Exterior Detail Between Poured Porch/Cold Cellar and Main Floor?
CZ6A, Ottawa ON
Having trouble figuring out how to handle exterior wall details at the front entrance where we have a poured slab over a cold cellar meeting up with the house at the front door. The slab will only be an inch or two below the front door (no distinct step, as flush as possible), but the main floor joists will need to sit on the foundation ledge 8″ or so below the slab level.
How is this handled from a wall standpoint? Does the wall extend below the slab? Should the foundation wall be level with the slab and the floor joists be hung from hangers? Is there a way to make this happen while keeping durability and thermal bridging in mind?
I’m having little luck searching for these details. Any body have any cross sections to share?
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Replies
Lance,
Are you designing a new house? Do you have any help from an architect? Perhaps you are an architect?
Or is this house already built?
There are several issues here, including structural issues, thermal barrier issues (where the insulation goes), and water entry issues.
The idea of using an entry slab as the roof of a basement room is problematic. It can be done, but it's tricky. You have to remember that this concrete slab needs to be insulated and to be covered with roofing. It's a roof. Then you need a durable walking surface above the insulation and roofing.
Moreover, you need good flashing details to prevent water entry where this slab meets the house.
Finally, you have to consider continuity questions for your thermal barrier, so that your basement wall insulation and the insulation on this concrete slab are continuous with the insulation layer and air barrier on your above-grade walls. All in all, it's a lot to detail right.
If you want a cold cellar room, it's usually easier to make such a room within the footprint of your rectangular basement than it is to create a foundation bump-out topped by a slab that acts as a roof.
Lance, I had a similar setup on a previous house I owned, it was a pain in the a$$, Martin is right flashing details are very important otherwise your going to have what I had , damp cold storage, good for nothing.
Eventually the slab above the room got a crack in it, which made things even worst. It was in Toronto, so freeze thaw and salt made it a pain. The original idea for that room when the house was built was coal storage, then the oil tank was placed there, finally I thought I would use it for storage. Not a good idea, I spent more time fighting water/mold then what it was worth.
Do what Martin recommends build it in the house.
I dont have a cross section for you, but did one with the foundation all at the same elevation. Integrated with the front door flashing and adjoining WRB was ice and water that rolled onto the metal decking and house. Metal decking over the porch which was covered, concrete was pitched and flashed at top of slab. The foundation was open to the house so it had a footing and basically an exterior 2x6 bearing wall to pick up the house load.
It was a terrible detail, it has numerous weak points. the builder framed a 2x4 ceiling with fiberglass batts and poly with drywall finish then the owners used it as an art studio, it had radiant tubes in it and was cold, so she added electric heat and they had condensation filling up the light fixtures within 2 weeks of living there, air leakage was frosting up the steel deck. After sealing the gaps around the light boxes they havent had any visible issues in 2 previous winters, as of last fall, but Im sure its got issues festering. The only thing helping is they are snow birds so most of the winter the indoor temps are low.
I also did a treated wood pitched cap around 2006 for wine, that also was same elevation foundation with a 2x6 bearing wall under the house. That one has held up fine and can be inspected through a suspended ceiling. If I remember we ice and watered that whole thing prior to forming and pouring the slab.
This is zone 6/7 border.
Thanks everyone for your input! Martin, I'm just a guy designing his house (starting the build soon), no fancy credentials here! I know this would be a considerable extra expense for our build, so I wanted to make sure it's planned for properly and done right.
Wow, I never thought they were this bad! My Mom's place has this same cold cellar idea, and while it's definitely not a glamorous room by any stretch of the imagination,it works. It must be said, however, that hers is nestled in a U shaped cavity between the house and the garage, and is fully covered by the roof so it never gets anything more than snow from foot traffic on it.
Our design has a four foot overhang over the front entrance between the first and second floors, so if we did do this it would be fairly well sheltered as well, though not nearly as well as Mom's is. We were planning to have an extra piece of foundation extend on the front of the house and have a covered concrete porch. I figured if the foundation is already there, why not make use of the space below? Having a suspended slab, it turns out, is a pricey detail that would be several thousand dollars on top of just a poured concrete porch. I'm not sure why the massive expense, but that's what the foundation company added to the quote for it.
Martin, I was racking my head over how the details would work which is what led me to posting this question. It sure seemed complicated to me, and I guess my gut feeling was right.
Dave B, Mom's place is in Port Perry, just outside of Toronto (North of Oshawa). We're building just West of Ottawa. Mom's cold storage is definitely on the damp side, but as far as I know there's no mold. Her slab is covered and protected from precipitation though, which may be the difference.
T Carlson, what a mess! While I had planned to do a full dividing and load bearing foundation wall with just a door penetration, it still seems like it could be problematic. After thinking about it a lot more I had figured the poured porch should be just below the level of the main foundation to keep any water accumulation from getting to the sill plate.
Are there any concerns with doing just a covered concrete porch similar to the attached picture? Our townhouse has this right now and is a very popular detail (here in Canada anyway).
With regards to building a cold cellar into the basement, how do I go about keeping it cool relative to the rest of the house if I don't want to breach my thermal envelope or introduce thermal bridging issues? I would think perhaps leaving the foundation walls non-insulated could be done, but what if I end up doing an ICF foundation?
If I was you, do a covered porch but not with a room below. Cheaper, easier, less chance of water issues.
As far as a cold room in the basement what is the plans for the room, wine, food, ???
because if its food, you need to vent to keep air movement ,from what I've been reading. So the room would need to be well sealed from the rest of the house. The room would have insulation on the walls facing the interior including ceiling.
Why ICF, I'm only doing it because I plan on doing that part myself which makes it cheaper. Poured wall is cheaper, then find used foam to put against it.
Good Luck.
Lance,
At some point his summer I have to try and rehab a similar porch where the area underneath is used as a wine cellar for a restaurant, and I'm not looking forward to it. The room has black mold and crystals at all the intersections between the slab and the concrete walls. The floor regularly floods, threatening the adjacent basement rooms. None of th options are particularly palatable. It's not a good detail.
It can be done, it has been done. Theres no reason for it to flood or stay so wet mold grows, The weak point is the slab to house flashing and controling frost buildup. I think you could devise solutions for both.
@Malcolm,
You might find this project useful as a reference. Scroll down to "Porch Slab Against Wall."
http://www.primesouth.org/detailHouse/residential_detailHouse.html
Steve,
Thanks for the link. The main source on water infiltration is between the slab and the walls furthest from the house. Quite a bit comes in where the concrete stairs meet the slab too, and some seeps through the slab.
There is no doubt you could make the porch the OP is proposing work. My preference is to choose building assemblies that are forgiving so you don't end up with elaborate details that have to be executed perfectly or things go wrong. My other problem with the proposed porch is architectural. The quality of the space you end up with as a cold room is not very pleasant. It isn't going to be somewhere you enjoy being in, and I don't think it's a good idea to design unpleasant spaces into new construction.
Lance, I struggled with this same detail. My front door steps out onto a concrete slab with cold storage below. the slab is +/-8' out from the door, and fully covered by the roof, so bulk water should be minimal.
Attached is the detail we're going with.
Thanks to all for their excellent input!
Looking at the facts which trend towards high risk and high cost, I think I'm going to plan on having a cold cellar area within the basement instead of beneath the porch. With a couple tweaks to our design I've come up with a plan that looks much easier to build, and our basement is large enough that I don't think I'll miss a small area. If anything my mechanical room will get smaller, but it was oversized to begin with.