Tongue and Groove Ceilings – Dripping, Leaking and Condensation


Tongue and groove ceilings – they’re beautiful, but like many beautiful things, they can be problematic.

(Photo shows a thermal scan of a ceiling where cold air is leaking in through the gaps – those are the black streaks down the photo)

Unfortunately, T&G ceilings have become the single most discussed items on Ted’s Energy Tips due to their overwhelming tendency to be associated with water, mold, or moisture problems.

Does your ceiling drip water?

I’ve probably heard this a hundred times – “Help! Water is leaking from my ceiling! We had a cold snap and now it’s a beautiful day and now it’s raining in my house!”
The caller/writer then tells me that the roofer came out to check for leaks and couldn’t find anything. If they took it a step further, someone pulled off some of the ceiling planks, saw the real problem (condensation) and told them that they need to ventilate the ceilings. If they’re unlucky, they spent thousands of dollars, added ventilation, and the water problems became worse!

Why do ceilings drip when there’s no leak?

Boil a pot of water with a lid on it. Wait a minute then lift the lid. What do you see? Lots of water on the underside of the lid. There’s no ‘leak’ but yet the lid is covered with water due to condensation forming on it. This is exactly what’s happening inside your ceiling.
Under the right conditions, when water vapor in the air comes in contact with a surface cooler than the air, the water vapor becomes liquid water. If this happens enough, the water builds up and forms large drop of water. Those get large enough and, PLOP!, it starts raining in your house!

All too often, a T&G ceiling is constructed in a way that virtually guarantees water problems. The roof is installed over the rafters. Fiberglass is shoved between the rafters. T&G boards are nailed to the rafters. Bingo – you get mold, water “leaks”, and thousands of dollars of repairs which probably don’t even fix the problem.

Why don’t normal ceilings drip?

A typical ceiling is made of sheetrock or plaster, and all the seams are carefully filled in. That’s then painted, usually with several coats of paint.
While some tiny amount of water vapor can get through the paint and sheetrock, it’s a very small quantity. Small enough that, under normal conditions, natural air flow and moisture transfer removes the moisture that does get into the ceiling cavity above the sheetrock.

However, even these ceilings often have problems when holes are cut in them for recessed lights. This allows that air and moisture to move much more quickly into the ceiling where it can condense and lead to mold growth and wood rot.
The key fact – if air gets into your ceiling, even through small holes, it will carry water vapor with it which is very likely to lead to moisture problems.

Why are T&G ceilings so bad?

Water vapor is tiny. Really, really tiny. So tiny, that even the smallest crack is billions and billions of times larger than the water molecule. Tongue and groove ceilings may look ‘tight’ but to a water molecule, it’s like a wide open door! So that water vapor simply flows right up through those cracks and into the ceiling cavity above.

To Make matters worse, most ceilings are stuffed with fiberglass insulation, which doesn’t stop the water vapor at all. So the vapor keeps wafting up past the insulation until it reaches the underside of your roof. That roof deck is cold in the winter and on clear nights. When the vapor hits it, it condenses into liquid water. This is when the real problems start…

When it’s below freezing, the water freezes into ice. More water vapor enters, contacts the ice, and adds to the ice. Before you know it, the entire underside of the roof and anything else cold enough in the ceiling cavity, is covered with a layer of ice. This could go on for days, weeks or even months until the ice melts and it starts raining in your living room!

Even when it’s not below freezing, in most climates, it gets cold enough and there’s enough water vapor carried by the air for condensation to form inside the ceiling. The wood can absorb some of that moisture and slowly transfer it out. But with a T&G ceiling, the water supply inside the house is nearly infinite, so it keeps building up. Pretty soon, the wood is saturated and larger water droplets form. They’ll drip down, following gravity, until they find a place to leak out. That’s why the drips often form far away from where the condensation is actually occurring, which is often high up the ceiling.

Will ventilation solve the problem?

Builders are taught that roofs have to be ventilated in order to ‘flush out’ the moisture. The problem is, that doesn’t work with T&G ceilings. In fact, it often makes problems worse.

Why? As the air moves through the cavity, it has a tendency to draw more air from the inside of the house into the cavity. Some of that is flushed out with the air moving through the cavity. But when the water vapor carried by the air comes in contact with cold surfaces, it basically sticks. If there’s enough, which there usually is in a house during winter, then you still have the condensation problem.

How do you prevent water vapor from getting in the ceiling?

Take a step back. What ceilings work properly? We started discussing how a typical drywall ceiling usually doesn’t have moisture problems like these (unless you put holes in it). So the easiest solution is to build a normal drywall ceiling over which you install the T&G boards – purely for aesthetic purposes. Drywall is cheap. Replacing your roof is expensive.

What about plastic sheeting?

