Garn cave

[SolarAndWood]

My house has a lower level walkout basement. At one end of the house, the far end of the pic below, is an old deck that is 15x22 with 15 on the walkout side. I am considering excavating the area under the deck out for a Garn cave. Two sides would be masonry against earth. One stick wall shared with the existing lower level with a door for inside access. The fourth wall would allow the boiler to slide in and have a large door probably big enough to stick the 5' bucket of my tractor through. The boiler would also vent through this wall. The ceiling would be concrete poured over a steel deck.

My biggest concern is that I am putting a big piece of steel that I plan to have for a long time in a concrete room with little ventilation and potentially not that dry. Second, the man cover access would have to go through the concrete deck. I think I can hide this with a table of sorts on top and from what I've read here I am not going to need access to it that often anyway.

Anyone think these are show stoppers? The big benefits are interior access for tending, exterior access for fuel, location is in the geographic center of the heat load and a perfect place for the dhw tank and I can vent horizontally.

 

[benjamin]

It looks like a nice solution to me.

The moisture issue can be dealt with. Tempurature is the biggest factor on your side. Condensation would be unavoidable if the water is allowed to cool off in the summer and there is any humid air leaking in. Any chance of wasting a little solar heat on keeping the garn a few degrees above the room temp all summer? Insulation and air sealing will help also. Assuming this faces south, why not find two 48" patio door panels for the door, or use glass for the rest of that wall.

The other possibility is using pressure treated wood for the two walls and a rubber roof, probably more expensive, but maybe easier to insulate. Although foam insulation on four sides of the masonry would give you a nice moisture buffer.

 

[dogwood]

Could you post a picture of the location where the deck is, and where you are going to excavate? I am picturing you building something similar to a root cellar up against one side of your house with the door side exposed for the Garn to go in, same side as your white doors and windows, with the grade sloping up towards the front of your home. I used to and may still have a plan for such a stucture (root cellar) ventilated to keep the food stored on the inside dry. Insulating and sealing the masonry walls on the outside may cut down on moisture from condensation and the dampness that many basements are prone to. I think I'd insulate on the inside too for the same purpose.

Mike

 

[SolarAndWood]

The house is kind of flat U-shaped with the pic above being the bottom of the flat U. I did 2/3 of the trusses last year that you can see in the pic. That existing deck isn't on the blue print but is on the main level of the house to the left of the long row of windows and in front of the double high set of windows. The windows on the walkout level in the first pic aren't on the blueprint.

The thinking with the concrete as opposed to roof is that I will end up with a masonry terrace where the existing deck is. With only 15' to play with, I am thinking the Garn would take half the exposed wall and the other half would be a double set of hinged doors. Maybe ICF the walls and put a couple inches of foam on the ceiling.

 

[Gooserider]

If this is an existing concrete deck that was built on solid ground originally, I'd have doubts about excavating under it to that much of a degree - it probably doesn't have the support or reinforcement to serve as a roof w/o all the ground under it that was there when it was poured.... I would certainly want to talk to a GOOD structural engineer before doing anything like this...

That said the idea itself sounds great - I'd be more inclined to excavate the area, junking the concrete deck, and replacing it with a wooden deck that is designed to double as the much lighter and appropriately sized / designed ceiling of the "Garn cave", w/ built in access for the manhole cover, appropriate ventilation / insulation, etc... I think it would be much easier and possibly less expensive than trying to preserve the old concrete deck.

Gooserider

 

[SolarAndWood]

Glad to have you back GR, best wishes on your recovery.

The existing deck is wood with concrete pier footers. The earth is just below the frame at the top side and probably 6' below at the bottom graded roughly linearly. I would demo the existing deck and excavate the entire area, pour new footers for a frost wall and then either block or ICF on top of it. The concrete deck I am proposing would be the ceiling for the cave. I would need to frame in the man hole access through this new pour.

This line of thinking started as I need to demo the existing building and dig a new foundation to support the new trusses over the wall with the double row of windows. The cave won't be too bad to include in that project but I doubt I would ever do it on its own in the future. This seems to be the only feasible Garn location on my suburban lot without significant structural work and a vertical stack to get it in the existing lower walkout level. And once its in there, it would be stuck in there as opposed to this configuration which would allow me to slide it out if needed.

 

[dogwood]

Can visualize your project better now with pics. As Gooserider suggested getting some expert advice would do you no harm. The roof, if exposed to the weather, will tend to crack over time and leak as concrete does. If it were me I wouldn't fool with the concrete roof, and would put in a deck elsewhere. Location seems perfect however for your Garn. An exposed double-duty roof/deck out of concrete would be geting beyond my skill level and I'd anticipate unanticipated problems without some engineering advice on loads, construction, roof water-proofing etc.. Not to say it couldn't be done and I've no idea of your construction know-how. I'd be curious to find out what you end up doing and how you do it. Keep us posted. You've got an interesting idea there. Nice looking home too. Those trusses must have been a bit of work.

Mike

 

[SolarAndWood]

The trusses were entertaining, good friends and good beer. I built a bridge from the side street and we walked them onto the flat roof and stood them up.

A couple of guys and I just did a concrete deck over a 12x24 vault in a 4 car garage; it isn't really that different than pouring a patio. The real question is if the effort/cash to make the Garn work is worth it when a downdraft would go in the existing lower level without modification.

 

[begreen]

Garn aside, that is a really dramatic photo. Looks like there is already a major steam plant in the structure.

 

[dogwood]

Don't imagine it would be too hard to pour. It's the weathering and response to live loads over time that would concern me if moisture infiltration is your issue. It would be easy enough to seal if you didn't want the top side of the concrete exposed for aesthetic reasons. Maybe an impermeable membrane on the underside of the slab would do the trick to keep moisture out. Good luck.

Mike

 

[RobC]

Doogwood followup:
I also feel that the concrete ceiling / deck could be problematic. I'm not sure exactly what your design for the cieling is but....
Isn't the metal decking that they pour concrete on used for interior floors on office parks where temps are relatively stable ?
I landscaped a rooftop lawn that had a heated garage under. In that scenario the concrete was foam board insulated on top, covered with EDPM membrane, and topped with some sort of wicking material(s). Then 6" loam and sod. The concrete below all this was the pre-stressed T beams that you see in municipal garages. ( correct budget was not an issue ).
What I'm getting at is concrete doesn't fair well in cold climates when exposed, even as a patio or walk etc. What your proposing is eliminating the stability, structural & temperature, that being on the ground offers. Being in contact with the ground or in my rooftop lawn the concrete was protected from temperature swings by earth mass.
As also mentioned you could look at wood construction for ceiling with a deck overhead you could build with a very low pitch. Wood wood breathe and do better with the temperature differential of boiler room and winter weather.
Great cave placement where you can drive tractor to door.
Beautiful home and location too.

On another note: Goose it's really good to read your words again. You were definitely missed. Best wishes for a continued recovery.
Sincerely, Rob.

 

[SolarAndWood]

Thanks for the kind words and sharing your thoughts on this. Going with a downdraft gasser/propane tank for storage setup just inside the door on the right in the first pic is probably a lot easier, cheaper and carries less risk.