Home on slab... but with a small cellar accessed by trap door in the kitchen?

Hi folks,

I'm planning a small passive solar home, on a slab. The home is 24ft x 32ft. We're about to submit our plans to the drafter, but I'm REALLY wanting a cold room/cellar. I built one in the north east corner of the basement in our current home, and I LOVE IT (it stays 3-6'C in the winter, and about 10' in the summer- like a huge free walk-in cooler!). I do not want a basement in the new home for a lot of reasons, and although I know I can build a outdoor cold cellar, I would really, really prefer one with kitchen access for the convenience. I'm envisioning a trap door in the kitchen floor with a steep stair set (or like a ships ladder) down into the room below.

Is it possible (and by that, I mean relatively economically feasible) to dig an 8ft x 10ft or so hole, use forms and pour concrete for the walls, and then create and poor the rest of the slab? The "ceiling" of this sunken cold room would not need have slab above it (it is on the North side of the home, wont get direct sun so doesn't need the thermal mass) but rather just joists/framing to support a plywood or tongue and groove subfloor for the kitchen above.

Has anyone done something like this? Am I being optimistic in terms of the added cost of this cold room compared to the relatively affordable option of simple slab on grade/rubble trench? Or is this a reasonable goal to pursue further?

Thanks for any thoughts/insight/experience!

Welcome to Permies Andrea!
Interesting idea.
My two cents is a cold basement under a tongue and groove floor will make for a serious cold spot in the house. If you are in a hot climate, that might be neat, if you are in a cold climate, you may not like it.

Pearl Sutton wrote:Welcome to Permies Andrea!
Interesting idea.
My two cents is a cold basement under a tongue and groove floor will make for a serious cold spot in the house. If you are in a hot climate, that might be neat, if you are in a cold climate, you may not like it.

Oh yes- the ceiling above the cold room/kitchen floor would be highly insulated with a vapour barrier etc, just as external walls would be. We've done it up this way in our current home and it works great- the floor of the room above our current cold room isn't any different than other areas of the home (our current home has an unheated basement, that is otherwise used for laundry and storage, and is insulated R-24 but is quite cool. We use a wood stove on the main floor 90% of the time, and the oil furnace (which is in the basement and does have a duct in the basement too) only comes on if the fire goes out because we're away for the weekend). I do think  putting a sunken/underhouse cold room via trap door would potentially be a trade-off with some heat loss in an otherwise tight and efficient home, and could potentially impart some humidity considerations- it woudn't likely be perfect.

Now my house is a typical build here and therefore does not have concrete under it, but we do have a cellar, it's got vents (and windows) to the outside and holds around 10C most of the year climbing up to 15 or so in high summer. BUT and this is a fairly big but, the floor over it is freezing on the feet, part of that is actually deliberately used as one of the kitchen cupboards that sits over the cellar has racks in it for storing vegetables as it's so much colder there, but I can say that standing at the counter is not nice on the feet. My point is to make sure you insulate the cieling of the cellar or you will freeze your feet.

What about ventilation? My cellar has two small windows one of which has an always open vent in it.

I have seen old-time homes with a kitchen trap door and root cellar underneath. This was very effective before refrigeration was available.

The only caveat I can see is the possible buildup of radon. Old houses had lots of accidental ventilation (they were drafty). A passive solar house would likely be tightly sealed. I would suggest investigating further before doing this.

Having read all the posts,I suggest you pour a concrete ceiling over the cellar when the overall slab is being poured.
Steep stairs may be difficult to use when carrying food up or down.
I suggest after excavation, pour footings for a hollow block wall, use reinforcement rods inside the blocks and then fill with concrete.
When the floor slab is poured, step it down a bit over the cellar and that step will help prevent to walls being pushed over by the soil.

Also, keep the open earth on the floor. Maybe have a wooden open slat floor, or gravel.
Include vent pipes through to the outside and maybe one up through a wall in the house to create draw if its needed.

Radon investigation would be a good starting point, but if that does not prove to be an issue, I would start with a floor slab in the cold room (no deeper than you need for headroom) and put up a concrete block wall to main floor slab level. Many concrete companies have minimum batch delivery sizes and you would not have to pay a premium for a small amount of concrete. Interior basement walls that are relatively compact and not subject to frost heaving do not need to be reinforced concrete. Running vertical rebar in a few cores spaced across each wall and filling those cores with concrete will give plenty of strength for insurance. I like dry stacking block and surface bonding on both sides - as strong as ordinary mortared joints and much faster. Then backfill around the finished cold room walls (with vapor barrier) and pour your main floor slab.

Glenn, many root cellars have open floors and no moisture barriers to ensure a high humidity can be sustained in the cellar.
From; https://commonsensehome.com/root-cellars-101/
"Make sure fresh air can get in, stale air can get out, and air can circulate around the produce.
Earth-shelter: The soil insulates and maintains a cooler temperature.
A packed earth floor or gravel floor is better than concrete for keeping moisture (humidity) levels higher.19 Feb 2015"

Since I am not experienced with root cellars yet, I will bow to accumulated knowledge about floors and humidity. I presume you would want moisture/vapor resistant materials to keep humidity from affecting the floor structure above.

