Attic insulation problems and solutions – Part 2

Fiberglass insulation against roof deck plus moisture = roof failure

In the first post, we looked at how adding insulation could lead to frozen pipes if the insulation was put in the wrong location. But, just like a sweater, if you put everything you want to be warm inside the insulation, you can keep your house and pipes happy and energy efficient.

This time, we’re looking at how to install insulation properly so that you don’t rot out your roof. Unfortunately, the photo above shows how not to insulate under your roof!

About moisture

Excess moisture is a home’s worst enemy. It can lead to mold, wood rot, health problems and even destroy the house. The good news is, the physics of moisture is well understood, and if you understand a few things about moisture, you can avoid a lot of frustration and expense.

The most important thing to know is that warm air can hold a lot more water than cold air. If you remember this, then you can figure out just about everything else!

This is clearly demonstrated when you take a shower. Inside the shower, the air is hot and saturated with water vapor. When you pull the shower-curtain aside and let that air into the bathroom, you see a cloud move out of the shower. After a few moments, the mirror and especially cold windows are covered with fog or even dripping water.

When the warm air from the shower comes in contact with the cooler glass, the air cools and can no longer hold that moisture. The result is condensation – water vapor condenses into liquid water. What you don’t see is that the water vapor is condensing on every cool surface – wood, drywall, etc. You just see it on windows and mirrors because it’s more visible. Also, other surfaces tend to absorb water, so you typically don’t notice the moisture. But in your cold attic, it’s a different story.

Frost on the underside of the roof

During the winter, the roof at night is the coldest surface around the house, so if moisture problems are going to show, they’ll show there first.

If you’ve ever been in a cold attic during the winter, you’ve probably seen something like this. Rusty nails poking through the roof deck, dripping with water or even with little snow caps.

If the moisture is really high, you’ll even see frost all over the wood.

This is exactly the same as the bathroom window – water from warmer, higher moisture air condensing on the cold surfaces like the nails or roof deck.

Where does the moisture come from?

The important questions are:

  • where does attic moisture come from?
  • how do you avoid moisture problems in attics?

Follow this closely. I can’t tell you how many times I’ve tried to explain this to people who absolutely insist on things that violate the laws of physics. Don’t be one of those people!

The moisture is carried by warm air from the living space leaking into the attic. It’s as simple as that.

Remember the bathroom window? Warm, humid air coming in contact with a cold surface condenses because cold air can’t hold as much moisture. So, even normally humid air in the house at 70 degrees, when it comes in contact with those nails or the roof, condenses into liquid water. If this happens enough, the wood gets saturated with water and you’ll see frost.

How about outside air?

During the winter, the outside air is cold, so it holds much less moisture than the warm air from your house. So if that cold, dry air enters your attic, usually, nothing happens. In fact, the attic is almost always warmer than the outside, so it warms the air, allowing it to hold more moisture. If anything, outside air passing through the attic tends to dry it out because that cold, dry air sucks the excess moisture from the attic. This is why attics are ventilated with outside air.

Bad attic fan, bad!

I hate attic fans. They waste energy and often cause more harm than good. Why? Because they pull so much air from the house.

Usually attic fans are installed to eliminate some issue, either hot attics in the summer or moisture problems in the winter. But if you blow out lots of air, where does the fresh air in the attic come from? If the floor of the attic was perfectly air-tight, then all the air would have to come from the outside. But attics are almost never built this way. You have attic ladders and hatches, leaky ducts, sometimes bath fans venting right into the attic, leaky recessed lights and a hundred other holes and air leaks from the house up to the attic. So when you turn on an attic fan, it’s going to suck a hundred times as much warm, humid air into the attic. In the process, it’s likely to make matters even worse!

Adding a ridge vent can be bad too

For the last 20 years, ridge vents have gotten popular. People thought that they’d be the cure-all for attic moisture problems. After all, warm air rises, so shouldn’t a ridge vent take all that nasty, humid air and send it outside where it belongs?

