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Ted, we live 1 block from the beach in Southern California, house finished in June 2021, one story. Cathedral ceilings with a 1.5:12 roof slope. Single slope roof, high wall side is approx. 16 feet, low side is 11 feet. Asphalt shingles, no truss venting, no eave venting, simply not ventilated, cedar t+g ceilings with R-30 batt insulation inside. Getting some condensation drip marks on the high side wall drywall surface in 4 areas. High side corners of master bedroom and both sides of the insert fireplace in the living room. Have a roof deck built over the cathedral roof trusses in the back of house all sealed tight with stucco on the sides and watertight Dex-O-Seal on the top surface. A small attic space holding 2 heaters partially 50% under the roof deck/roof trusses in the master bathroom and the other 50% under the roof truss assembly in the laundry, powder bath, master closet. I was thinking the best solution is a TPO membrane “hot unvented roof” over PolyIso at R-36 with an adhered moisture barrier first directly on top the roof decking. Leave the R-30 batt insulation in the ceilings, which is likely not tight to the roof decking so has lots of air gaps, we have Nora Lighting Iolite LEDs in the cathedral ceilings (don’t think these are airtight). What do you recommend. Want to avoid mold, drywall damage, condensation issues and/or rotting out my trusses and roof prematurely. Thank you, Henri
Hi Henri,
Thanks for such a detailed description, it really helps. Your instinct is a good one: rigid insulation above the deck is the right way to fix an unvented low-slope roof. R-36 of polyiso over your R-30 batts would keep the roof sheathing warm enough that it shouldn’t condense. Before you commit, though, I’d want to be sure we’re chasing the right problem.
Is it condensation or a leak? Asphalt shingles at 1.5:12 are below the 2:12 minimum that shingle makers and the code allow, and low-slope shingles tend to leak where the roof meets a high wall and around a chimney or flue. Those are exactly your four spots. One simple test is to note when the marks show up. After rain points to a leak. After cool, clear, damp nights with no rain points to condensation. The high side also fits condensation, because warm, moist indoor air slips through the joints in the cedar boards and around the can lights, rises to the top of each rafter bay, and hits cold sheathing there. Near the ocean, a clear night can chill a roof below the outdoor air temperature.
You can look inside without much damage: pull a can light near one of the problem spots and look up the cavity with a phone camera. Dark staining or wet wood on the underside of the sheathing tells you a lot. A cheap indoor hygrometer is worth having too. If indoor humidity is above about 50% in the cooler months, run the bath and kitchen fans more.
If you go ahead with the new roof, here’s what I’d check:
When the shingles come off, look for wet or soft sheathing, and let it dry or replace it before you seal it in. With the membrane and foil-faced polyiso on top, that deck can only dry downward.
Make sure there’s no plastic sheet behind the cedar ceiling. Plastic on both sides would trap the deck.
R-36 is generous. The code minimum above the deck in your climate zone is only about R-5, but with the marine air I’d stay above R-15 or so. With R-36 over R-30, the sheathing stays a little warmer than halfway between indoor and outdoor temperatures, comfortably above the indoor dew point. Thinner foam might make the edge, high-wall and roof deck flashing easier.
The area under your roof deck won’t get the new insulation, so it stays in the risky situation it’s in now. Air sealing the ceiling below it is the main defense there.
If the attic with the two furnaces ends up inside the sealed roof, check that they’re sealed-combustion units or that they keep a proper combustion air supply.
Air seal the can lights either way. Some recessed LEDs are rated airtight, but the gap between the trim and the wood boards can still leak, so a gasket or an airtight-rated replacement helps.
Since the house was finished in 2021, it may also be worth asking the builder about the shingles at that slope.
I hope this helps, and I’d love to hear what you find when you look inside.
Ted
I have a tongue and groove ceiling in a log cabin that is built exactly as you described in the article. Rigid foam insulation jammed in between with no air gap. I have an ice and water shield on the plywood deck and metal roofing on top of that. This is the first summer. I have had this constructed this way as we replaced the roof last summer. Starting to have moisture problems as of the other day and heavy dripping. What is the best way to fix this without removing the roof?
What’s the weather and temperature been like recently? Day and night. Where I am, this time of year is tough. Lots of things are still holding cold, then the morning hits and it might be much warmer and foggy. That immediately condenses on the cold surfaces and they sweat. Dripping wet. Is this the kind of thing that you might be experiencing?
it’s been the standard Pacific Northwest weather ha ha which means 75 in the day and sometimes down to 45 at night. But the day it started dripping was actually the first really warm day. I also own a roofing company and have seen this happen when moisture builds up in a space and when it gets really warm, it tries to evaporate, but instead comes back down. My main concern is I just bought this house and put a 50 year metal roof on it, now I feel like it’s going to rot from the inside out.
Well, I tried to reply but it seems like it deleted it. The conditions you describe sound like the same ones. When it gets warm, the air holds a lot more moisture and that will easily condense on any cool surface and then come dripping down. It sounds like the air is getting underneath the metal roof, which means it can also get out and evaporate. My main concern is whether it be dripping down into other building materials and soaking in there as liquid water and building up and causing rot in those materials.
Ted,
Happy Spring finally, hope you’re well! Along the back of my house my floor joists cantilever 2′ out from the basement wall. Currently I have the cantilever blocked off and air-sealed from the interior basement. Within the cantilevered bays, I have 2″ rigid foam sealed up against the underside of the floor, then R-30 fiberglass insulation, then plywood on the bottom of the floor joists, enclosing the bays. I thought I had it insulated pretty well, but I noticed a discernible difference in the temperate of the floor over the basement as compared to over the cantilever this winter. Would it be better to use rigid foam on the bottom of the floor joist cantilever, to separate from the outside, and remove the blocking to open the bays to the interior of the basement, so heat from the basement could flow into the cantilever, helping to keep the floor warm from underneath? Or will it be colder regardless of what I do, simply due to the nature of it being a cantilever?
It was a long, cold winter!
The short reply is put insulation right up tight against the subfloor.
I have a 5’ overhang like that and had them try to blow in insulation when they were doing the walls. That was a failure is it all fell to the lower surface where it was useless! So now we have cold feet all winter. If I was able, I’d redo it.
You could use a couple inches of XPS then pack in fiberglass under it. Don’t bother opening the bays to the warmer sections, that’ll just waste the energy.
Whatever you do, do it tight because you’ll be fighting gravity and any air gap will mean cold floors again.
Cheers