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Community and Q&A

Mold, moisture, and insulation issues

Brent_F | Posted in General Questions on
GBA Fellas,
I live in a single story ranch house with a full basement under it and I am in climate zone 5A.  I have attached a couple of photos to show you the mold that is growing in my daughters bedroom closet.  Since finding this mold we have also found mold in our master bedroom closet as well as our upper kitchen cupboards.  All of the locations where the mold is growing have two things in common, they are exterior walls, and they are in areas where air flow is minimal.  Our house has electric radiant ceiling heat (yes ceiling heat, not floor heat) however there isn’t heat in the closets where a majority of the mold is.  From my understanding it is installed to the bottom of the rafters and then drywall was placed over the top of it.  In the picture attached you can also see water droplets forming on the ceiling, this is uniform in all of the areas where the mold is growing as well.  I believe the problem stems from inadequate insulation in my attic, leading to cold ceiling, thus causing the warm air from the house to condense on the cold ceiling?
If that is the solution my next question is do you have any advice on adding blow in cellulose insulation, to a house with a hip roof and a pitch of 3 /12?  Getting into my attic in the first place is nearly an impossible feat, due to the electric radiant ceiling heat, there is no access up through the ceiling.  You have to climb up through the garage rafters and then squeeze through a small 2′ x 2′ opening to enter the attic, and since it is a hip room with a 3 / 12 pitch, once you are in there, space to work is pretty much non-existent.  There are also no vent baffles in my attic, the blown insulation that is there currently maybe getting blown back off the outer edge of the wall but do to the pitch of the roof I can’t see if that is the case or not. 
There maybe a glimmer of hope though, the shingles on our roof are worn out and I plan to put a new asphalt shingles this year.  As I have thought about this a lot (mourning may be the correct term)  I had the idea of while my roof is off, why not add insulation from above?  My house has board sheathing on it so I think I might be able to pop of the bottom 4′ or so and add insulation from above?  In that case would you put cellulose back in there or would you cut pieces of rigid foam and then cover that with cellulose? Or pull the board sheathing off enough to put vent baffles in every other rafter bay, block of the the bays without a baffle, and then cut a temporary roof access from the outside, drop down in the attic with the hose and start blowing cellulose? 

I had to run a dehumidifier all winter long on the main level of our home due to humidity levels around 80%.  I assume the high humidity is part of my mold problem as well.  I have an unfinished basement, with concrete block foundation walls.  The block walls are painted but other than that there is nothing.  The basement walls aren’t insulated and there is no insulation between the basement and main floor.  We also have a shower in the basement that ins’t vented, just shower curtains in the corner over the floor drain.  I’m assuming this basement configuration is a reason for so much moisture in my main level but I’m unsure of the best approach of resolving this. 

I’m not sure if my main problem is in the basement or in the attic or if they are both part of the issue? 

Any advice on how to get a handle on the mold would be very helpful.  

Please let me know if you have any follow up questions.

St. Charles, Iowa 

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  1. onslow | | #1


    I feel your pain. I did a rehab on a brick ranch with only slightly higher slope on a hip roof with similar issues. First off, no amount of attic insulation will cure your current closet mold issues, but that I will address second.

    Insulation should help with the ceiling, but before planning on blowing in cellulose in the attic you need to check into safety issues with the radiant panels. I grew up with them in one part of our home and found them deficient in many ways. The particular units installed actually set flush to the ceiling surface and looked a bit like random suspended ceiling panels. I recall that they were very heavy since the heating elements were embedded in a plaster like layer to even out the heat. Think upside down heated floor tile.

    As far as I can recall, the rehab of that part of the house did not entail removing the original ceiling. The radiant panels went on the surface and my Dad laid out sleepers which carried a second layer of drywall. Wiring was done below the ceiling using the gap created by the sleepers. All the extra insulation in the attic was kept separate from the panels and electrical connections. I mention all this because you might have a similar situation.