Drywall is cheap. Replacing your roof is expensive.
Some builders try to ‘cheap out’ by just putting plastic up, usually with thousands of staples, then nailing the T&G boards to the rafters, with thousands more nails.
Do you think a sheet of plastic with thousands of holes in it is going to stop the movement of water vapor, one of the tiniest molecules in nature, from getting into the ceiling? Maybe it will take 5 years to rot out the roof instead of 3. But eventually, you’re going to be spending many thousands of dollars for a new roof, just because the builder decided to take the cheap approach and save a little bit of drywall.

Ok, Ted, how would you build a T&G ceiling?

Lots of foam insulation. Works great!

Glad you asked. Here’s how I did it when we replaced our sunroom.
We used high density spray foam. The foam completely fills the rafter bays, as in the photo above. Any remaining cracks are caulked. Then we installed the T&G ceiling to the rafters, like in the photo below:

Adding the T&G ceiling. No vapor barrier required with closed cell polyurethane.

We’ve had this ceiling for more than 10 years now with zero issues. It’s on the kitchen which gets humid from cooking, but still, no problems.

Would you do it differently if you did it again?

Good question! I might add a thin sheet of foil faced polyiso sheet foam across the rafters before installing the T&G ceiling to add another layer of moisture barrier and to reduce thermal bridging through the wooden rafters. In extremely cold climates, this would be highly recommended as you could still get condensation on the exposed rafters because they transmit the cold much more than the insulation. But in eastern PA where I live, it’s not cold enough to warrant that extra work for the minimal gain given this construction.

I already installed my ceiling, can I seal the wood or the seams?

In a word – no. That might slow the process slightly, but eventually you’re going to regret the decision when your roof rots out and your ceiling is filled with mold. Shortcuts don’t work with T&G ceilings. You have to do them right, or you’ll pay the price eventually.

Can I install recessed lights in a T&G ceiling?

If you’ve read much on this, or any other energy efficiency and building science website, you’ll know that recessed lights are nightmares. Yes, they look clean and some are even rated ‘air tight’, but those ratings are BS. They are filled with holes which let ample moisture through. That moisture will go into your ceiling and cause the same issues.

The compromise I’ll make is if you do a full foam job like I did and then install low-profile LED lights that don’t require reducing the insulation. Since the T&G ceiling isn’t an air barrier anyway, cutting holes in it to mount these lights doesn’t matter. The low profile LED lights look just like normal recessed lights but they don’t protrude into the ceiling cavity. Here’s an example of a ‘canless’ recessed light at Home Depot.

9 thoughts on “Tongue and Groove Ceilings – Dripping, Leaking and Condensation

  1. Hi Ted,

    Last year, we had a T&G ceiling installed in a cupola that was added to a geodesic dome. It was installed right up against the rafters with fiberglass insulation. Very recently we’ve noticed two issues. Two areas of pine that were sealed to the drywall part of the ceiling with caulk have cracked (likely due to the pine shrinking). Almost immediately, water has been dripping out, and a small amount of mildew is developing.

    The other issue is the pine itself. Moisture is developing on the boards themselves that clearly looks like condensation and not a roof leak. The pine is mildewing very very quickly as a result. Within about 10 days nearly a third is covered.

    Based on my research before we had the T&G installed and after this incident, I feel the problem is that cupola is improperly sealed off.

    My contractor, however, believes it is an HVAC issue. We’ve had two companies come out (one specifically recommended by our contractor), and both have said our systems our fine and adequately cooling and drying the area. We also run three humidifiers 24/7 in hour home–we live in north central Florida. Our measurements indicate 45% humidity.

    I’m having an architect friend take a look later today, but they probably will not be able to provide any guidance.

    It is imperative we fix this issue immediately before irreparable damage is done to the roof, but I’m personally stumped as to how to proceed. I do NOT want to go the route of installing expensive ventilation when I’m 99% certain it’s not the cause.

    Should we just pull off the T&G and start over? Maybe skip the T&G all together and go drywall? If you do remote consults, I’d be happy to discuss the terms.

    • Yikes!
      First – you say you run three humidifiers – I’m guessing you really mean dehumidifiers since you wouldn’t want more humidity in Florida during the summer.

      45% humidity sounds great for Florida. My house is 49% right now and we never have moisture problems.

      For condensation to be forming, you need a surface that is cool enough. If you’re not getting condensation on your walls near the ceiling, you shouldn’t be getting it on the ceiling. However, it may be that the drywall is soaking in the moisture more so you don’t notice it. Still, if it’s so bad as to cause all this mold growth on the ceiling, I would think that you’d be seeing it on the walls.
      You are correct that you want to fix this ASAP.

      I’m very concerned about the water dripping out at the crack. The ceiling cavity must be a mess.

      With 45% relative humidity in the house, condensation requires a much cooler surface than room temperature. Are you in a part of Florida with wide day-night temperature swings? I usually only hear about that in the desert climates. The only way I can think of to get conditions that cause condensation in Florida would be if the air conditioner vents were blowing onto the ceiling, cooling it 15-20 degrees. You can also get it if the ductwork is running in the cavity and lots of cold air is coming out, chilling the ceiling.

