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Community and Q&A

Unfinished third floor insulation

crosby72 | Posted in GBA Pro Help on

I have a fairly new house (2004) and I am convinced something is wrong with it. Located in NH, it’s a basic square colonial with a full shed dormer rear in the attic, and a normal sloped (cape style) flat roof in front (no dormers at all).

The roof is not insulated, but rather the floor of this third floor/attic, and there is 3/4 inch sheathing covering the floor joists and faced insulation fiberglas batts. Mold has been discovered in the insulation, below the subfloor, and more so near the back wall, on the board that goes from one end of the house to the other other, tying all of these floor joists together.

The builder originally told me 9 years ago this was due to the fact that we didn’t finish the third floor. The relative humidity is generally very high up there, sometimes over 70% according to the cheap humidity meter I bought at Lowes and stuck on the wall. Just this past weekend I found so much condensation on these back windows (of the third floor) that I couldn’t even see out them.

Is there supposed to be a vapor barrier preventing heat/humidity from escaping the living space into the attic?

The builder also chose to put strapping perpendicular to the joists, and the ceiling drywall below is mounted to that, leaving a nice half-inch or greater area below the joist for all of this heat and moisture to spread throughout (I am guessing).

What can I do to keep the humidity down in the attic/unfinished third floor, and what should be done as far the current setup (not using the third floor)? I just want to fix the problem and move on as we plan to sell this house and move in a year or two at most.

So far I have removed and replaced the moldy insulation, replaced the standard cheapo bathroom vent fan hoses (two second floor bathrooms) with insulated vent hoses, which vent out the back wall of the house, and sealed the bathroom vent fan units with expanding foam, as there was clearly a gap around both, about 1/4″ wide in some spots. Still, the humidity is very high and I am worried that may not have been the main problem.

Thanks very much for any help you can provide.

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  1. GBA Editor
    Martin Holladay | | #1

    The situation you describe is fairly common. There are usually two factors contributing to the problem:

    1. A damp basement.

    2. The lack of an air barrier separating the second floor from the attic.

    Here is the mechanism: The ceiling has leaks, so there is no barrier preventing warm, humid indoor air from escaping from your home into the attic. When the warm humid air contacts cold surfaces, the moisture in the air condenses, leading to mold.

    The driving force is the stack effect. Escaping warm air at the top of your house is replaced by entering cold air at the bottom of your house. Usually, the air enters your house through cracks at your rim joist area in your basement.

    To fix this problem, you need to perform the following work:

    1. Seal the air leaks in your second floor ceiling (your attic floor). Here is a link to an article that tells you what you need to do: Air Sealing an Attic.

    2. Seal the air leaks in your basement. Here is a link to an article that tells you what you need to do: Air Sealing a Basement.

    3. Investigate whether your basement is damp, and, if necessary, perform the work that needs to be done to lower the indoor relative humidity (RH) in your basement. Here is a link to an article that tells you what you need to do: Fixing a Wet Basement.

    4. Investigate whether your house has other sources of high humidity contributing to the problem. Possible moisture sources include: a humidifier (running a humidifier is always risky); lots of firewood stored indoors; a hobby like keeping lots of tropical fish; many houseplants; a large family that takes frequent showers without operating the bath exhaust fan.

  2. Expert Member
    Dana Dorsett | | #2

    If the humidity is high enough for copious condensation on the windows at recent weather temperatures it means the attic probably has NO ventilation to the exterior of any type. If it was intended to be conditioned space, putting the insulation on the floor without adding ventilation for the space above the insulation is an error, and may not be fully resolved simply by air sealing the attic floor.

    If you put screens in the attic windows and crack them a bit, the attic temperature will drop a bit, but the average humidity should drop a LOT.

  3. crosby72 | | #3

    Thank you both for your detailed responses. The house does have very good ventilation in the attic itself, with venting all the way across the soffit front and back, and of course the ridge vent. This is why there's not mold all over the walls and interior of the attic, because the humid air can get out the roof. (As far as I understand). I think the first post touches on some very pertinent issues, and I am excited to pursue that further. The basement isn't actually damp, however. I do need to consider the rim joists, and will get more cheapo humidity measuring devices tonight. I will also read the air sealing document too. There is no humidifier, no firewood at all, no tropical fish, but we do have 5 people taking showers, and more often than not I have to urge them to turn on the fans. Even still, I am sure they don't leave them on long enough, and want to swap out the existing controls for timers or maybe even the automatic humidity detecting kind. Thanks again for your help, hopefully I can get this straightened out soon.

  4. GBA Editor
    Martin Holladay | | #4

    A basement can be a significant source of moisture into a house even when the basement doesn't seem damp. What happens is that entering moisture evaporates quickly. The walls and floor might seem dry, but they might still be sources of moisture.

