Bay Window, Rodents, and Mold

[Fredalbee123]

Hello all,

I am in the process of finishing my basement. In a search of the forum, I found some other threads regarding bay window and insulating the joists below, but nothing that is directly applicable to my situation. Please reference photos and details below.

I moved into the house 5 years ago and started finishing the basement myself last winter. At some point in the homes past, it had an infestation of rodents (looks like both mice and Rats were present based on the droppings). There were significant amounts of rodent droppings above the insulation in the floor joist bays that were in line with where the bay window is located.

When looking down into the cavity from the basement, there appears to be OSB fiberboard that is now sagging. I tried to pull the OSB back up into place, but was unsuccessful. Instead, I screwed a wire mesh screen onto the sides of the floor joists. I wanted to make sure this area was fully sealed off from future rodent entry, so I cut 1x1’s and screwed them to the sides of the floor joists. I cut plywood to size and was intending on screwing the plywood to the 1x1’s as extra protection.

Then, I temporarily stopped working on the basement over the summer and just started resuming the work.

I had laid some of the pieces of plywood down on the mesh screen over the summer. When I looked at them the other day, I noticed that the side facing down had mold growing on them. The weird thing is that I don’t see any mold growing on the floor joists or the plywood which is my 1st floor flooring.

We have had several weeks of really rainy weather here on the east coast (Lancaster, PA). Looking down into the cavity, I do not see any water or moisture, but I cannot see what is below the OSB. On the outside of my house, I have the bay window and there is no way for me to access the bottom of the bay window as the front porch goes directly up to the window.

My thought is that the water table level has risen or that warm humid air is rising up under the porch and bay window and condensing.

Does anyone have any experience in this, could confirm the reason mold may be growing, and the best way to remediate or insulate.

Thank you!!
Fred

 

[Dana]

It looks like the floor joists are cantilevered out over a straight foundation, but open to a cavity behind the brick veneer skirting under the bay window. Is the cavity under the bay window full height, extending down to the basement slab, or is there a small footing &/or slab at roughly grade level holding up the brick veneer under the bay window?

Brick stores rain & dew, and can release that moisture in high-intensity bursts when warmed by the sun. Most brick veneers have a 1" gap (or deeper) to the structural sheathing of the house, and that gap is vented at the bottom with weep holes ever few bricks, and open at the top or with vents under eaves or sometimes into a vented attic. You may have to get creative on how to vent the under-window cavity to the exterior, while still air-sealing it to the basement interior. Cut'n'cobbled sheet foam board on the bottom of the cantilevered joists to replace the sagging OSB, caulked air-tight would be a decent air & moisture barrier, keeping the wood drier, but may be impossible to do well. You may have to settle for cut'n'cobbled foam board slipped between the joists and caulked in place, leaving the joist bottoms still exposed to the brick veneer cavity, which would make venting the cavity to the outdoors even more important.

There are several ways to insulate a basement, many of which will turn it into a mold farm. The first an most important thing to get a handle on this is to air-seal the basement as tightly as possible from the outdoors. The basement temperatures are below the outdoor dew point temperatures in summer, which causes the moisture content of wood at that temperature to rise. The wood with most direct exposure to the incoming outdoor air will have more moisture accumulation than the average, and the wood at the lowest temperature (usually nearer the floor) will too. Some variation on this theme:

While the drawing specifies XPS, using foil faced polyiso is both easier to deal with and a "greener" option, due to HFC blowing agents used for XPS. The foil facers are a better moisture barrier than bare polystyrene, polyiso will deliver R5.5-R6.5/inch of thickness whereas XPS is only warranteed to perform R4.5/inch (but labeled a R5/inch, the initial few-years performance average.)

