto existing 10" steel purlins on 60" centers (yea I know, they did not add a thermal break, something about not screwing with the edge alignment). They plan to install R-38 open cell foam, which will be 10" thick between the purlins and had not considered the bottom of the purlins. This alone will make a huge difference compared to the previous leaky metal roof with the torn and sagging R-19 fiberglass insulation.

Knowing the temperature of the purlin bottoms will be the same as the top (no R-value to steel), and the design temperature (97.5%) for the region in winter will be +2 deg F, I believe insulation needs to be added to the exposed purlin. Condensation will occur if the surface temperature is below the interior dew point, which could be 23 deg F (High plains desert, interior 70 deg F, 15% RH). The manufacturer's literature state that the air permeability of the material "at a
minimum thickness of 3.5”, is considered air-impermeable insulation
in accordance with section R806.4 of the IRC, based on testing in
accordance with ASTM E 283 and ASTM E 2178." 

I believe if they specify a minimum 3.5" of insulation (R-13.3) below the purlin, then the insulation will be "air-impermeable" and the chance of air reaching the purlin is reduced and the surface of the purlin should not drip condensation down on the new suspended ceiling. Water vapor transmission at 10" is 2.3 Perms (class 3 retarder) and the system should be able to dry to the interior. Does this group think this would be an effective strategy?

They are adding R-19 insulation on the 4 ft high bare block walls above the suspended ceiling as a change order to the roofing project. While the insulator is on site, it only made $ense to insulate the current R-2 wall.  Insulating beneath the purlins and the walls will create a continuous air/thermal barrier above the suspended ceiling. I have already suggested that they reseal the flex ducts to the ceiling registers and air handlers with mastic since the 'duct tape' as fallen off. Imagine that!! With new insulation, a sealed air distribution system and a functioning heat pump, their comfort level should increase and the energy usage should decrease. 

Any and all comments gladly welcome.

David 
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