It turns out, in many situations, ridge vents can make matters worse. Why?

Remember those holes between the house and the attic? Well, they’re still there. As the ridge vent sucks air from the attic, it’s acting just like the fan. Air is going to take the path of least resistance. So year round, that ridge vent is sucking that air from your house – air that you’re paying so much to heat and cool.

Ideally, when you install a ridge vent, you’re supposed to install matching, continuous soffit vents. Unfortunately, this usually doesn’t happen. If you’re lucky, somebody will drill a few holes and install a few round soffit vents. But have you ever looked at those? Do you think lots of air is going to flow through those little slits? How about after you paint your house and cover the vents with a couple coats of paint? Frankly, most soffit vent retrofits are worthless. Except for the Cor-A-Vent system. That system is awesome. I used it on my house and it works great. But you’re not so lucky. You probably have either no soffit vents or one so inadequate that adding a ridge vent just makes attic moisture problems worse.

What happens when you do it wrong?

In the worst case, you rip your roof off because it’s so rotten. Look at the photo at the top of this page. That’s insulation in the space between an attic bedroom ceiling and the roof. Remember our first lesson – cold air can hold less moisture than warm air. So when you insulate that space, you’re making the bedroom ceiling warmer (good) and the roof deck colder (not necessarily bad). In this particular case, it’s really bad.

In this case, the space above the bedroom has a little attic with a ridge vent. Since the attic isn’t otherwise ventilated, the ridge vent sucks air from the space where I took this photo. So imagine a constant airflow from the house, into the first attic space, then up through this fiberglass insulation. The warm, humid air from the house passes right by the very cold roof deck and condenses. To make matters worse, the fiberglass traps the water like a sponge, so now you have wet fiberglass pressed against wood. All winter long, this process continues and before you know it, your roof is rotted out.

If you’re lucky, this roof faces the sun during the day. Plywood is pretty tolerant of getting a little wet if it dries out quickly. During the day, the sun shining on the roof can bake out the water, letting it flow up and out. This process can continue for decades without damage. But a shaded roof or one facing away from the sun doesn’t stand a chance. That water just sits in there all winter long.

Vent chutes might not fix the problem

Insulation Vent Chutes

Because of problems like this, builders created another band-aid – the vent chute. The idea is that if you keep the insulation away from the roof deck, it will allow air to flow better and flush out the moisture.

Would this have helped in the situation above? Probably not. Why? Physics!

Remember, we have warm, humid air, moving from the house, into the attic then up past that cold roof deck. The water will still condense as the air cools. I’ve seen proof of this plenty of times – nice, moldy roof decks exactly where the air channels through the vent chutes.

How to fix attic moisture problems

The solution is extremely simple in theory – don’t allow warm, humid air from the house into places where it will cool off and condense.

The most important first step is to vent that damned bath fan directly out through the roof. Check every bath fan and ensure that it’s vented, through insulated duct, up and out the roof. Not out the soffit. Out the roof. Bath fans vented into the attic are almost guaranteed to destroy your roof. So if Bubba, your friendly handyman, installs a bath fan and vents it into the attic, his incompetence will probably cost you about $20,000 to replace your roof. And another $20,000 when you have to replace it again. If after reading this, you still insist on venting a bath fan into your attic, well, you deserve what you get.

Next, wherever you have attic access, install an air-tight gasket. I wrote an article showing how on my other site. You can also buy a commercial product from ESS Energy Products. I’ve installed several of these and they’re great. Top notch. Much better than anything else I’ve seen for this purpose.

Another big problem with many homes are access ports/doors in knee-walls. Those are the walk-in attic spaces next to an upstairs bedroom. Since they’re indoors, people usually install interior doors that don’t have weatherstripping or insulation. Horrible. Really, these doors should be just like your exterior door – insulated and air tight. But if you don’t want to (or can’t) install a real door, figure out some way of air sealing the opening. You might make an inner hatch with foam board or something else. But the key is, air-tight.