To insulate basement wall, air sealing the band joist & foundation sill is the first step. Then install at least as much rigid foam insulation against the foundation behind the studwall that it takes for wintertime dew point control on the studwall cavity insulation, so that you won't need an interior side vapor retarder, and use unfaced fiberglass or rock wool batts. The IRC code prescribes different amounts for different climate zones & studwall depths. York PA is on the cold edge of US climate zone 4A, where above-grade walls don't require any exterior insulation to be able to skip the interior side vapor retarder, but in a basement you need at least an inch of foam board (any type) to limit high humidity from ground moisture wicking up from the footing. Cheap 1" box store EPS is good enough, seams taped, edges sealed. If there is a bit of ledge at the top of the foundation between the foundation sill and wall foam, fill it in with a strip of 1.5" thick foam board, taped seams, caulked edges"

Rather than sill gasket/sealer under the bottom plate, install an inch of EPS foam board as a capillary & thermal break, which will keep the moisture content of the bottom plate lower. The wall isn't structural (it's not holding up the house), so the density/compressive strength of the EPS doesn't matter here- cheap stuff is fine. With EPS under the bottom plate it doesn't need to be pressure treated, since it can't wick moisture from the slab, and will remain above the dew point temperature of the basement air in summer.

 

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[Fredalbee123]

Thank you for your response Dana!

To your question: if the cavity under the bay window full height, extending down to the basement slab, or is there a small footing &/or slab at roughly grade level holding up the brick veneer under the bay window

I don’t know. My guess is that it does not go all the way down.

I am not hopeful for any weep holes based on the sub-par construction that was done on the rest of the house.

I guess the best thing for me to do is drill a hole in the OSB large enough to get the lens of a camera/flash in the hole to see what exactly is down there.

How about if I do that and report back to see if the mitigation would be different.

Thanks again!!
Fred

 

[Dana]

Creating retrofit weep/vent holes doesn't take a rocket science degree or a journeyman level of fine craft. There are retrofit weep hole screens for keeping critters from using the weeps & vents as the front door to their cavity-condo.

A weep every 3-4 bricks (24-32") on the course of bricks one brick height above walkway surface ( so that the walkway doesn't drain into the cavity in a heavy rain) and a corresponding vent just below the overhanging corbel/sill course allows it to convection-vent to the outdoors. A whole-house brick veneer view would look like this, but even the ~18" of height under the bay window will provide a modest rate of convective air purging of the cavity (way better than what you have now, anyway.)

When opening up the brick veneer with weep vents the air tightness of the rest becomes even more critical from a heat & moisture movement path perspective. It dries out the cavity, but it's important that it not become a wind-tunnel too. When sealing the band joist to the framing & subfloor with caulk, be sure to seal the seams of the odd-angle joints of the band joist visible in your photo too, not just the floor joist to the band joist, and joist to subfloor, etc.

 

[Fredalbee123]

Thank you again Dana!

How are weep holes installed where windows are installed?

Thanks,
Fred

 

[Dana]

The weep holes under the windows get cut into the second course of brick under the korbel, the sloped overhanging brick sill. That would be the course of brick below the one marked "Section mode".

The reason it's not the top course of non-sloped brick is to avoid damaging any pre-existing window flashing (the green line crossing the gap in this picture.) Flashing is there to direct bulk water that gets through the window unit outward, away from the house, not into the gap, unless there is a drain for the gap out the bottom, which probably doesn't exist in your assembly.

With weep holes top and bottom it's not a disaster if some amount of rain driven water gets by the flashing, but if the flashing is there it's better to preserve rather than impede it's function.

 

[Fredalbee123]

Thanks again Dana!

It’s weird that from what I can visually see at least, mold has not grown on the OSB (I can see top side), the plywood flooring above, or the floor joists.

I don’t know how I would ventilate the brick since my porch is out front. I also can’t belive how much moisture is accumulating there considering the porch is covered.

If I were to paint everything I can access with mold resistant paint, keep the mesh screen against the OSB, cut n cob xps (I have a lot left over from the walls), then foam seal the edges....then stuff rock whoop in the cantilevered hoist bays, and finally put xps against those 1x that I was going to originally put the plywood, that would be good?

Thanks,
Fred