    One way to test these surfaces is to tape a square of polyethylene on the concrete wall or concrete slab -- and to examine the polyethylene after 24 hours. If you can see beads of moisture under the poly, then the concrete is contributing moisture to your indoor air.

  5. crosby72 | | #5

    great point, I just bought some clear plastic (drop cloth material for painting) that I hope is polyethylene... Also had a thought while driving home. Our furnace has a pipe that allows fresh air to enter the basement from outside, about 4 inches in diameter. This was installed many years ago I think because the furnace was starved for air because the house was so "tight". I am betting that is a big contributor if not "the" contributor. Maybe I can run the same insulated piping I bought for my bathroom vent fans, to the intake on the furnace. Do they sell intakes you can configure like this to prevent the incoming air/moisture to accumulate like I think it has been? I moved my cheapo humidity detector out of the attic (was at 60% when I just got home), and it is now in my bedroom (2nd floor). It jumped up to 80% but now it has come down in the 15 minutes or so I have been home, to about 63%. All of which of course are too high, but it seems the whole house, living space and all might be high. Could it be that the humidity is coming in through the basement, maybe that furnace air intake, and making the whole house humid? I also have a Weather Channel device that tells you outdoors and indoors statistics, one of which is relative humidity (that I assume is indoor?) - but that says 44%? The cheap humidity meter I bought now says 57%, maybe it's still coming down. Anyhow, I will let that acclimate and watch the values for a day or two, and then do the same in the basement. Hopefully I can find an air intake assembly for my furnace, which is actually fairly new since the house was built in 2004. Thanks again for all of your help, hopefully I will get this straightened out soon.

  6. crosby72 | | #6

    wow - I have been monitoring the relative humidity in the basement, and it's not very high like the attic. the master bedroom (2nd floor) and the basement are at an "okay" level of about 40-45% it seems. does that refute the argument that moisture is coming in through the basement/sill and propagating up the exterior walls? Or is it possible that such humidity isn't shown in the basement but still can creep up inside the exterior walls? Thanks very much for your help everyone.


  7. charlie_sullivan | | #7

    If the basement is cooler than the main conditioned space, and the same relative humidity, that means it has lower absolute humidity and it is unlikely to be the main source of the humidity in the house. If, on the other hand, it's warm down there, perhaps because of poorly insulated heating equipment, it could still be the source.

    Do try the clear plastic test. The drop cloth material probably is polyethylene, but it will work even if it isn't.

    The air intake should not be adding moisture--the outside air during the winter has lower moisture content than indoors.

  8. crosby72 | | #8

    Hello again. The basement is roughly between 60 and 64 degrees on average, while the house is normally 70 degrees, kicked down to 62 degrees when we aren't home and at night by programmable thermostats. So I would guess the basement itself shouldn't be the source of extreme humidity. I am doing the poly test on a wall and the floor, but don't expect a positive result as it's very dusty down there. I did seal a good portion of the basement walls (about 70 percent on the basement side, zero on the garage side of the actual basement - we have a garage under) with dry-lok several years ago. I never finished the basement side - but it seems pretty dry in there. I moved the humidity monitor back up into the attic thinking maybe sealing the bathroom fan units and using the insulated hoses helped remove the humidity and it just needed time to vent out the soffit to ridge vent flow? Probably not but I am curious to see if the humidity has been reduced at all. Still plenty to do but I really need to find the principal source of humidity. I will post an update shortly, thanks very much!

  9. crosby72 | | #9

    So I've done the poly test, and when pulling up the plastic, there's no noticeable moisture underneath. Is that all I should be looking for, trapped moisture under the plastic when I pull it up? It was dry as a bone, in fact that whole basement is very very dry. Dust everywhere it seems. So it wouldn't make sense to me that the humidity is getting in or is being caused by the basement, and traveling up the exterior walls somehow. Any other thoughts? I do still have to review and act on the "sealing the basement" and "sealing the attic" documents (thanks for these great resources), but don't I need to determine the source of the humidity first? Would the warm air from the living space escaping into the attic be enough even with normal (relative) humidity of about 45% cause the 75% humidity in the attic (much colder currently)? Thanks so much everyone, I think we're maybe an answer or two away from the source!


  10. GBA Editor
    Martin Holladay | | #10

    If you take air at 72°F and 45% RH and cool the air to 56°F or 57°F, the air will have an RH of 75%. (That's what I get reading the psychrometric chart -- these figures are approximate).

    Indoor RH of 45% is high during the winter. You still need (a) to identify the sources of moisture in your home and attempt to reduce them, and (b) to perform air sealing work in your basement and attic.

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