Beyond that, you’ll want to seal up all the other air leaks into the attic. Electrical boxes. Recessed lights. Ceiling fans. All of these are sources of moisture that can lead to roof damage. But start with that bath fan. That will put a thousand times as much moisture in the attic as these other things.

What if moisture problems persist?

Here’s where it gets tricky. Keep in mind that there are millions of homes with insulation pushed up against the roof  where it’s NOT a problem.  Why? Because the warm, moist air from the house isn’t getting up there. No house air, no moisture problem.

Second, most of the houses built like this were built by people who actually understood what they were doing. Look at older homes and they have well vented attics. Not with ridge vents and soffit vents, but with big gable vents. Or, instead of tight plywood roofs, they had nailing strips that allow a lot of fresh air to flow, flushing out the moisture. But as people built tighter homes, for some reason they built tighter attics. The worst case scenario is the “dead air space”  with a bath fan venting into it. A dead air space is an attic space with no ventilation at all. No gable vents. No soffit vents. Nothing but air leaking from the house through an access door, carrying moisture into the walk-in attic. This is pretty much guaranteed to lead to serious moisture problems.

The solution? These spaces need good balanced ventilation. A place for air to get in and an equal sized space for air to leave. In a knee-wall attic, that means two gable vents, one on either side. That allows the cold, dry air to flow through the space and flush out the moisture that gets in there. Or, you can use a system like Cor-A-Vent that gives you proper soffit to ridge vent air flow.

An alternate solution – closed cell spray foam under the roof deck

This solution always causes some people to freak-out. Literally. I’ve seen roofers heads explode when I suggest this. The problem is, people remember half-truths like “always ventilate your roof deck or it will rot.” You know better because now you understand why rot happens.

Remember the physics of moisture. Moisture problems occur when warm, humid air comes in contact with a cold surface. If warm air can’t come in contact with cold surface, you have no condensation problem.

The idea here is to prevent warm air from ever touching the cold roof. If you spray foam that is not water permeable (closed cell spray foam) to that roof, there is no way the moisture can accumulate.

Disclaimer – this can be done incorrectly and your roof can still rot out. Also, if you have a roof leak from above, you may not notice it because the foam will hide the leak. Also, in very cold climates, it is better to allow soffit-to-ridge venting between the insulation and roof deck in order to reduce the chance of ice dams.

To repeat – the laws of physics tell us that condensation can not occur if the water vapor is prevented from coming in contact with a surface that is cold enough for the water to condense.

References showing various attic insulation practices:


If you understand the basic principle of how condensation occurs, you can avoid many of the moisture related problems that lead to horrible mold and problems in homes. Just remember your bathroom window – warm, humid air coming in contact with cold surfaces equals water condensation and condensation can lead to mold and wood rot. Now, go check out your attic!

34 thoughts on “Attic insulation problems and solutions – Part 2

  1. Hello, we have a wood framed flat roofed Victorian that we had spray insulation blown onto the rafters as spray on foam. I can’t tell if it has helped. I now want to add some sound insulation batts onto the ceiling due to noise coming across from adjacent rooms. would this cause any moisture issues when I add additional batt insulation – likely rock wool batts? Thank you.

    • If I understand, you want to add sound deadening insulation to the ceiling. Then there’s a gap up to the underside of the roof and then the spray foam?
      If that’s the situation then that would indeed raise the risk of condensation because it would make the space colder since you’d be reducing the heat flowing into it. In turn, this would make the bottom of the insulation colder which would encourage condensation formation on it.

