R-values, U-factors and Thermal Mass
Easily 80 percent of the comments and questions that I receive in response to this column pertain to the thermal characteristics of adobe. The inquiries usually relate to insulation as it affects the plaster, specifically mud plaster, on the exterior.
It is the norm now to spray polyurethane on the outside of adobe walls, thus making it necessary to lath the building to receive whatever plaster is contemplated. Were the walls left bare, a mud plaster could be applied directly over non-stabilized adobe. But, the inquiry continues, if you don't insulate with polyurethane, what should you use? In my view, nothing -- despite the current building code.
Heat is transferred in three ways: It is conducted through a solid material by the direct contact of molecules energizing their neighbors (as in the iron skillet on the stove); it moves by convection, wherein heated molecules move from place to place (warm air rising); or it can travel through space electromagnetically in the process of radiation (the campfire that still warms on a windy night).
Conductive heat transfer is measured, at least in the building trades, by the R (resistance to heat gain) value of a material. The higher the R-value number the more effective the insulation. We know that dense materials get hotter faster than less-dense ones; that's why we use oven mitts to grab the hot pan. Cotton, being porous, is more resistant to conductivity than iron. The U-factor is a measure of the flow of heat-thermal transmittance -- through a material, given a difference in temperature on either side. It is the inverse of the R(esistance)-value. Good insulators have a high R-value and a correspondingly low U-factor. "Steady-state" heat flow refers to the ideal situation in which both sides of the wall are at constant temperatures, in which case -- assuming a lower outside temperature - conductivity will carry the heat out of the building at a predictable rate.
But materials have another property that can affect energy performance, and that is heat capacity. Heat capacity is a measure of how much heat a material can store. This really throws R-value calculations for a loop, the result being the introduction of yet another (scientifically soft) term, "effective R-value". The term attempts to define the ability of high-mass materials to achieve better energy performance than would be expected if only the steady-state R-value or U-factor of the material were considered.
In real life, inside and outside temperatures are seldom constant, thus the steady-state calculations themselves become bogus. If a particular climate is prone to dramatic diurnal temperature swings, the conductivity cycle can reverse in the course of 24 hours. In the Southwest, for example, the outside of a south wall may approach 80 degrees on a sunny January afternoon and by 4 a.m. be in the teens. In a high-mass wall these swings result in an "effective" R-value that is higher than the steady-state R-value upon which codes are based.
It is unfortunate that these rates have never been standardized, or even thoroughly examined, and therefore cannot be codified. Anyone who has lived in an un-insulated adobe house can attest to the benefits of the "mass effect" which results in amazingly stable inside temperatures with very moderate heating bills. (That is, assuming the builder followed traditional wisdom in siting and the judicious placement of openings.)
We do know that, depending on climate, high-mass walls can significantly outperform low-mass walls that have comparable steady-state R-values. However, if you are building code-compliant and insulate the exterior of the wall, the cycle is broken and the inherent advantage of thermal mass is lost.
Of course, the argument for exterior insulation is that the high mass is then free to warm up and stay warm because it is protected from the onslaught of cold air from outside. But the trade-off is the loss of the systems, both thermal and mechanical (the use of mud or lime plasters) for which unsullied adobe appropriately became renowned.
Unbeknownst to most builders, and architects for that matter, the building code in New Mexico can be met for adobe buildings without using an exterior insulation. We have built single wythe adobe homes without it by conferring with the folks at the Department of Energy, Minerals and Natural Resources who are eager to help find ways to manipulate design elements to compensate for the absence of insulation in the walls. By over-insulating the roof and floor, and by avoiding openings in the north wall and increasing them in south and west walls, a trade can be quickly negotiated that allows you can build un-insulated adobe and be code-compliant.
Of course, the ideal method is to build a double-wythe cavity wall whose thermal performance is second to no other building material.