Best worst-case-scenario attic room fix
Hi!
This forum is an exceptional wealth of knowledge. Thank you!
Two years ago we bought a house built in 1922 with a converted attic space making it one and a half stories. The house, located in Seattle (4c), is essentially a giant rectangle, open gable roof. The attic space has a climate zone of its own that we have been mitigating with fans and dehumidifier. There are four can lights. At some point we noticed water condensing in the can lights and bubbles under the paint in the area of the lights. Ugh.
This weekend, we cut out a section of drywall next to the can light exposing the excess moisture from the ridge to about a foot/foot and a half down. It also became evident there are no baffles, only fiberglass insulation and the ridge vent seen from the outside of the new roof (3-4 years) had no vent along the ridge – in fact, it was covered with shingles with the ridge vent and more shingles on top. We removed this decorative ridge vent, cut an actual vent in the sheathing, and reapplied the ridge vent properly. That night, the dehumidifier actually turned off on its own for the first time in months.
I’ve been reading extensively and trying to determine a best case retrofit scenario:
1) Remove can lights and seal.
2) Cover and seal the fiberglass insulation loosely laid in the kneewall space with rigid foam insulation to prevent air flow from the first story into the kneewall cavity and also open up the soffit vents currently covered with fiberglass insulation.
3) Cover and seal the fiberglass insulation along the kneewall to prevent air flow from half story room into kneewall cavity.
Then there is option step four which I am not sure which is best:
a) Retrofit baffles constructed from cardboard and furring strips over the fiberglass insulation;
b) Retrofit baffles constructed from thin rigid foam insulation over the fiberglass insulation;
c) remove fiberglass insulation between sheathing and sheetrock and use cut and cobble method to snug fit with rigid foam insulation between rafters leaving a 1-1/2 inch gap at top for air flow.
Or would you go an entirely different direction?
PS: I have access to quite a bit of R6 rigid foam insulation for cheap or nothing as well as more time than money.
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It sounds like you are on the right track. If you give us your location and climate zone, it will help people here give more appropriate advice.
Thank you for the feedback, I will amend my original post.
For the cathedralized ceiling approach to really work well (whether cut'n' cobbled foam or cardboard & fiberglass) there needs to be soffit to ridge venting, not just ridge vent. A ridge vent alone depressurizes the attic, drawing air from conditioned space. Ideally the free area of the soffit vents will be something like 1.5x more than the ridge vent area, (60% soffit area / 40% ridge area ) which will limit the pressure difference between the vent channels & kneewall attic spaces and the conditioned space.
https://www.greenbuildingadvisor.com/article/lstibureks-rules-for-venting-roofs
https://www.greenbuildingadvisor.com/article/all-about-attic-venting
https://www.buildingscience.com/sites/default/files/migrate/pdf/PA_Crash_Course_Roof_Venting_FHB.pdf
Hi HolleeCee ,
Condensation near the ridge of cathedral ceilings is a phenomenon that has been covered in this article: https://www.greenbuildingadvisor.com/article/summertime-condensation-near-peak-cathedral-ceiling
Of course the first step is to make sure that it is not actually a roof leak. Assuming that is not the case, there are a few possible solutions that the article covers.
However, it seems that you have a lot of work to do to get your space properly air sealed and insulated, whether you retrofit a vented roof or choose to go with an unvented assembly. Here are two other articles that will be helpful. Once you've had a chance to read them, please let us know if you have any other questions.
https://www.greenbuildingadvisor.com/article/insulating-a-cape-cod-house
https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling
Thanks Dana and Brian - I'd read all the other articles but the Cape Cod one is new to me and quite informative.
Part of covering over/sealing the fiberglass insulation on the floor of the knee wall cavity was to also define the thermal boundary and expose the soffit vents so air can flow up the baffles to the ridge vent. Currently, someone just shoved as much pink insulation as they could in the knee wall space and called it good.
Based on the calculation of 60/40 if I have a continuous soffit vent and continuous ridge vent, would you adjust this by only baffling 40 percent of the rafter spaces then?
I've attached a picture of the house to show that my roof line does extend past the walls of the home.
Oh and the condensation is only in the area around the can lights. After reading the advice to remove them and run a dump truck over them, I concur.
>"Based on the calculation of 60/40 if I have a continuous soffit vent and continuous ridge vent, would you adjust this by only baffling 40 percent of the rafter spaces then?"
Nope.
Figure out the free area of the existing ridge vent, and add soffit vent area that that is 50% bigger. Throttling it in the middle along the rafter bays isn't going to do the right thing.
The soffit vent free-area can be more than 50% bigger than the ridge vent area, or somewhat less than 50% bigger. But in the ideal case it will at least be bigger, even if not by much.
Thanks! That makes more sense.
I've been reading and reading and reading, one article leads to another... and another questions comes to mind.
From what I have gleaned air leakage is the most likely cause of moisture transfer. Taped drywall is an air barrier. Lath and plaster is not.
With this information:
Is it reasonable to consider extending the XPS foam board and baffle from the ceiling cavity beyond the knee wall to the top of the lath and plaster exterior wall cavity, thus enclosing the knee wall cavity into the envelope of the house, if the floor of the knee wall cavity is essentially lath and plaster ceiling with a plank "walkway" lying atop the joists? The air from the soffit vents would be flowing directly into the baffle to the ridge vent.