  2. I live in a “Southern Colonial” the 2 story house with the dormers in the front. built in 1986 with 2×6 exterior studs 2×4 interior. My question is about the triangle shaped walk in attics on the front and back sides of the second floor. Right now the insulation is on BOTH sides of the triangle the roof side and the room side. The room side has faced insulation with the paper next to the drywall, (the right way). The roof side has the unfaced side of the insulation next to the roof sheathing.
    I intend to replace most of the insulation because of a rodent problem… So what is there, doesn’t really matter.
    I have also been having an issue with my Shingles over these walk in attic spaces on the North side of the house. The CertainTeed architectural roof is only 6 years old and has developed a serious mold/algae problem. CertainTeed claim has been filed & they are sending me money to clean not replace the shingles.
    I’m thinking that the roof side maybe does not need to be insulated? there is little or no ventilation in this area, two little (18×24) vents on the ends of the house. no vents in the soffit.
    Insulate the roof side too? Don’t insulate the roof side?

    • Good catch! Best to only insulate the part that is directly adjacent to the living space. The roof insulation isn’t doing anything positive.
      The algae on the shingles is common, most common on shaded roofs because in the shade they stay wet and provide the environment for algae growth.

  3. Hi there – I have an unfinished garage and I want to insulate the attic rafters with Roxul 6″ thick Comfortbatt mineral wool. I’m in Denver, CO so we don’t have the humidity problems like they do back east and down south. It was built in 1983, and curiously none of the eaves have soffit vents. There is also attic space above the bedroom at the other end of the house I am considering doing this to the south rafters (to help muffle increasing traffic noise nearby). The east attic has a couple gable vents, and a couple static roof vents as well. I’ve read that Roxul is hydrophobic – do you foresee any issues as far as possible damage to the roof deck if I do this?

    • Garages don’t really need a lot of ventilation because they’re so leaky as-is. While a properly designed roofing system has a ridge vent paired with a soffit vent, it’s not strictly necessary in this situation. And, if you have a gable vent, you really don’t need it.
      Generally, I wouldn’t worry about moisture buildup in the garage. However, just to be safe, you might want to install the Roxul with a little gap between the roof deck and the Roxul. This will minimize the chance of any moisture getting held against the roof by the Roxul.
      As you noted, Roxul doesn’t really absorb moisture, but it can still hold it. Consider a piece of steel wool. If it sits in water, it will still be wet. If you put that on the counter, the steel wool will trap the moisture against the counter, potentially damaging the surface.

      The attic space above the bedroom is a totally different situation. You never want to insulate two different areas, like the attic floor AND under the roof deck. You’d be better off adding more insulation to the attic floor which will give you a better insulated house and some sound abatement.

      • Thank you Sir! Good point on the bedroom attic. I don’t think I mentioned that the garage attic has no ventilation, except for a very small ‘gable vent’ that I put in the east wall that connects to an adjacent vented clerestory (I did that shortly after buying the house because the garage was hot as Hades). My main reason for wanting to insulate is mostly to make it useable during the warmer months and warmer in the winter. I have chosen not to finish the garage ceiling in favor of unrestricted access to the attic among other things. I must confess that a few years ago I packed the rafter bays with
        fiberglass R19 & R13 and last fall of the batts started falling down, and I noticed that they were moisture laden. The winter before I had also used propane heaters, and I don’t know how much they contributed to that. After pulling it all down, I did see some minor delamination in areas of roof deck. I’m hoping it was the result of those two factors and not something else. I’m thinking if I switch to Roxul and a large kerosene heater instead, that I can avoid exacerbating the issue…..Thoughts?

      • I am so glad you mentioned this! In most garages, there are no sources of moisture, so the moisture levels inside the garage matches that of the outdoors. The only time there can be problems is during temperature swings when it gets really humid out and the garage is still cold.
        HOWEVER you pointed out that you’re using a propane heater in there. When you burn propane or natural gas, the exhaust contains massive amounts of water/steam. This definitely will cause condensation issues in a garage because of the cold interior surfaces, especially inside the insulation and anything above it.
        The only proper way to heat a garage is with a heater that vents the exhaust (and moisture) directly outside. The problem is, those are expensive. However, they’re far LESS expensive than replacing the roof when it rots out.