    However, if the panels are directly mounted to ceiling joists with insulation directly in contact, it would be best to investigate if code requirements will be at play. Cellulose can get pretty heavy and it would be unfortunate if the weight cracked a panel. Digging through cellulose in a tall attic to find wiring is the pits, doing so in a cramped one sounds worse. If you should ever need to remove or replace a panel then finding it from below will be interesting and the resultant hole will bath you in a shower of cellulose.

    Have you considered installing electric baseboard? Wall heaters? or Cove heaters? The panel already has positions for the current needed. Based on my prior experience with the ceiling panels heating mostly the top of one's head, you might find a better level of comfort. Disconnecting the ceiling panels might allow for a greater depth of cellulose to be safely put in. Either way, when putting cellulose in the attic, managing the venting at the eaves of a low pitched roof will be a royal pain.

    If your construction is much like the one I dealt with, the roof rafters will have been birdsmouthed onto the top plate with maybe 4 1/2" of clearance under the sheathing as you go past the exterior sheathing. My fight was getting a bath fan vent out to the eave and trying not to drive roofing nails into my head while working down in the wedge. Your fight will be easier if you pull boards probably three feet up from the exterior wall line is enough. If four feet makes using new plywood sheathing a cheaper option, that's fine too.Then you can staple your choice of vent chutes to the sides of the rafters with a "tail" piece bent down and stapled to the top plate. I would do every bay personally. This will still leave the cellulose very thin at the ceiling wall intersect, but with a 3/12 roof you don't have a lot of choice.

    It will be hard to spray into the wedge, but at least the vent chutes should keep you from snuffing out all attic air flow. It may be a good idea to plan the roof peel to rip back all the sides, set the vent chutes, reset the sheathing everywhere except one for, as you note, one rafter bay where the boards are taken off half way to the ridge. Then a (skinny) person can drop into the attic with the hose and aim outward to all corners, backing out as the fill gets to proper levels. It will also allow the sprayer air to vent more easily than out the eave vents. By the way I do hope you have large eave overhangs so you can fit in vent panels easily.

    Back to the closets; two houses I have lived in had the same issue. Keeping things away from the wall and leaving the closet doors open were the only ways we found to get through the winter without major mold farms. One house had no wall insulation, the other standard R-13 batts. Yes, the lack of air exchange from the warmer room air is what allows the losses through the wall to reach the condensation point. Better wall insulation is the better answer, but since that is probably not on the table, better air exchange with the heated room is your only choice.

    If you can move the clothes rod forward to keep hanging items further from the wall, that will help. Same with boxes on shelves above the clothes rod if they exist. Keep that area as open as possible. The corner by the ceiling is a major heat leak in most homes due to the shortest path to outside temps with no intervening insulation. Energy heel trussed help with this issue, but still fail to keep closet corners as warm as they need to be at the top plates in cold climates.

    Not sure what to say about the basement situation. Not using the shower might help a lot. Insulating the rim under the floor will help keep the chill at bay in the closets. Generally keeping the humidity under 40% will go a long way to reducing the condensation point on the wall areas not getting good heat. I remember that we had mold behind the sofa which was pushed up tight to the wall, so don't forget to look beyond closets and kitchen cabinets. Under the sink or behind dishwashers are also places to check.

    Hope this helps guide your thoughts about what to do.

  2. Jon_R | | #2

    In the Winter, you should increase ventilation with an HRV (with the dehumidifier as backup) - humidity needs to be kept low (< 35%) in cool weather and ~50% in mild to hot weather.

    You probably have a lot more mold than you know about.

    EPS on the basement walls will decrease moisture gain from the basement. And maybe put epoxy on the floor.

  3. Expert Member
    Dana Dorsett | | #3

    >"I had to run a dehumidifier all winter long on the main level of our home due to humidity levels around 80%. "

    That's an insanely high wintertime indoor RH for a zone 5A location, which if that measurement is true indicates a LARGE indoor moisture source, not merely low ventilation. (Both could be true.) How are you measuring the indoor RH?

    Jon R has it right- controlling indoor humidity levels by varying the ventilation works, and would solve this problem if the source of moisture is better bounded. And if the basement isn't already finished, air sealing and insulating the foundation walls to IRC code minimum with R15 rigid foam board (3.5" EPS) or 1.5" EPS trapped to the foundation with a 2x4/R13 studwall will reduce ground moisture sources of indoor humidity, and lower the amount of infiltration borne humidity during muggy summertime weather as well.

