Hybrid Questions and Answers Involving Foam

Kelly Hart is your host at greenhomebuilding.com, and has built his own home using a hybrid earthbag/papercrete technique, which can be seen on the Earthbag page. He has adapted the concepts popularized by Nader Khalili and his "superadobe" building, by filling the bags primarily with crushed volcanic rock. This creates insulated walls that are similar to strawbale, except that they are completely impervious to damage from moisture, insects or rodents. The earthbags are plastered both inside and outside with papercrete. Kelly has produced a video, titled Building with Bags: How We Made Our Experimental Earthbag/Papercrete House, which chronicles the adventure of building this house, and shows other earthbag houses as well. Another video program that he produced is A Sampler of Alternative Homes: Approaching Sustainable Architecture, which explores a whole range of building concepts that are earth friendly. One of the homes shown in this video is a hybrid strawbale/wood framed home. Kelly spent many years as a professional remodeler, and is available to answer questions about what he has done, or consult about other hybrid projects.

Questions and Answers

Q: I am about to build a house in Thailand, where there are three seasons: hot, hotter, and hottest (100-110 F). A friend of mine built a house using bricks that are made of a mixture of cement and recycled styrofoam. He recommends a cheaper alternative for our exterior wall construction: a 'sandwich' made of two vertical layers of cement blocks with a recycled styrofoam in the center of the 'sandwich.' He has made one wall like this and it stays very cool. The white foam pieces he uses in the center are recycled lids for coolers. He uses two of them, back to back, for each piece of insulation, sliding them down between the cement block walls. My primary concern is toxic gasses in the event of a fire. But I would appreciate any thoughts you might share.

A: This sounds like an excellent strategy for building in your region. Isolating the two layers of thermal mass walls like that will keep the heat from migrating into the inner layer, while allowing the inside mass to remain cooler and help buffer interior temperatures. Your concern about toxic fumes in a fire seem like a minor likely problem to me. First of all, the masonry blocks obviously will not burn, and the relative lack of oxygen in the wall cavity would keep the foam from burning much either. Furthermore, tests done on SIPs that have EPS cores have shown that the foam does not tend to burn when encased even in wood fiber materials, and when it does, the fumes are no more toxic than wood smoke.

A: (Owen Geiger) I agree with Kelly's summary of your building system.  One additional consideration is ceiling insulation.  Most roofs in Thailand are not insulated and get very very hot.  If roof insulation is not addressed, the house will quickly over heat.  One good technique for the region is to fill polypropylene rice bags with rice hulls.  Rice hulls are fire resistant, don't attract insects, provide excellent insulation, and are virtually free in Thailand.  The bags can be suspended from the roof structure with wire, wood poles, etc. and covered with reed or bamboo mats, or plaster, etc. if fire is a concern.

C: Our house is going to be a one-story, Mediterranean style, U-shaped with a central courtyard. You are correct about most Thai houses lacking insulation. We plan to have ridge vents and soffit vents. We had planned to use conventional insulation, as well, but the rice hull idea sounds great. The living room & dining room will have a cathedral ceiling, so we would have to use conventional insulation there, with a ventilation chute above (linking soffit vents & ridge vent). The other rooms could certainly have rice hull insulation in the attic (in addition to soffit vents & ridge vent).

A: (Owen Geiger) Consider using natural building materials, such as adobe or pressed earth block, instead of concrete block.  If you're concerned about moisture damage to the exterior, consider using natural materials just on the interior. You can use the rice hull idea on cathedral ceilings.  One option is to build parallel trusses where the top and bottom chords are parallel.  Another option is scissor trusses.  Both create adequate space for the earthbags/rice hulls.  Be sure to leave an air gap above the earthbags to vent the hot air.