      • On my soap-box – any manufacturer that sells these ventless heaters should be subject to a class-action lawsuit. It should be illegal to sell a ventless heater. I also forgot to mention that ventless heaters also spew all their carbon monoxide into your space, so you’re breathing that in. Granted, the CO levels should be low with propane and gas furnaces that are properly working, but the slightest combustion problem can lead to incapacitating or lethal CO levels.

    • I didn’t see a ‘Reply’ icon on your last comment, so I’m using this one ;-). I was thinking you might say that about the propane heater, and in my gut I suspected that was where the moisture came from also. This is even AFTER reading several garage forums where guys cautioned about condensation on tools, etc – so shame on me, but lesson learned!

      So if I take the propane heater out and just use kerosene for now, do you think I’m okay to repack the rafter bays with 6″ Roxul?

      • Hi Ted – just wanted to give you an update since we last corresponded. I decided to get rid of my ventless propane garage heater, and put a radiant barrier last spring. It kept the garage comfortable all through the summer. I also cut pcs of R-15 Roxul and put them between the rafters on the wall top plate, effectively sealing the garage from the cold air in the eaves. So far this winter, that has REALLY made a difference. Now when it’s cold out, my garage stays around 20 degrees warmer than outside temp.

        As for the other attic, I installed a radiant barrier, and put down two layers of R-15 criss-crossing Roxul on the attic floor. When it’s cold out I usually keep the house in the low 70’s then turn it down to 60 at bedtime. In past years I’d hear the furnace kick on a few times through the night. Now, during our recent October cold spell (teens and single digits at night) I’ve woken up to see the house temp in the high 60’s – never getting cold enough for the furnace to kick on once!! During winter my bedroom used to be the coldest room in house, but not anymore. Also, the basement is now very comfortable and not the ice box it used to be.

        I’ve read about the Stack Effect before, but it is now very apparent just how big a factor it is if not fixed!!

  4. Hi
    I have two fixed double glazed windows. Their frames are made of aluminium. In winter. I always have water condensation inside on both glass. When this happens, the temperature at night time must drop below 0 degree Celsius. At the moment I let my reverse cycle air-conditioning run at night but curtain barrier the warm air to get through. I installed an outside curtain and drop them at night to minimise cold air on the outside surface of my windows. This solution works! However, I thought double glazed should prevent this condensation but it is not. My question is how and why this happens in such situation. I do not cook or bath inside. So where does the humidity com from? And why the aluminium frame are so cold? Does this condensation occur because of cold air transferring from outside to inside through the aluminium?


    • First, good job on finding a solution! Window condensation can be difficult to eliminate. It doesn’t sound like you have excess moisture in the house but it’s still possible that moisture levels are elevated due to moisture coming up through the ground and foundation of the house. Soil around the house is usually very moist.
      There are other possibilities too. In cold weather, I often get condensation even on my very efficient double glazed super insulated windows. Even the best windows can get cold enough to have condensation because they just don’t insulate very well. With aluminum frames, as you have guessed, the aluminum is a very good conductor of the cold. However, I have vinyl insulated windows that get condensation almost every cold day in my bathroom when I use the curtains to keep the bathroom warmer.
      When you have condensation on the windows, do you have inside curtains in front of the windows? The condensation will be worst when using insulated curtains or shaded because these prevent the warmth from getting to the window, allowing them to get very cold. Cold enough for ice to form on the glass.
      Window condensation is very challenging to eliminate complete without going to the levels you have. But most solutions involve two things: 1) reducing moisture in the house; 2) keeping the glass warm enough to prevent condensation.

      • Hi Inoue
        Thanks a lot for replying my question! My timber floor is almost 1 metre above ground level and it has insulation rate 4 bat (Canberra – Australia means 4/6 level). Its wall is made of 75mm foam and acrylic render outside. It is very quiet inside when close its sliding door. This means the insulation is very good to me! Like you said and guessed, my rollers are very good blocking light and temperature. That I always pull them up since I installed two other rollers for outside cover. This prevents condensation very well with this way. I worked out that these outside curtains block the exchanging air which is dry air inside and wet air outside so the entropy of the dry air reduced then reduces transmission of cold air to glass(?).