      There are a few things I would do if this were my home.
      1 – check the easy thing – air conditioning vents. Are they chilling the ceiling and causing the condensation?
      2 – check behind the area where water is dripping out. You say it’s at a junction between drywall and drywall. Carefully cutting out a section, maybe 1-2 feet square, would let you poke around in the ceiling cavity and determine if it’s soaking wet in there. If it is, you need to dry it out ASAP or you’re going to experience a lot of wood rot and expensive repairs.
      3 – until you determine the problem, you can blow a fan or fans at the ceiling where the moisture is building up the most. This will help dry out the condensation without cooling it off like blowing air conditioned air would. This could buy you some time.

      I hope this helps. You’re absolutely right – you really need to get to the bottom of this soon.

  2. Ted, I have spray foam insulation and T&G ceiling. But I still fight condensation especially in the hot summer months. My home is less than two years old. Last summer the the T&G started buckling and we fixed those boards. Now we are seeing bucking return and drips coming from the ceiling again. My ceiling at the highest peak is 23 feet. Should we consider exhaust fans between the ceiling and roof?

    • Summer condensation is a different beast from winter condensation though of course the physics is the same – humidity in air coming in contact with a cooler surface.
      During the summer, warm air can contain a tremendous amount of moisture. That warm moist air rises up to the ceiling. Probably, at night time, the roof cools off enough to allow the condensation to form on the ceiling.
      Are you opening the house up at any time during the day to air it out? Sometimes what happens is people will do this when the air is cool in the morning or in the evening and that allows saturated air to fill the house which leads to condensation problems. If you are doing that, I would suggest not opening the house up and using the air conditioner because that will keep the moisture lower in the house and minimize the chance of condensation.

      I had one client who had terrible problems with floors warping throughout the year. They never opened their house and ran the air conditioner during the summer, but the humidity inside the house remained high in the summer and low in the winter. I traced this to a disconnected duct connection in the garage, where the air conditioner was mounted. The system sucked in air from the garage, essentially just outdoor air, and distributed that around the house leading to all the problems. In addition to moisture problems, it drove up their electric bill some 30%! So it’s possible that your ductwork is very leaky, leading to these problems.

      If you don’t think your problem is due to opening your house for fresh air, then I strongly recommend having your duct system pressure tested. This will determine how leaky your ducts are and allow you to attack the actual source of the condensation problem.

  3. Ted, thanks for your excellent articles and Q&A, including tedsenergytips.com/2011/03/13 Cathedral Ceilings.
    I am installing everything above the beams, prioritizing flame retardant Rockwool insulation, and location is CZ2 Novato CA, with lows rarely below 40 degrees F.

    As an alternative to foil faced foam or sheetrock with taped joints as a vapor barrier just above the T&G ceiling, you ever consider impermeable peel-and-stick membranes, directly over the T&G, and why or why not?
    What would you change about the setup below?

    From the top down….
    -2:12 single pitch standing seam metal roof, with minimal venting at top and bottom eaves;
    -Toprock DD (rockwool) double layer of 4″ panels, staggered, ~ R30;
    -Peel-and-stick impermeable membrane – adhered to top of T&G ceiling – but at exterior walls near top and bottom eaves, interrupts T&G by diving through the T&G and lapping to the Tyvek home wrap.
    -2×6 T&G ceiling
    -6×10 beams fully exposed, 4′ o.c.

    Would it improve or worsen the risk of condensation inside the roof, to include an air gap under the roofing, and allow venting at the top and bottom eaves?

    Thanks, Brian

    • Hi Brian, thanks for the words of encouragement.
      My visualization of what you describe is like an Escher! The Peel and stick adhered to the T&G? Then lapping to the Tyvek? I don’t get how that happens. Sorry if I’m being thick, it’s probably obvious but I just can’t visualize this.
      What I could see is the peel and stick facing the insulation and rafters, with the T&G nailed through the membrane to the rafters. That would certainly be highly moisture blocking as long as the seams were carefully sealed. I would think the foil-faced foam board would be easier to work with and to seal as well as less costly per square foot, so I’m not sure of any advantage that the membrane would have. But again, since I’m not really visualizing your proposal properly, I’m probably missing what you’re thinking of.
      Oh, and the air gap under the roof would help things as it would help flush out any moisture that invariably works its way into the sealed cavity.

      • Ted, I appreciate your response.

        I borrowed the membrane idea from another builder who did include a picture: https://tinyurl.com/y22k2sb4

        I’m more confident now about venting this type of assembly; I’ll incorporate bug screen and allow air flow under the metal roof with a 3d mesh underlayment, or wood strips.

        Thanks,
        Brian

  4. We built our house back in the early 80s tongue and groove on the ceiling but we sheet rocked first mud and tape to see you all air gaps and aid in fire resistance.
    That’s a no brainer especially today in these times in the ages of information and education. If that was a contractors build he should be demoted down to building dog houses and bird houses.

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