I feel like some kind of walkout, even if you still do have access from inside, would be beneficial. If I were to dig a room under my house I'd like the options that would give me for access and ease of natural air circulation. Is that possible or does that make sense in your setting? Ideally the whole floor would slightly slope to that outward access area in case any weird water issues. Construction would be hollow concrete block walls w/ footers and re-bar thru like already mentioned and a gravel floor (wood planks over it maybe in some sorts). I was wondering about filling the blocks with gravel too or maybe even concrete the 1st/2nd base row in?

I'd have to excavate/build mine one wall at a time since my tiny house sits on blocks as it's support already, and might still need 1 or 2 block pillars in the middle of the room from ground to underside of house floor.

I guess it's always good to multi-purpose? Storm shelter comes to mind...my plans involve using the existing underside of the "shed/house" as my ceiling, which isn't ideal if a tornado took it away. But, underground, up against the earthen/block wall is better than nothing.

Andrea Munroe wrote:Hi folks,

I'm planning a small passive solar home, on a slab. The home is 24ft x 32ft. We're about to submit our plans to the drafter, but I'm REALLY wanting a cold room/cellar. I built one in the north east corner of the basement in our current home, and I LOVE IT (it stays 3-6'C in the winter, and about 10' in the summer- like a huge free walk-in cooler!). I do not want a basement in the new home for a lot of reasons, and although I know I can build a outdoor cold cellar, I would really, really prefer one with kitchen access for the convenience. I'm envisioning a trap door in the kitchen floor with a steep stair set (or like a ships ladder) down into the room below.

Is it possible (and by that, I mean relatively economically feasible) to dig an 8ft x 10ft or so hole, use forms and pour concrete for the walls, and then create and poor the rest of the slab? The "ceiling" of this sunken cold room would not need have slab above it (it is on the North side of the home, wont get direct sun so doesn't need the thermal mass) but rather just joists/framing to support a plywood or tongue and groove subfloor for the kitchen above.

Has anyone done something like this? Am I being optimistic in terms of the added cost of this cold room compared to the relatively affordable option of simple slab on grade/rubble trench? Or is this a reasonable goal to pursue further?

Thanks for any thoughts/insight/experience!

Hi Andrea, Doable? Yes for sure. Reletively inexpensive... Nope. Usually small slab on grades like yours would fly right through permitting with standard cookie cutter engineering. Slabs work as a single floating unit... When you modify it you will need to get an engineer involved to size out any extra reinforcing required as well as the cement beams for the now suspended slab above the coldroom. To do it you would excavate the room, pour footings, form up the walls, pour them, strip it out, insulate, moisture proof it then backfill and compact ,then form the ceiling and beams, then move on to the slab tying in all the rebar from the walls. If it was me I would pour and burry in a mound a small building or build a mini full basement for a shed. Less headache and less separate pours ...
Cheers, David

I do not agree with your pessimism David.

When you modify it you will need to get an engineer involved to size out any extra reinforcing required as well as the cement beams for the now suspended slab above the coldroom. To do it you would excavate the room, pour footings, form up the walls, pour them, strip it out, insulate, moisture proof it then backfill and compact ,then form the ceiling and beams, then move on to the slab tying in all the rebar from the walls.

None of these issues are a major issue, moisture proofing is not needed, any additional reinforcement and tying can be easily handled, additional design work will be minimal.

John C Daley wrote:I do not agree with your pessimism David.

When you modify it you will need to get an engineer involved to size out any extra reinforcing required as well as the cement beams for the now suspended slab above the coldroom. To do it you would excavate the room, pour footings, form up the walls, pour them, strip it out, insulate, moisture proof it then backfill and compact ,then form the ceiling and beams, then move on to the slab tying in all the rebar from the walls.

None of these issues are a major issue, moisture proofing is not needed, any additional reinforcement and tying can be easily handled, additional design work will be minimal.

did it come across as pessimism? That wasn't the goal. Maybe.

Thats Ok Dave, I see opportunities for action everywhere compared with others.

What about sinking a large road-grade culvert in the ground, vertically, as the structural walls of a cold room? I have seen this done for water wells with packed gravel crush all around, and it was strong and stable. Then pour the slab around it. This approach is also used for pipeline valve vaults in industry.

Also, is it really necessary for a person to climb into the cellar? Could several narrow culverts be used in a "dumb waiter" style? It would reduce cost and distribute the forces much better, with packed gravel between them, with less risk of cracking the slab.

If you make it round it will require less material and should be better at holding back dirt.
I have a partial basement in my old house.

I heat with wood and on cold nights like this I roast the place up over 100F.
A thermoswitch starts a fan in a duct running from the ceiling of the hot living room over by the burner
and blows the hottest air into the basement where it
stratifies up to the wood ceiling of the basement.
Keeping the floor and living room warm through the night.
Water pipes are also run high in the basement and it helps keep them from freezing.

The basement floor is still cold enough to store food.

Hi Andrea, welcome to Permies. I know a vapor barrier is imperative as you and all mentioned. Good luck with your new home build, sounds lovely.

Could you share what you did in your current home basement, with instructions by any chance? I have planned one, and almost bought a used walk in at one point, and commercial steel table was free but could not get up to fetch it. I would love to see your ideas and implement them in my northeast corner of mine for next year or another place I hope to get a farm etc.  Thank you for your time.

Craig, very clever arrangement, well done!