Insulation around skylight tunnel

sbkim

I am curious what everyone recommends for insulating skylight tunnel. Currently there is 2x4 frame and the builder is proposing R21 batt but not sure how it will stay in place. is it advisable to use small R 15 and perhaps put foam board around the exterior frame?

Thanks

gregzoll

Really need pictures. If it is in the attic space, there is really nothing you can do, other than build a box around it and insulate said box. Then the top would still conduct temp diff.

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Gary in WA

Your location is helpful as well.....

Gary

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Windows on Wash

Rigid foam is better to the extent that it is not subject to wind washing but a combination of both will work as would a properly installed batt with some sort of air barrier on it.

+1 to the other observations too!

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sbkim

Thank you and my apologies for not making myself clearer. I am located in Chicago so I am not sure what R value is required for "knee wall" areas such as this. The skylight is located in unconditioned attic space. This isn't actual picture of our house but very close to what we have:

sbkim

One issue I see is 2x4 construction with the tunnel. Seems like there won't be enough room for both rigid foam... As mentioned previously, I think my installer was going to just put R21 with some insulation just sticking out. What sort of air barrier would you guys recommend? Plywood sheathing, Tyvek?

Windows on Wash

Insulated between the studs with unfaced R13 batts (or better yet something like Roxul because it won't sluff or fall down over time.

Install rigid foam over the studs to thermally uncouple them from the attic (minimum layer or 1" for complete thermal break). I would shoot for 2" as they get hot that far up.

Make sure the foam is covered or code compliant to be exposed.

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Gary in WA

If you go with the R-21 batt and no foamboard (taped and air-sealed), use a asphalt-coated paper faced batt required for your Zone 5: http://publicecodes.citation.com/ico...001_par001.htm

R-38 is required in ceilings there; http://publicecodes.citation.com/ico..._11_sec002.htm
The skylight shaft is similar to a ceiling as the rising warm air collects there (add to that the solar gain...). Best to insulate the side walls well to cover any major heat loss (more than required wall insulation). ADA the drywall also: http://www.buildingscience.com/docum...wall-approach/

With the low R-value the skylight itself provides, you need all the help you can get.

Gary

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sbkim

Hi Gary,

Thank you so much for the additional information! I will definitely push for as much of ADA as possible. A few questions:

1. I am still a bit confused about how much R value is really required under the code for tunnel or knee wall - is it R38 or R21 for zone 5 (Chicago)?

2. I would love to see R38 on the tunnel but just not sure how feasible that would be. I can ask them to add another layer of 2x6 frame to allow total of 10" to allow R38 sticking out 2".

3. I am most curious whether using thicker batt which sticks out is better than using thinner lower R value batt with some sheathing.

4. He is using Kraft backed batt - is this the same as asphalt-coated? or do I need to specifically request?

Sorry for so many questions and thanks for everyone's help!

Windows on Wash

The shaft is considered wall (more like kneewall) as compared to attic floor by most inspectors.

Use rigid foam. It works far better and is not susceptible to wind wash and other issues that fiberglass suffers from.

The money spent in additional framing will easily be more expensive than just using foam board.

Kraft paper is more proper in this case as compared to a PSK fiberglass.

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Gary in WA

It is a side wall, so the minimum insulation for walls is required (my point was the great heat loss in the shaft/hole in the ceiling-plane), with a vapor retarder. Code also accepts foam board exterior of the cavity insulation in which case the vapor retarder is omitted. The dew-point of the cavity changes because the foam board keeps the wall warmer.

Use the faced as you said (without the foam board), though a high density batt is much better at stopping convective loops within low density batt;
"R-13 HD at 1.0# density
R-19 at 0.55# density
R-21 HD at 0.90# density
R-30 at 0.57# density
R-38 at 0.53# density

All the ones (low density) listed without a HD (High Density) will have inherent convective loops in a wall or attic floor installation.
Taken from my info collected and posted here: The "biggest loser" in fiberglass insulation....

The foam board is much better as it stops air movement degrading f.g. batts; http://www.aecb.net/PDFs/Impact_of_thermal_bypass.pdf

The paper facing should be asphalt coated to meet the vapor retarder (Class 2) requirements per code:http://publicecodes.citation.com/ico...001_par003.htm

http://publicecodes.citation.com/ico...001_par005.htm

OR, add R-5 foam board (that stops convective looping) with 2x4 framing:http://publicecodes.citation.com/ico...001_par004.htm

The dew-point changes: http://www.greenbuildingadvisor.com/...ally-necessary

The science behind it: http://www.buildingscience.com/docum...apor-retarders

If no f.b. and only Kraft-paper faced, one type has the coating and one doesn't (paper alone won't stop moisture). Use the coated one as it has variable permeability: http://www.buildingscience.com/docum...vapor-barriers

Without the f.b., at least add a housewrap to the shaft on the attic side, over the f.g.- like Tyvek or similar permeability to let the moisture out from any air gaps in the ADA or regular diffusion (minimal).

Gary

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sbkim

Thank you! Great info but I am still dazed by all of the info... If you don't mind can I confirm a few takeaways from this post?

1. If I decide to use foam board, do I not use unfaced batt of R13 for 2x4 wall?

2. R13 + R5 = R18 (foam board) < R21 ... I guess eliminating air movement is more important than just R value for this application? I just want to ensure no condensation along the shaft and to keep the cold/heat out as much as possible.

3. If I decide to go with coated Kraft paper batt (R21) which sticks out a bit, can I still wrap it with Tyvek? I feel as though I might be "squeezing" the insulation while wrapping/taping...

Thank you

Windows on Wash

If you go with foam, used unfaced fiberglass.

If you are worried about condensation, foam board is the only way to go. It breaks up the thermal bridging of the studs and keeps them much closer to room temp and therefore away from dew point.

Skip the R21 batt idea and use the foam. Just check the codes for exposed foams and once you have one layer on it, I would just as soon double it up.

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sbkim

Got it - thanks!

Would something like Thermax 2" work?

http://building.dow.com/na/en/produc...xsheathing.htm

Windows on Wash

Very much so.

Put the foil facing to outside and realize that the foil facing is a class I vapor retarder so make sure the drywall is air tight and there are no leaks where moisture can get into that space.

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