  5. Hi, i just recently had insulation in the attic and walls by a low income program. We never had a problem with leaks in the our whole roof is leaking in different areas. This was not happening prior to the work done? And its definitely not condensation. Do you know what could have happened? Please help i’m so upset, the new insulation got wet and now stains all my bedroom ceilings, that we just remodeled.

    • this almost has to be condensation. Nothing they would have done unless they worked on the roof itself would have caused actual leaks. Condensation drips in these situations are fairly common unfortunately. it’s caused by lots of excess moisture getting up into the attic which is now cold. Do you have bathroom fans that spill out into the attic? That’s the most common problem

  6. Hi TD very simple question. In my attic above my shower there is no drywall or vapor seal, only pink insulation with your typical spray in attic insulation on top, is this typical or should I add drywall and vapor seal over this area of my attic as is the case throughout the rest of my attic. I live in Canada and like Game of Thrones Winter is Coming!! LOL. Seriously though I would really like to know the answer to this question. Regards.

    • If I understand correctly, when you look down from the attic, you can see the top of the shower enclosure? I’ve seen that before and it’s quite bad construction practice. I’m actually surprised it makes it past the building inspector. By “spray in insulation”, do you mean blown in loose insulation or hard spray foam insulation?

      I would definitely want to seal it off to minimize the moisture flow up to the attic if it’s the loose fill insulation. Even though the shower enclosure may seem water tight, I wouldn’t trust it. Plus, it allows the frigid air to enter the space which can be dangerous to pipes, even with the pink stuff.

  7. Hi TD, I am using fiberglass kraft faced batts in my attic. I will air seal before laying the insulation of course. The batts will be face down providing some moisture barrier between the ceiling below and the uninsulated attic space. Question is; I live in a hot humid climate (south Carolina), other than spray foaming the roof, how can I keep moisture levels down in the attic? all of the hot humid air is pulled in through the soffits and can create moisture problems. Any suggestions?

  8. I have a ranch house with a large attic and a stairway going to the attic with a insulated door to the attic. Outside the insulated door to the attic, there are 2 globe light fixtures on each side of the door. I found water in one of the globes several years ago and removed the globe. The lights still worked until this winter and recently they went out. Any suggestions? A new light bulb didn’t solve the problem and no fuse is blown. Thank you.

    • That doesn’t sound good! Since water flows downhill, I would look for any water staining above the fixtures and see if you can track the source of the water.
      Probably the fixtures electrical connections have corroded and an electrician would have to install new fixtures. But first deal with the water or you’ll keep having issues.

  9. Hello, I am building a small lofted cabin in NE. Tennessee with no attic space at all. I’m using freshly cut lumber which will shed m moisture in the months to come. My plan is to allow the wood to dry during this summer and in the fall when I’m ready to install R30 roll insulation I will apply a poly moisture barrier under the rafters and insulation area. Is this a good idea or do you recommend something else? Thanks

  10. Ted, I think your advice is very sound, But in cases where people have installed large quantities of insulation, allowing the attic to reach thermal equilibrium, as the roof cools at night you can still reach dew point . The quantity of water that will condense on the roof sheathing in the Midwest may be less than by the sea but still water. This process can be additive with the sheathing absorbing enough water over time to become saturated if thermal equilibrium prevents evaporation. Another phenomenon that needs some consideration in talking about attic moisture is night radiative cooling. This process can drop the roof sheathing temperature very quickly at night by 20 or 30 degrees below the soffit vent temperature. Under the right conditions this can cause a considerable amount condensation to form on the roof sheathing.

