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Mold / insulation / ventilation problem

carolehunter | Posted in Energy Efficiency and Durability on

My problem is related to Bryan’s in his question “Can lighting roof moisture problem” posted on July 1 2018 but with the addition of mold and high RH. Here’s the situation:

I own a 1970ish 24’ x 28’ 2 story alpine cottage style house in Vermont. Construction is as follows:

1st floor – approximately 2’ of the building walls are below grade in the back 1/3 of the building – so most of it is above grade and there are no liquid water leaks.
Slab on grade – no insulation below.
Wall assembly – 8” concrete block with 2×4 wall inside – there is fiberglass batt between studs which appears to be in very good condition. Poly vapor barrier and 1/2” GWB

2nd floor floor – 2×8 floor joists – no insulation

2nd floor – – – -2×4 walls with fiberglass batt in good condition and poly vapor barrier. 1/2” GWB or masonite paneling inside, 1/2” T-111 siding
At top of wall there is a 4×12 beam that is continuous the length of the building and serves as top plate and window header
There are solid 4×4 posts in each corner and under roof beams. These continuous beams cantilever out and form a 3’ overhang – the walls are literally never wet by rain

Roof 3/12 pitch
2×8, 1/2” sheathing unfortunately completely covered by grace ice and water and then a new 5 yo asphalt roof which I prefer not to remove.
There are ridge and soffit vents but fiberglass batt insulation with no air space means they don’t work. Roof is supported by 2 additional 4×12 beams under the rafters and about 3’ down from ridge.

electric basebd heat, supplemented by direct vent propane stove

really crappy aluminum frame sliding windows – condensation and ice in winter, frames and tracks are moldy in summer

Here’s the problem:
In the summer I have the same problem as Bryan – roof insulation is wet and where the one light fixture is it is wet enough to turn the sheetrock to mush and cave in. Very moldy up there. Downstairs always smells musty but no visible mold

In the winter on the second floor, mold forms on the walls in all corners, behind furniture – ie wherever there is poor air flow and/or where walls are colder because of uninsulated 4×12 beams or 4×4 posts. Ice too. And liquid condensation. The relative humidity on the 2nd floor averages an unbelievable 60 -65%. I am one woman who rarely cooks, takes one shower a day (with fan running) and I don’t have an aquarium. Interestingly, the RH on the first floor averages about 45%. There is a notable difference. Room temperature on first floor is kept lower than on 2nd floor. Door to 2nd floor is always kept closed.

The mold problem is bad. Of course I’d like to minimize expense. Here is how I think I might address these issues:

Roof – remove fiberglass batt – install propavent or equal the entire length of rafter bay to provide air channel from soffit to ridge vent. Fill bays with open cell foam. Install 1” XPS rigid foam on inside of rafters to help with thermal bridging – tape joints – strap and 1/2” GWB no light fixtures in ceilings.
2nd floor Walls – leave existing fiberglass batts and poly vapor barrier. Install Henry Blueskin¯ VP100 over existing T-111 siding. then 3” layer of Rockwool Comfort Board, strapping and siding.
Install new windows and doors
Insulate band joist with combination of XPS and spray can foam
1st floor walls – cut out a horizontal strip of GWB and hook out as much fiberglass as possible. Install Pourable closed cell foam in 2×4 wall cavity
1st Floor floor – remove existing wall to wall carpeting. Install 1/2” XPS, with taped seams and vinyl strip flooring

Replace electric baseboard heat with mini splits – one up and one down

Add Lunos e2 HRV – one pair up and one pair down

Will all this help solve my mold and humidity problems or am I missing something vital? If I have mini splits do I need the Lunos HRVs as well or can I do without them? And why oh why is the RH upstairs far greater than the RH downstairs and where does all that moisture come from? Does anyone have any better ideas?

thank you

Carole Hunter
carolehunter.net

Proctorsville, VT 05153

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Carole,
    A site visit would be helpful to understand what's going on, but it certainly sounds like you have a leaky, poorly insulated 1970s house built on a damp slab.

    Your plan to install 1/2 inch XPS on the slab is the right idea, but there is more that you can do.

    1. Assess the grading at the exterior of the house to make sure that rain is directed away from your slab. On the uphill side, you may need a swale to make sure that the grade slopes away from the house in all directions.

    2. You should definitely install at least 2 inches of vertical rigid foam (XPS will work, but EPS is more environmentally friendly) at the exterior perimeter of your slab. This rigid foam should extend at least 16 inches below grade. The above-grade portions of the rigid foam should be protected with a durable layer to limit exposure to sunlight and physical abuse, and there should be Z-flashing at the horizontal seam between the rigid foam and the bottom of your siding.

    Once these measures have been taken, the rest of the work is run-of-the-mill weatherization work: namely, blower-door-directed air sealing and insulation improvements. A ventilation system (like the Lunos fans you hope to install) is essential. Good luck.

  2. GBA Editor
    Martin Holladay | | #2

    Carole,
    Q. "If I have minisplits do I need the Lunos HRVs as well or can I do without them?"

    A. If you tighten up your house, you'll definitely need a ventilation system. Minisplits have nothing to do with ventilation -- they provide space heating in winter and air conditioning in summer, without introducing any outdoor air.

    For more information on your options for ventilation, see these two articles:

    "Designing a Good Ventilation System"

    Revisiting Ventilation"

    Q. "Why oh why is the RH upstairs far greater than the RH downstairs and where does all that moisture come from?"

    A. There are two reasons why the relative humidity is greater upstairs: the stack effect (which introduces dryer outdoor air through cracks at the lower levels of your house) and (perhaps) hygric buoyancy. For more information on hygric buoyancy, see "High Humidity in Unvented Conditioned Attics."

