Repair A New-Home Skylight, Found Fully Wrapped, But Only R2

Here is an as-found view of a new-home attic in Portland, Oregon. Standing atop the usual OSB rim about a hallway ceiling drywall-plunker-access, all is booby-trapped against usefulness. Pulled-up HVAC and fan ducts are in the way, obstructively kinked, everywhere. An adventure has begun, to make the huge attic useful. This post addresses repair of a ten-ft tall bathroom skylight with defective insulation, seen in this view. A full set of captioned photos of the process is consulted.

I imagine a City Of Portland building official inspected and approved the insulation in this home, relying upon 2009 International Energy Conservation Code in effect as the 2014 Oregon Energy Efficiency Specialty Code . See that such code is not accessible despite intentions; there is nothing at followed links. My 2015 IECC book at the Index, Skylights, calls out: R303.1.3 (U-values for glazing), R402.1.2 (skylight U-factor of 0.55, wood frame wall R20), R402.3 (words about fenestration, n/a walls),Table R405.5.2 (1)(standard reference design specifications, for skylights: None.). Defined: R202 (Glass or other transparent or translucent glazing material installed at a slope of less than 60 degrees from horizontal.)

An inspector might have looked for and found R20 at the skylight walls, but I will show that that is only wishful thinking. Find better requirements a Portland, Oregon insulation installer would have been required to follow, under better supervision of Specifications of Energy Trust of Oregon. These say:

skylight—Any window unit in an opening in the roof assembly, including one that is installed at a slope of 15 degrees from vertical or greater.

AT 1.13—Vertical Walls in Attic Spaces
Any vertical wall in an attic that separates conditioned space from unconditioned space shall be sealed for air leaks and  shall be insulated to fill the cavity. Insulation shall be secured and covered with a vapor-permeable air barrier. Vertical walls may include side walls of vaults, skylights, transitions in ceiling height or other surfaces. See AT 2.6 for program requirements for knee wall insulation.


AT 2.6:
Knee walls shall be sealed for air leaks and shall be insulated and covered with a vapor-permeable air barrier.
Rebate Rules:
R-15 for 2x4 cavities; R21 for 2x6 cavities; cover attic side with vapor permeable air barrier.

Where this skylight has some 2x6 boards, R21 might be commanded. It isn't clear.

I persist in challenging that attic walls may be as thick as one wishes, unlike exterior walls. Better rules will deal with confusions including so much on-flat framing at a skylight. Where attic walls are extremely challenged by Summer heat, they should be insulated much better than exterior walls. I insulate attic walls to modest R25 (with R30 batts 7.5" thickness and with lessened framing thermal shorts). The vapor-permeable air barrier specified, can't be simpler than a thorough covering of good 3/8" plywood. I offer well-practiced means of attaching the plywood covering.

R21 was not achieved here.
Here is  my math for the skylight with insulation merely draped over on-flat 2x4s of the trusses:
Say total inside area of skylight drywall is 100 sq ft. Skylight insulation is in contact with drywall only at end walls, 25 sq ft called R21. On side walls insulation has full effect only where draped over on-flat 2x4s, effectively R21 over 11 sq ft. Remaining area, 64 sq ft, has no insulation filling cavities, and at worst has no value.

100/(Reff + 3) = 25/24 + 11/24 + 64/3; Reff = 1.4 for the entire 100 sq ft area. 

My insulation math at 95% furnace efficiency and natural gas cost of $2 per therm, says heat cost per year is: $2.4*100*.88/.95*(1/(Reff + 3)).

The penalty with loose draping, or any draping, is large. Draped insulation must not be allowed.

Accepting that draped batts do resist heat convection, allow Reff = 7, for payback math, vs. better insulation to be demanded and to be achieved. I believe that is a sufficiently large number to avoid over-estimate of weatherization savings. As in the title of this post, accuse that better than Reff = 1.4, can not be demonstrated.  Energy savings at skylights is relatively easy, and comes with better safety too. An outside air barrier is required and is nicely provided in 3/8” plywood hard covering. 7/16" OSB saves some money, but scrap must be disposed as trash. In some homes the hard covering of a skylight prevents a deadly fall in an attic tumble against shaft drywall. Where one wants a useful attic and may not use a respirator, a nearly-complete covering of insulation is appreciated. Where my allocated cost of the skylight covered insulation is between $300 and $500, and savings are $15 or more, estimate an acceptable less-than-twenty years recovery of cost.

Mostly, be grateful for better summertime comfort, perhaps continuing to avoid the need and cost of air conditioning.

Pull the nice but useless Certainteed R21 kraft-faced batts from the skylight side walls. Build simple wall extensions of 3/8" plywood cuts 7 1/4" wide that efficiently consume 96" panel length, and stitch them to the 1 1/2" truss depth wherever they fit and align with 2x2 nailer positions, that will support hard siding of 3/8" plywood. In the truss rows, add 2x4's to increase depth for added end wall insulation. Ensure that the plywood cuts forming top and side constraints of the side wall insulation, are very nearly air tight.

Use new Certainteed R13 unfaced batts 15"x96", accurately cut, to cover skylight drywall. Stuff through the floor  gaps,  completing R60 insulation about the skylight below the floor that is up 21.6" from the attic drywall floor. Where the skylight peak is nearly 10 ft tall, be grateful for the elevation boost of the higher attic floor of 5/8" plywood. A stool for added reach is safely used upon a flat and complete attic floor.

The found R21 batts are stripped of their ripped and crumpled kraft facing, and nearly complete the R30 insulation fill, now easily cut to fit.

Use insulation scraps to fill in low spots of the R30 insulation.

The 3/8" plywood side panels were cut to fit, before the insulation was placed. See that R30 insulation is complete and pillows to greater than the 7.5" thickness defined by 2x2 nailers. See thorough already-completed R30 end wall insulation.

See progress cutting the next skylight face plywood. This end of the now-floored attic is a quite nice, brightly-lit wood shop, for now. I love my Milwaukee saw with laser light well  suited to my right-handedness. All sawdust and wood scrap will be recycled as bark mulch. Then don't use less-expensive OSB that must be disposed as trash.

Pre-fit the second skylight face, set with screws.

See 2x6 uprights tied into the truss planes at this end of the skylight. With 2x4 nailers now secured to the side walls, I may release end wall plywood facing to trim the 2x6.

Skylight finished now. The brightly-lighted attic is the best place to do brush-on paint coats of the R15 access cover.


















Where Energy Trust Of Oregon chooses to do nothing upon notice of this post, I have edited post Labels to include "Voids Under Attic Insulation."   Emphasize then that voids behind some insulation are more sure of being energy hogs, than voids under insulation. The energy dissipation is in convection, air flow past an energetic surface, at temperature different from ambient. Smooth skylight drywall is a superior energy convector of air in motion through gaps. Gaps are made worse where out-of-contact insulation has smooth or deformed facing. The situation is surely better than worst case R2 average, but may not be better than R7. There is no basis for treating insulation as fully effective. There clearly is not compliance with Energy Trust Specifications, in loosely-written new-home building code. Energy Trust calls for treatment of a skylght shaft, R15 insulation, same as for 2x4 exterior walls. New-home building code does not mention skylights. Look in the 2015 IECC code table below to see that a new home skylight in Zone 4 should have insulation of 2x6 construction. In fact, non-specific instructions, are ignored.