Condensation Concern for Skylight in Scissor Truss
Cliffhaven_Rob
| Posted in General Questions on
We are in the design phase for a small coastal cottage in Downeast Maine climate zone 6. We are intending to build tight (<=1 ACH50, 2×6 walls with dense-packed cellulose with Zip sheathing as the primary air barrier and 3″ rockwool exterior continuous insulation with a rainscreen). The cottage is about 850 sq. ft. on one level and will be occupied by two people. There will be a great room with a vaulted ceiling (26 ft. x 16ft.) under a scissor truss with a 7:12 external and 5:12 internal pitch and with a 12″ energy heel. The remainder of the floor area will have an 8′ ceiling and conventional vented attic. We intend to have a vented roof in the scissor truss area insulated with blown-in cellulose to R60 (about 17″). To control humidity and IAQ, we intend to use a Lunos E2 HRV pair with one vent in the great room and one at the other end of the house in the flat ceiling area. The floorplan is mostly open. We also intend to have a Panasonic Whisper Green exhaust fan to control humidity in the bathroom and a vent hood fan (~250 cfm) to exhaust kitchen odors intermittently. The lot is heavily treed and we are in the shoreland zone where tree cutting is quite restricted. We would like to have two skylights, one on either side of the ridge, in the vaulted ceiling to provide additional natural lighting.
I have designed the position of the skylights so that they are far enough up the roof that a ventilation channel should be open around the skylight box. This should hopefully allow for attic air circulation in the truss bay containing the skylights.
My questions:
1) What happens on the inside of the skylight box up near the skylight itself when it gets very cold in the attic? This looks like an area that will likely have heavy condensation inside from any humidity left in the inside air because the skylight box is just separated from the vented attic by drywall above the cellulose.
2) Should the skylight box be insulated in the attic right to the roof deck with EPS or closed cell foam or something else?
3) Is this just a terrible idea and we should drop the idea of skylights?
4) Am I just being paranoid?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Rob, skylight shafts should be insulated like any exterior wall.
I have been in a lot of New England houses and have never seen a conventional skylight that did not show signs of condensation. It's not always a problem, i.e. resulting in mold growth or paint damage, but it can be a problem. It's best to use a skylight designed to be energy efficient and with condensation resistance built in, such as these: https://foursevenfive.com/products/fenestration/fakro-pitched-roof-skylights/.
You definitely need to plan ahead for insulation of the skylight shaft walls. You won't have much access to these after the ceiling drywall is up, and pretty much no way to verify that they were done right. Also, getting enough cellulose in the area behind the shafts can be a challenge with low attic access. Make a plan for all of this before the drywall goes up.
Thanks Peter,
I totally agree. I have read more times than I can count here on GBA about planning the insulation details fully before starting construction and that is what I am trying to do.
Thanks Michael,
I didn't mention it but these would be fixed skylights and I was initially looking at a Velux fixed skylight with 9994 glass which has U=0.38. Slightly less than the Fakro skylights you pointed me to (=0.41). Do you know of other features of the Fakro skylights that would make them a better choice than the Velux?
Also, can you, or anyone else comment on the advantages/problems with moving the skylight lower on the roof to ensure that it sits fully within the cellulose insulation? I'm thinking this might be a good idea to keep the skylight box as warm as possible. The tradeoff would be that I would lose the attic venting between the eave and the skylight box in that roof bay. I understand it's a tradeoff and would welcome any comments on other's experiences.
I have installed many Velux skylights here in MN with success. Peter makes a good point about getting the skylight shaft insulated before the drywall. This is where I like to use Thermax or equivalent to wrap the skylight shaft and rafter framing. Also I like to counter flash the skylight with ice and water shield when installing the roofing. This provides an airtight seal around the skylight. I have found the vent flap version is not needed. I do however install the operable skylights for summer ventilation options.
There is an energy penalty for using skylights but so is there for each window and door opening in the house. I am big on natural light in my building projects and will use skylights where a window can't be used.
Thanks Doug,
I'll look at Thermax so I'll be prepared to discuss this with my GC when the time comes.
If it is useful, here are the basics of insulating skylight shaft walls:
Building America Solution Center-Skylight Shaft Walls
https://basc.pnnl.gov/resource-guides/skylight-shaft-walls
Hi Kohta,
Thanks for this excellent reference! In all of the reading I have been doing I have not come across this yet. Fortunately my GC is very good, I am just trying to learn as much as I can so I can provide meaningful feedback on his proposals when they come up for discussion.