    • Those are good points Jay. I often worry about unintended consequences which is why I try to make exactly the same points about insulated attics – moisture control becomes ever more important in attics and within the homes.
      Thanks for pointing out the roof sheathing cooling issues. These are the types of subtle effects that often get overlooked.

  11. I have a client in the Maryland/Delaware area with a horrible attic moisture/humidity level problem. We’ve only just become acquainted but he’s apparently had this problem for many years. I’m trying to help him out of the kindness of my heart because he’s a senior citizen and has tried for many years to find someone to help him with no luck. It seems however that I’ve exhausted (no pun intended) all of the options I could think of. All of his bathroom/kitchen exhaust vents are up through the roof. He has new vented soffit, ridge vent and gable vents. He has the attic access door enclosed with a specialty built box to limit air escaping into the attic there. His attic moisture is so bad that his insulation is actually wet in the morning! He did mention that when it is a very windy day or night the humidity/moisture levels drop dramatically. He has two layers of insulation. Since I can’t think of anything else that could be causing this problem what are your thoughts on the possibility of his having too much insulation? Is that even possible? He did say there is an area of his attic where he installed the insulation himself with the paper side facing up – isn’t that supposed to be down against the floor of the attic? Any help you could give would be greatly appreciated.

    • Easy answer first – yes, the paper should go down towards the warm house in our climate zone.
      Is your client close to the ocean? I grew up on Cape Cod and the air was so heavy every morning that everything would get saturated with dew. I can see humidity getting into the attic and condensing. If that is the case then it would be hard to combat without sealing the attic and insulating under the roof.
      I have to go. Let me think about this some more.

  12. I’m in Des Moines, IA. Several years ago I had the old insulation removed from in between the attic joists. We then had the floor spray foamed with a 1-2″ skim coat to seal off the living space from the attic and 18″ of blown in fiberglass over top of the spray foam. I build a foam board hatch for the walk up attic and have both bathroom vents exhausted up throu the roof. I switched out the typical exhaust vent hose and replaced it with one that had extra insulation. The soffits are blocked off from the house so there are no soffit vents and only a gable vent at both ends of the attic. I felt like we did everything right to seal off the attic space but for two winters I have had moisture problems on the underside of the exposed roof deck. Any ideas of what could be going on? Thanks.

    • Sounds like you did things right but chances are there’s got to be some air leaks left between the living space and the attic. Don’t feel bad, I knew an insulation contractor who did great work and had the same problem with his own home. In that case, the attic became a dead air space (no venting of any sort) and he left openings around recessed lights so all the moisture came up through the lights then condensed on the cold exposed roof deck.
      Even with your gable vents open, moisture coming up from the house can condense on the cold roof, especially on roofing nails poking through since they are great cold conductors. You’ve got a couple options to track down the problem:
      1 – get a local energy auditor to come in with their blower door and thermal imaging camera and have them do a trouble-shooting visit. They should be able to locate areas where air can move between the attic and the living space. The easiest way may be to have them pressurize the house (blower door tests usually blow air out of the house). Then, go up to the attic while the fan is blowing into the house and see where warm air is escaping into the attic using the thermal camera. This should work like a charm. The blown in insulation will be a real pain in the butt, but all the more reason to go after it with the proper tools.
      2 – wait until it gets cold enough for you to have moisture problems again, then go up there early in the morning, before the sun can warm the roof, and look for the areas of the roof that shows the most moisture problem. The source of the air leak from your living space should be right under that area. The you’ll have to dig through all the insulation to figure out where the air’s coming from.

      Likely culprits:
      vent stacks from drain pipes that come from bathrooms up through the roof. Bathrooms generate a ton of moisture and vent stacks are usually big open pathways from the bathroom to the attic.
      Bath fans with incomplete sealing
      Recessed lights in bathrooms
      Anything else cut into the bathroom ceiling.
      Sometimes the space above a shower stall is left open right to the attic. I’ve seen more than one house where this happened, resulting in exactly the same issue that you’re seeing.

      Good luck!

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