    If the closet corners with the condensation & mold are exterior, where two exterior walls meet the corner framing may be uninsulated (common), with a significant thermal bridge making that corner extra-cold, and may have nothing to do with attic insulation. It may take some drilling &/or exploratory house surgery to figure out how your corners are framed, but if there's an uninsulated mini-cavity on the exterior it may be possible to squirt some cellulose or low-rise closed cell foam in there. In typical 3-stud corners in new construction there is sometimes batt or board insulation stuffed in there:

    Even if it's not an exterior corner, the framing at the partition wall may have left a gap leaving it extra-cold (especially if it's leaking outdoor air) a gap that may be treatable with spot blown insulation or slow rise foam:

    In new construction there are other options for insulated corners, which probably isn't how your house was built:

  4. Expert Member
    Dana Dorsett | | #4

    BTW: Is there any sign of high moisture coming through the CMU foundation walls, such as failing paint or efflorescence? Has there ever been flooding or puddling of water on the basement floor? How is the basement heated (if at all)?

  5. Brent_F | | #5

    Thanks for some of your feedback guys, I'll try to answer some of the questions you have asked.

    The paint on the CMU foundation walls is in good shape and there is no water puddling on the floor. We do however have an interior foundation drain system that was installed last summer. This leaves roughly a half inch gap between the basement floor and the basement wall all the way around the perimeter of the basement. (See Attachment) This leads to a sealed sump pump and is then pumped out of the basement.

    I have used the cut and cobble method with 2" XPS and canned spray foam to insulate my RIM joist all around the basement. We do have 3 small basement windows that are single pane with a metal frame, they stay frosted over most of the winter, not sure how much they may contribute to the moisture also.

    I'm currently trying to insulate my basement. Starting first with a dimple membrane that will tuck down into the footing drain, and then I plan on applying 2" XPS to the dimple membrane and then framing up a 2" x 4" wall filled with R-13 batts as you mentioned.

    As far as how the Humidity of 80% was determined, our dehumidifier has a gauge on it. It displays the room humidity and the desired humidity levels. In the morning it would be full and the reading would be between 70% and as high as 80%. There is no shower on the main level, just a bath, which only gets used 3 to 4 times a week.

    Also, the basement isn't heated at all. We aren't sure what we are going to do for that yet, we are planning on finishing it to add a third bedroom to our house. My desire is to do a mini-split heat pump but I haven't explored that option thoroughly yet.

    I have drilled into two of our outside corners from the inside at an angle, aiming for the outside corner and have hit wood all the way. I am assuming it is a 3 stud corner.

    Do you think a bath fan above the shower in the basement would be a small start in helping with the cause? I'll have to run it about 20 feet to outlet it through the RIM joist, going the other way will require going through brick or going 5' but through 3 floor joists. My assumption was avoiding cutting through floor joists would be better, that is why that length is necessary.

    Thanks for you feedback also. I really don't know much about the radiant ceiling heat. I believe it is a type of coil type system though. The reason I say that is because you can take a temperature reading gun and see higher and lower temperatures in a consistent pattern. I think they may be mounted to the underside of the chord on the roof truss. There might be sleepers also keeping the sheet rock off of them. The house was built in 1962, there is no plaster in it, it is all sheet rock.

    We did end up finding a lot of mold under one of our kitchen cabinets in the corner, where once again is an outside wall, as you mentioned we probably would.

    I saw an article on Fine Home Building awhile ago about retrofit roof baffles from above that looked like a pretty slick approach. I was planning on removing 4' of the existing room sheathing so I could put plywood sheathing back down after installing the baffles. The article call for making the baffles out of 1" rigid foam, to also give you a little added R value in such a cramped space. I don't think our trusses have a bird mouth in them but it is still a very tight space, I'd say roughly 4 - 5 inches.

