Using cargo containers as constuction units — concerned about dew point
We are building a community house for aid orphan children for a charity in Johannesburg, South Africa, with the use of international shipping containers. They are interconnected to form a housing unit for 12 plus children. We envisage using a high-density polystyrene boards internally for insulation.This board is plastered with a polyester fabric in the plaster to prevent cracking.
This gives the board rigidity with high impact strength. The board is used as a cladding to the internal face of steel container wall with a 10 to 15 mm cavity.
Has any of your advisors any experience with the use of containers for dwelling use? The containers are interconnected two storey high inclusive of windows and doors etc. Roof consists of corrugated iron sheeting with a 4 inch open cavity between top of containers and the underside of corrugated roof sheeting. the ceiling in the containers ate boarded with insulation.
We are concerned with the possibility of condensation forming within the cavity of the inside claddings.
Any previous information or recommendations would be highly appreciated.
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Make sure it has adequate fire protection: http://en.wikipedia.org/wiki/Sealand_Youth_Training_Center_Fire
Bernard: I admire you for your efforts to help the kids. I don't think you will find anyone here who has used shipping containers for houses, but I suspect the containers will last a long time. Do your best to air seal the rigid foam sheets w/ caulking and tape before you plaster over them. Best of luck to all of you. john
I lived in a steel shipping container for one year, in the mountains of Armenia near Stepanavan, when I was helping build housing in response to the earthquake of 1988. The container was insulated on the interior -- according to my memory, with thin fiberglass batts -- as you propose.
1. Exterior insulation is always better than interior insulation. Shipping containers aren't particularly big, and ceilings are low. Moreover, exterior insulation solves the condensation problem in winter.
2. Our shelters were temporary, so we weren't too concerned about durability or occasional examples of condensation.
3. If you follow your plan, do a careful job of air sealing on the interior to limit the chance that warm, humid interior air can contact the cold steel during the winter.
(I bet I surprised you, John Klingel.)
Holy Thermal Bridge Batman!
In layman's terms, the containers will get very hot due to the sun beating on them. Are you planning on AC? Your best bet is to insulate the outside with a plaster or adobe skin over the insulation. If you can't do that, paint them white.
"We are concerned with the possibility of condensation forming within the cavity of the inside claddings."
I think you're right to be concerned and however well you air seal the internal surface you can expect some moisture in either liquid or vapor form to find its way into the cavity over the lifetime of the building. The questions are:
1. will it do any damage and
2. can it escape rather than accumulate.
My two cents: the insulation material itself is unlikely to be damaged by small amounts of moisture. You don't say how the cavity is formed - steel hat sections might be better than wood furring strips to hold the insulation off the container wall. And from what I read of local climate conditions it seems you get enough sun to burn off accumulated condensate on a regular basis. If you have the opportunity to vent the cavity to the exterior this would allow the resultant vapor to escape before it does any damage. You just have to break the hermetic seal of the steel box: a series of small holes (3/8"?) in the troughs of the exterior skin at the top of the section where it is sheltered by the top flange should do the trick. Don't worry about drain holes at the bottom of the section. You should not get enough accumulated liquid water to become a problem.
PS - I wonder if the corrugated metal over-roof might have a greater overhang than appears on your rendering, to provide better solar shading and general weather protection. A minimum of 18" or so all round would not seem excessive?
• the condensate risk will vary by room usage, with bathing, cooking and sleeping rooms being the most obvious candidates. The public rooms with no plumbing and large areas of glass are unlikely to be a problem, so any treatment could be locally rather than globally considered.
• The treatment suggested above could be applied as a retrofit if problems occur rather than in initial construction. This approach presupposes that ongoing monitoring of cavity humidity levels is an option.