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My cathedral “F” up…. Now what? Retro foam? Airtight cans?

changetherate | Posted in Green Building Techniques on

I will try to not bore you all with the details behind the insulation choices that were made for our 1935 Cape Cod’s Unvented Cathedral ceiling. The house is in DC so we have hot/humid summers and cold winters. So of course we thought we were going above and beyond and it turns out after reading article after article is that we made the ultimate cathedral ceiling sin.

What it had before we “upgraded”
The house had R10-R15 fiberglass from the 50’s or 60’s with a particle board interior. Very, very hot upstairs in the summer, hence wanting an affordable upgrade (could not afford spray foam).

What we installed:
R21C fiberglass (specific to the 5.5 inches of available space) between the rafters
RMax 2 inch Rigid Foam board R13.1 over the rafters for more R value but also vapor barrier.
3/4 Inch Drywall over Rigid Foam.
Paint over drywall.

The Cardinal Sin:
Installing Twelve 4 inch LED Recessed cans into the beautiful new ceiling, which now all drip from condensation (assuming from the mixing of hot attic air with very cold AC coming out of mini split)

Now what to do?
Replace all of the recessed lights with air tight cans seems a good start… but should I pull out all the damp fiberglass (as much as I can reach) and then sprayfoam behind all of the recessed lights? (They will be IC rated and air tight)… or should I just repack with fiberglass or rockwool?

Open to other ideas as well, including if this would be a job for retrofitting spray foam behind it (if I can afford it)

Keep in mind the house was built with no roof venting or soffit vents for any airflow.

I really appreciate all of the great advice you guys give and am very eager to hear options of fixing this blunder. Also is there any way to dry out the damp fiberglass or to just let it air out over time?

Thanks in advance.

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  1. GBA Editor
    Martin Holladay | | #1

    If your recessed cans are dripping while your air conditioner is running, it means that hot outdoor air is leaking into your rafter bays. When this hot outdoor air contacts the relatively cold recessed cans, condensation results. (Although you wrote that "the house was built with no roof venting or soffit vents for any airflow," trust me -- there is airflow due to unsealed cracks. In fact, there is enough air flow to cause condensation on your recessed cans.)

    The best solution would involve air sealing work. You need to figure out how outdoor air is entering your rafter bays. A good guess would be via the soffit area. It's also likely that air is leaving (or entering) via the ridge, where there may be accidental leaks or even a ridge vent.

    Before you can tackle the challenges ahead, you need to answer some very basic questions, including (1) How big is your budget? and (2) Do you want a quick fix or a robust solution?

    As you might have guessed, your roof assembly is leaky and poorly insulated. A real fix will be expensive. Even if you seal up the air leaks that are probably there -- leaks at the soffit and ridge -- you'll still have an unvented roof assembly insulated with the wrong kind of insulation, and punctuated by air leaks in the ceiling (the recessed cans).

    To seal air leaks at the soffit, it's often possible to work from outdoors. Disassemble the soffit and see what's going on at the base of your rafter bays.

    To seal air leaks at the ridge, climb up there are remove the ridge cap or cap shingles.

    None of this makes sense, however, unless you plan to install a sufficiently thick layer of rigid foam on the exterior side of your roof sheathing.

  2. GBA Editor
    Martin Holladay | | #2

    Instead of converting your failing roof assembly to an unvented insulated roof assembly, as I suggested in my first response, it might also be possible to convert your failing roof assembly to a vented insulated roof assembly -- but that would only be possible if you are willing to remove all of your ceilings and all of the existing fiberglass insulation.

    If you go this route, you would start by removing everything, exposing the roof sheathing from the interior. Your next step would be to install airtight ventilation baffles near the top of each rafter bay. I can't stress enough that airtight methods are required here -- that is, if you want to avoid the type of condensation you are now facing. (To read more about installing these ventilation baffles, see this article: "Site-Built Ventilation Baffles for Roofs.")

    After the ventilation baffles are installed, you could insulate each rafter bay with fiberglass. Needless to say, you would probably want to choose a different lighting method -- one that did not involve recessed can lights.

    For more information, see "How to Build an Insulated Cathedral Ceiling."

  3. GBA Editor
    Martin Holladay | | #3

    If you can't afford a robust solution to your problem, you could simply remove all of your recessed can lights, and do your best to patch all of the ceiling holes. At that point, you wouldn't have any dripping cans. But you would still have rafter bays that are extremely leaky, with exterior air flowing between the rafters, undermining the R-value of your roof insulation.

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