    We did end up just leaving our closed doors open after we cleaned up the mold and it did help from coming back so quickly. With having the radiant heat we don't have much air movement which doesn't help. We would try to turn the ceiling fans on to promote air flow in those rooms. I'm considering louvered closet doors so we don't have to keep them open all the time but not sure that would allow enough air to exchange or not.

    Thanks again for you feedback guys, I look forward to your ideas after you receive this additional info.


  6. Expert Member
    Dana Dorsett | | #6

    IRC Code min for zone 5A the rim joists need to be insulated to R20. The 2" XPS is only warranteed for R9, and at full depletion of it's climate damaging HFC blowing agents will drop to R8.4, the same as EPS of similar density. But R8.4-R9 is plenty of dew point margin for high density R15 batts, which would bring it north of R20. From a fire hazard point of view it's better to cover the XPS with a rated timed assembly (like half-inch gypsum board), but most inspectors would give it a pass without it. But if the R15 batts are rock wool it provides a substantial improvement in fire resistance even without a code-specified thermal barrier such as half inch gypsum board.

    If the painted CMU shows no signs of flaking or efflorescence ground moisture coming through the foundation walls is not a major contributor to your indoor humidity issues. It's generally safe to insulate it with foam board + insulated studwall, provided there is sufficient dew point control at the foam/fiber insulation boundary on the above-grade section, which in climate zone 5 would be R5+. While 2" of XPS would meet that requirement, it's one of the LEAST green options, due to it's HFC blowing agents.

    XPS is blown primarily with HFC134a (automotive AC refrigerant) which is an extremely powerful greenhouse gas (~1400x CO2 @ 100 years). EPS and polyisocyanurate are blown with far more benign hydrocarbon blowing agents, usually variants of pentane at about 7x CO2. In the EPS case the vast majority of that pentane escapes the foam and is recaptured at the factory long before the product arrives at the distributor, often burned for process heat, so most of it doesn't even get into the environment. Rather than XPS, using a minimum of 1.5" of EPS (any density, faced, unfaced) or polyiso (any density or facer) would be preferable, and usually cheaper per labeled-R.

    From a zone 5 IRC compliant thermal performance point of view the basement wall insulation needs to be equivalent to R15 continuous insulation or better, which you'd be able to hit with 1.5" of EPS + 2x4/R13. At 1.5" low density Type-I EPS runs about R5.8, Type-II EPS would be R6.3, fiber faced roofing polyiso would be R8.5, and foil faced polyiso R9+. It's often easy to find reclaimed 2" roofing polyiso (R11+) for well under the box store price of 1.5" Type 1 EPS which takes a big bite out of the financial sting. (With a ZIP code I might be able to point you to some.)

    When the slab isn't insulated it's temperature will be below the outdoor air's dew point for much of the summer, which allows moisture to accumulate in the bottom plate of the studwall if it's resting on concrete. Cutting strips of 1-1.5" EPS (not polyiso) to install under the bottom plate keeps the wood warmer (=drier) and provides a bit of capillary break for wicking moisture (a complete capillary break if the EPS has a plastic or foil facer). If polyiso is used for the wall-foam leave a gap between the slab and the polyiso- if there's ground moisture in the slab it can wick into the polyiso. If putting EPS under the bottom plate of the studs, extend it a bit under the polyiso with a bit of a gap beween the edge of the EPS and wall as a channel for bulk moisture.

    The humidity sensors in dehumidifiers aren't exactly a NIST calibrated measuring instrument- it's common for them to be WAY off the mark. Even a $10 battery operated AcuRite humidity monitor is likely to be closer, at least in the middle ranges (not so much at RH north of 85%). It's worth investing in a few of those to track rough humidity levels in the rooms with the moldy closets, and the room with the dehumidifier.

    Installing a shower fan would be a good start, if it's being used regularly. Does the basement bathroom have mold & mildew issues too? Keep the duct runs short. If you're careful it's not that hard to make a 4" hole in a brick using a hammerdrill and a hand chisel. (It works fine on CMU too, even poured concrete, though that takes a bit more time.) This guy's dryer vent installation video is a pretty good demonstration:

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