greenspree.ca at The Sietch

Heat rises. Such is the oft-quoted bit of wisdom used to justify preoccupation with attic insulation in houses and dismissal of the importance of basement insulation. This basic axiom is not entirely accurate however. Warm fluids (gases or liquids) will rise, this describes how heat moves via convection. Convection heat losses occur from leaks in a non airtight building envelope and in air exchanges through mechanical systems and envelope openings (door and windows). The proper use of air barriers and sealants eliminates or reduces this heat loss. This is not however the only manner of heat loss. Heat also moves via conduction and radiation.Conduction occurs when heat travels through solid matter, from an area of higher temperature to an area of lower temperature until a uniform temperature is achieved. This transfer of heat can happen in any direction, up or down, and always from hot to cold. This is the main source of heat loss in a foundation as the concrete foundation walls are in contact with the warm air inside the house and the cold ground outside the basement walls. The heat inside the house will want to travel tot he cold side of the foundation to achieve equilibrium, however the ground being an almost infinite thermal mass will continue to draw heat from the basement. The use of insulation, which is merely any material with a series of voids which breaks the thermal bridge used in conduction, stops or limits this transfer of heat.

Radiant heat losses occur when electromagnetic radiation is released from hot matter (the filament of light bulbs, element in electric heaters or combustion of fuel). This radiation travels in a straight line from it’s source and radiates out like light (which is a form of radiation as well). This radiation is absorbed at different rates by different forms of matter, it travels through air quite easily losing little of it’s intensity while our bodies would absorb more of it and reflective surfaces like foil or mirrors actually reflect and eliminate it’s loss. Normal insulation, being made up of high amounts of voids or air spaces perform poorly in stopping radiant heat losses while large thermal masses like masonry or solid wood absorb a lot of the radiant heat as it passes through it.

So you can see, heat losses are much more complicated than simply believing “heat rises”!  An effective strategy to combat heat loss in building MUST take into account all three methods of heat loss.  In our home we have an effective barrier against convective heat losses via a continuous stucco finish on the wall system inside and out and a sealed vapour barrier and air barrier in the attic space.  We are tight enough that on a still day if we have the range hood on we can’t light the wood stove without drawing smoke back into the house!  We have to wait till the stove is hot enough to create it’s own draw then turn the range hood back on.

To stop conductive heat loss our house has 14″ thick straw bale walls (est.R36), 21″ cellulose in the attic (R78), 2″ EPS foam under the slab (R8) and 4-1/2″ of EPS along the perimeter edge of the slab (R18).  R value is the measue of resistance to conductive thermal transfer, typical new construction on exterior walls has an average R value of about R16 and R40 in the attic.

To stop radiant heat loss our house has thick thermal masses in the slab (8″ of concrete), walls 2-6″ earthen stucco and since our largest radiant heat source (our wood stove) is on the first floor we have the second floor system (4×8 solid wood joists at 19″ centers and 1-1/2″ solid wood decking) and the second floor solid wood ceiling (3/4″ thick) to absorb most of the radiant heat released in that direction.

So if you are building a new home or renovating an existing one, make sure you consider ALL the ways heat is transferred and whether your building plan takes them into account, your wallet will thank you with lower energy bills!