    It's impossible to tell where the moisture is coming from without a site visit, but the most likely sources are your slab and the concrete block wall that is partially below grade.

  3. GBA Editor
    Martin Holladay | | #3

    Carole,
    Your plan for the cathedral ceilings will work, but open-cell spray foam is not the only option. Moreover, your plan to install XPS on the interior is not ideal from an environmental perspective. Most green builders avoid the use of XPS, because it is manufactured with a blowing agent that has a high global warming potential. For more information on this issue, see "Choosing Rigid Foam."

    For more information on all of your options for cathedral ceiling insulation, see "How to Build an Insulated Cathedral Ceiling."

  4. GBA Editor
    Martin Holladay | | #4

    Carole,
    Your plan for the first-floor walls -- to try to "hook out as much fiberglass as possible" and then "install pourable closed cell foam" -- is not an approach I would recommend.

    Both of these steps are tricky. It will be easier to just remove all of the drywall and start with open stud bays.

    Ideally, there is an air gap between the concrete block walls and the studs. If that's the case, you should hire a spray foam contractor to install closed-cell spray foam in the stud bays. You want to instruct the contractor to make sure that the spray foam fills the gap between the concrete blocks and the studs.

  5. Expert Member
    Dana Dorsett | | #5

    It's not clear what is meant by " Install 1” XPS rigid foam on inside of rafters to help with thermal bridging...", but it sounds like the foam would be installed on both sides of each rafter(?), which would barely move the needle on thermal bridging. (Modeling it with THERM would tell just how much the needle moves.) Installing 1.5" of polyiso on the rafter EDGES, would cut the heat flow through the 2x8 rafters by more than half. You'd lose 1.5" of headroom, but you would have another 1.5" of depth for cavity fill insulation too.

    A 2lb pour of closed cell foam in the wall cavities would be a nearly-irreversable mistake. An inch of HFO-blown closed cell foam on the interior side of the CMU wall would be sufficient to control moisture drives from the masonry, and would be enough to provide dew point control for fiberglass or rock wool filling the remaining 2.5-3" of cavity depth, with NO interior side vapor barriers (just standard latex paint on new drywall.) This might even be cheaper than a full cavity fill of 2lb pour, and a LOT greener, but it is far more disruptive if you're trying to live in the place while under construction. An alternative, less-disruptive approach would be to install 2" or more of EPS on the exterior of the CMU finished with an EIFS, but that wouldn't block ground moisture wicking up the CMU from getting into the studwall. It's possible that on the first floor the interior side polyethylene vapor barrier is aggravating rather than reducing the moisture problems by keeping the studs & fiberglass at a higher moisture content. This is something that might be determined by a site visit, doing a bit of exploratory disassembly in a few places taking moisture content measurements.and looking for other indicators. The drainage and back fill around the below grade section needs to be investigated too.

  6. carolehunter | | #6

    Thank you for reading my long question and spending the time to answer in such detail. I can get myself turned in knots so appreciate the help. I would welcome a site visit. Please email me details for that.

    1.1 grading at the house is pretty good - those 3' overhangs keep most of the water well away from the building
    1.2 great idea!
    2.A I guess I thought that since mini splits bring in fresh air (they do, don't they?) that might cover it - but fresh air is essential and Lunos is a relatively small investment so they will be part of the program!
    3. yes - I guess I could just as easily use rockwool or cellulose or something between the bays. Re the green-ness of XPS - sigh - I guess since I don't have children I think I should get some sort of karmic rebate or green-ness credit but I will revisit other options. LOL.
    4&5 money is not unlimited so I was hoping to avoid gutting the downstairs - it sounds easy to just remove the GWB - but as you know, that exposes all kinds of other issues and soon the budget is way beyond blown and the question becomes - why not just tear it down and build a smaller, more comfortable house that suits me better? Believe me, if I thought I could remotely afford it I would. But I'm glad to be saved from the mistake of pourable fill - I sure don't want to make anything worse. Re rigid board and rafters - I meant not to wrap rafters but to install a continuous sheet of rigid below rafters to serve as vapor barrier and a layer of insulation between rafters and GWB - 1" isn't much but sure better than nothing - polyiso would work fine

    thank you again for your help. It's a complicated subject and easy to get confused and that confusion can lead to some very costly mistakes. So many people claim to be experts but actually are not - Glad I finally came to the source!

    CH

  7. Expert Member
    Dana Dorsett | | #7

    >"...thought that since mini splits bring in fresh air (they do, don't they?)..."

    No, they don't. They circulate room air entering at the top, and blow it out the front.

    The thing about XPS aside from the envirnmental impacts is that as it's blowing agents dissipate out over a few decades doing it's longer term damage it's performance drops to that of EPS of simlilar density, about R4.2/inch. The vendors will warrantee "90% of original R-value" (which would be R4.5/inch) for 20 years, or even life, but they know NOBODY is going to disassemble their house and have the material tested to collect on that warranty. EPS doesn't have that problem- the majority of it's low-impact hydrocarbon blowing agent leaves the foam (and usually recovered) at the factory, and plays no part in it's thermal performance- it's R-value is stable over time.

  8. carolehunter | | #8

    thanks Dana - i guess when you think about it it's obvious that minisplits don't bring in inside air - there's no air vent to the outside - duh.

    re foam - ok thanks for clearing up the eps-xps thing - it makes more sense now. If you end up in the same place it makes more sense to go with the safer product.

    re installing any sort of rigid foam on the outside of the building - I've always been worried about bugs and this house does have a carpenter ant infestation in one location which should be remedied when everything dries out - but if I add outside foam am I giving the blighters another place to live? Bugs are neck and neck with mold in the eewww -- ick department.

  9. carolehunter | | #9

    nevermind the bug question - I see that there are several articles here - I'm going to read more and will get back to you if I need more help - thx
    CH

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