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Community and Q&A

What the heck do I do with this room?

cmag | Posted in General Questions on

My 100 year old home has a cold storage room off of the basement which I would like to bring into the building envelope. The previous owners put rigid foam between the studs and ceiling joists, then put up drywall, which had turned into a moldy mess by the time I bought the house (they did little to no air sealing, which I’m assuming was the problem. I’d like to spray foam the thing, then finish it, but I’m wondering what to do with the ceiling–It’s just the tongue and groove porch flooring sitting directly on the ceiling joists. The porch gets little to no rain on it as it’s well sheltered, but if someone takes a hose to it, the water will definitely drip through. I don’t think I can afford to re-do the whole porch at the moment, so do I just waterproof the underside in some way and hope for the best? Should I look into one of those under-deck roofing systems? Scratching my head here . . .

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  1. GBA Editor
    Martin Holladay | | #1

    I don't recommend your attempt to bring an under-porch room inside your thermal envelope. This room belongs outside your thermal envelope. I would rip down the ceiling and moldy materials and throw the materials away. Then I would install a good quality insulated exterior door to separate this cellar from my basement.

    This is the foundation under your porch. It's not a basement. Leave it outside.

  2. cmag | | #2

    Thanks, Martin--

    Currently there is no source of ventilation in the room. Should I add something, maybe a bathroom fan? I can live without the space being insulated, I just want to lower my mold risk as much as possible.

  3. GBA Editor
    Martin Holladay | | #3

    Ventilation often adds moisture to this type of room rather than removing it.

    I assume that the walls are concrete, right? This is the foundation under a porch. It's outdoors, as I said. If you remove the gypsum drywall, there won't be much mold food available. And if there is a spot of mold on the concrete occasionally -- who cares? I feel like I need to keep repeating: It's the foundation under your porch. Leave the door shut. Better still, lock the door so you aren't tempted to peek.

  4. cmag | | #4

    Well I kind of care if there's mold in there since no door is perfectly air tight and we use the basement quite a lot. Furthermore, one of the four walls is a shared stud wall with the rest of the basement--the foundation for the porch is contiguous with the foundation for the rest of the house. I'm not sure sealing this spot off is that straightforward.

  5. GBA Editor
    Martin Holladay | | #5

    If you want to monitor it for mold after removing the moldy materials, go ahead.

    To keep the area dryer, treat it like a crawl space. Seal it up. Improve grading around the porch to make sure moisture flows away from the house.

    If you really want to invest $$ to make sure your porch foundation stays dry, recurring mold can be tackled by installing a dehumidifier under your porch. I think that would be nuts -- especially since there is no ceiling, only leaky boards -- but you can do it if you want.

  6. GBA Editor
    Martin Holladay | | #6

    And by the way, it sounds is if your house is a classic example of a house without a clearly defined air barrier. The designer and builder had no idea what they were doing.

    Every house needs a clear, defined thermal boundary -- a boundary with insulation adjacent to an air barrier.

    The room under your porch has leaky boards above it, so it's outdoors, clearly. But that room needs to be separated from your basement with an airtight, insulated wall. It sounds like the builder forgot to include that airtight, insulated wall.

    If the builder were standing in front of me, I would ask: Where do you think the thermal boundary is?

  7. cmag | | #7

    The house was built in 1914, so I don't think that opportunity will present itself . . . So it sounds like I either need to turn the porch floor into a proper roof before spray-foaming it, or turn the interior shared wall into an exterior wall.

  8. GBA Editor
    Martin Holladay | | #8

    Yeah, I guess I won't ever get a chance to chew out your builder.

    If you want to build a roof over this room, you would have to remove the porch floor and install a real roof -- for example, a membrane roof with EPDM. Then you could install sleepers made out of rot-resistant wood, and you could reinstall your flooring. If there isn't much elevation difference between your door threshold and your porch deck, this could be tricky.

  9. GBA Editor
    Martin Holladay | | #9

    The other alternative you listed -- "turning the interior shared wall into an exterior wall" -- is certainly possible. Ideally, this would be a concrete block (CMU) wall on a concrete footing, to emphasize the fact that the wall denotes where your basement ends and the outdoors begins. This CMU wall would be built in an airtight fashion and would be insulated.

  10. charlie_sullivan | | #10

    Martin will have a busy afterlife if his agenda includes chewing out all the builders who have built low-performance buildings.

    I would think that a layer of EPS on the "exterior" side of the wall that is about to become an exterior wall could do the trick: keep the wood in the wall warm enough to avoid condensation potential and mold risk, and build the exterior part out of a material that doesn't support mold growth. But if that wall is already moldy and needs to be rebuilt, CMUs are a better choice.

    I don't think we heard what your climate is--that would affect details like the EPS thickness.

    If you are worried about breathing mold spores that come through leaks in that wall or up through the porch floor, I think your best bet is to thoroughly clean the concrete so that there's nothing under the porch that supports mold growth. You can also spray it with a mold inhibitor after you clean it. The trick would be doing that cleaning without getting more nasties airborne.

  11. cmag | | #11

    Other than this little oddity (which I'm sure served some sort of purpose in its day), it's actually an incredibly well-built house. I'm in Seattle, and the old-growth timber that makes up the framing is pretty indestructable. I'm actually leaning toward converting the ceiling into an actual roof, although I'm not sure how to find a competent contractor to make that happen. It's a decently big room, and having dry mold-free storage would be pretty wonderful.

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