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Insulating a skylight shaft

I was in a house with a skylight shaft that was insulated with standard wall batts on the attic side. The insulation is exposed. I would say the effective value was slightly above nil.

What would be an effective strategy for insulating the shaft? The shaft is about 6' in height and 4 foot to a side. As a retrofit project we are talking about moving materials to the 2nd floor of the house and through a scuttle in a walk-in closet.

What R level would you shoot for and how would you do it? Again it will be quite a chore to get materials to the workspace.

Asked by Robert Hronek
Posted Oct 29, 2010 8:51 PM ET


5 Answers

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Here are a couple of options you might consider.

1. Remove batts. Install a swath of mesh netting at the vertical midpoint of the wall. 2" Polyiso from drywall below to roof sheathing above, excluding a foot or so at the mesh netting. Use single-part foam to air-seal critical junctions as you're installing the foam (plates), but don't seal joints in foam yet. Dense-pack cellulose through the mesh netting and install missing swath of polyiso. Seal the perimeter of the foam board with single part foam. Assuming the skylight shaft is 2x4, the effective R value will be >20. If you're looking for more, you could always increase the cellulose thickness by installing a 2" horizontal strapping.

2. Quick and dirty patching of batts and sealing of critical joints. Fan-fold XPS tacked over the back of batts and sealed at joints. Fan-fold is a pretty popular choice with weatherization folks owing to the access problems inherent to the profession. The trade-off is that it's a much lower performing detail.

Answered by Jesse Smith
Posted Oct 29, 2010 10:02 PM ET


I would simply wrap the shaft in XPS to match or exceed the R-value of the rest of the ceiling. If 2' x 8' foam boards won't fit through the access hatch, then it can be precut and brought up in pieces. Tape the joints and use gun foam where the XPS meets the ceiling and roof.

Answered by Riversong
Posted Oct 29, 2010 10:18 PM ET


We are trying to expand our business into energy efficiency. We want to do things the right way and conceptually I understand what needs to be done.

The house is about 20 years old with 2x 6 cieling joists. In places fiberglass is 8 inches other places the top of the rafters are visible. I am trying to sell him on air sealing the tops plates, etc and to beef up insualtion with at least an additional R 25. I would like to see him in the range of R 50-60. Would be using cellulose over the existing fiberglass.

Back to the skylight shaft. Price wise I think is could be a hard sell to get him to R50. Would be quite thick and expensive. If I could get it to the mid 20's would that be enough?

I would rather sell him on letting us rake back the blown fiberglass and using can foam on the top plates, light fixtures, etc.. To me he would get more benefit than getting the skylight to R50

Answered by Robert Hronek
Posted Nov 4, 2010 12:57 PM ET


You don't provide your location or climate zone, so it's impossible to know what the code requirements for wall insulation are in your area.

For all intents and purposes, a skylight shaft is a wall. In climate zones 5 through 8, the 2006 IRC requires R-20 or R-21 insulation. So that should be your minimum goal if you are in one of those climate zones.

Exceeding the code is always a good idea, however, especially if you are performing energy retrofit work. The energy penalty attributable to a poorly insulated skylight shaft is likely to be more severe than that attributable to a poorly insulated first-floor wall.

Answered by Martin Holladay
Posted Nov 4, 2010 1:04 PM ET


Located in Nebraska. Climate zone 5.

In the original post I noted that is was standard batt insulation for 2x4 wall. Basically air is able to circulate on all sides as noting holds it tight to the back of the drywall. What is the R value, in my opinion not much. So going from R 3 (guess) to R 20+ is a major improvement. The much larger area is the ceiling. Loose cellulose with an R18 to R 24 range. OF course the effective R is lower.

Answered by Robert Hronek
Posted Nov 4, 2010 3:18 PM ET

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