I am interested in constructing metal frame houses in Brazil. I want to use 6 inch metal frame walls with R-19 insulation. Covered inside and out with Plaster. The roof will be metal truss with R-30. Typical construction is 4″ clay block with concrete post and beam. There is no insulation and the windows are non sealing. This is a very crude and long process of construction. The area I want to build is fairly dry and avg temp of 85 deg. I would like to use sealed windows and AC. Does someone have a reason why not to construct in this manner? What is the thermal value diff. between fiberglass insul and the clay block filled with conc? Any advice?
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Replies
The main problem with contruction as you described is that it is very susceptible to low levels of moisture. Masonry and wood construction have relatively high moisture capacities. By that I mean these hygroscopic materials will absorb a lot of moisture in a humid environment and relaese a lot of moisture in a dry environment. This storage capacity is a good property and helps to prevent normal swings in humidity from causing "wetting". When moisture in a building is in the liquid form either from leakge or condensation, problems occur and "things" grow, and the AC system is much less effective at removing the moisture. What all this means is that the vapor barrier and environmental control (AC) measure must be well done.
A pearlite filled, 8" concrete block has an R value of 2.1. A 2x6 wood framed wall (16" o.c.), with R-19 fg batts, will have an R value of about 15.2. With steel studs, about 14. Couting inside, outside surfaces, an air gap and 3/4" plaster add about 2.5 for the overall R value of the wall.
"Fairly dry" means little. You need design conditions and maximum wet bulb temeperatures to have someone design an AC system that will work.
Advice would be to design your building and have an engineer design the HVAC system to accomodate the construction and the environment.