Finishing Gypsum Walls

[bbradsby]

The naturally-occurring convection air currents in the occupied space evaporate any moisture that forms on the ceiling surface. It's trapped moisture (inside the wall system) that just sits and grows mold and destroys the gyp board and damages finishes. The ceilings stay pretty dry in the absence of a roof leak.

In hot climates, rain or humidity is absorbed into the exterior concrete/plaster/block masonry material like a sponge. Then solar insolation or just ambient exterior tropical heat drives the vapor through the wall toward the relatively drier & cooler inside air until it hits a surface with a temperature at or below the dew point. Thats where it collects. If it's inside a non-ventilated cavity (gyp on block wall), thats the problem. If it condenses on the inside finished surface (your typical, simple block & cement render wall) where the air currents evaporate it.

To minimize moisture problems, place insulation on the warm side of the wall assembly, as far as possible. In the tropics, that is the exterior side and outside the blockwork is ideal. Of course, no one does that, since building science is not practiced in LoS - just layperson knowledge that is passed from generation to generation. Was not a problem long as houses were built from tropical rainforest woods which are naturally resistant to fungal & insect attack. The problems started when all the forests were cut down and concrete block was introduced late in the 20th century. The folk techniques are not adapting, and the universities here dont appear to be developing any building science programs. Its all coming from the Western universities, institutes, and Western-influenced or managed local building material manufacturers. QCon would be an example.

[wpcoe]

Some of this is hard to follow, but I *think* I understand why they told me not to layer gypsum/drywall and insulation at the ceiling level: My particular application was to add the layers of gypsum/insulation beneath an existing concrete roof/ceiling, not beneath an open air cavity. What I was proposing would be similar to putting gypsum/insulation on the interior side of a plain, uninsulated brick exterior wall, no?

[bbradsby]

Yes, it would condense moisture on the cool side of the assembly if the existing concrete above does not have insulation above it.

Is the existing roof/ceiling assembly a low-slope concrete slab with a roofing membrane on top? If so, you should think about an (google it) 'inverted roof' assembly with extruded polystyrene insulation over your existing roof membrane, held down with basically a big net and gravel ballast. That will keep the mass of the concrete nice and cool, working for oyu versus against you as it is now. Just make sure your engineer/architect would agree to the added weight of the ballast needed to keep the exterior roof insulation from blowing away in the wind. The attached detail shows a green roof, even cooler as it provides evaporative cooling, if lots more weight; all you need is ballast.

Or just go with rigid insulation on the concrete deck to insulate the mass, cover it with roof sheathing, then do a torch-down single-ply roof.

Edited April 6, 2014 by bbradsby

[krisb]

Using fibreglass insulation is fine, tontine is also good which is the stuff you find inside pillows. Never over stuffthe walls with insulation, it should sit snug in the wall cavity, and not bulge out, or the gyprock sheets will want to pop.

CInemas for ie use 4layers of 16mm gyprock on their ceilings for sound proof. This can be done in any house if you really want, but 2 layers is plenty.

If you guys ever need any steel frame, suspended ceiling, insulation or gyprock advice, feel free to pm me anytime.

[transam]

Termites just love gypsum.

[wpcoe]

Yes, it would condense moisture on the cool side of the assembly if the existing concrete above does not have insulation above it.

Is the existing roof/ceiling assembly a low-slope concrete slab with a roofing membrane on top? If so, you should think about an (google it) 'inverted roof' assembly with extruded polystyrene insulation over your existing roof membrane, held down with basically a big net and gravel ballast. That will keep the mass of the concrete nice and cool, working for oyu versus against you as it is now. Just make sure your engineer/architect would agree to the added weight of the ballast needed to keep the exterior roof insulation from blowing away in the wind. The attached detail shows a green roof, even cooler as it provides evaporative cooling, if lots more weight; all you need is ballast.

Inverted roof pic.jpegInverted roof section.jpg

Or just go with rigid insulation on the concrete deck to insulate the mass, cover it with roof sheathing, then do a torch-down single-ply roof.

Thanks, so much. Yes, the traditional roof is a low-slope concrete slab with membrane on top. I think the green roof might be a bit much, but the initial idea without the plants sounds good. I'm house-hunting and looking for an old colonial home (this is the Yucatan in Mexico) to restore/remodel, and I will have the soundness of the existing roof before assessed I buy (depending on how much the house costs, I might not flinch to totally replace the roof, or sections of it) and consult an architect to see if it will support the "inverted roof" is an option.

I will file this away, as with my memory I will never remember this in a month or two.