Thermal Bridging

Thermal Bridging

Thermal bridging generally occurs when there is a break in the building envelope, namely in the insulation. Admittedly, the Omni Block block unit design does have a small amount of thermal bridging (see illustration below). That said, the design of the block minimally allows heat from entering through the completed wall.

Most consumers and some industry professionals are only concerned with a product’s R-value (resistance to heat flow). Most do not understand “heat”. Heat has energy and looks to move towards cooler temperatures. This is a principle of physics. To illustrate; a hot beverage does not stay hot. The “heat” leaves the hot beverage to predominately cooler ambient air temperatures.

How Omni Block “Works” in the Summer

On a typical summer day the sun rises creating solar heat that hits the east side of an Omni Block wall (face shell 2; R-2.2) and granted, heat starts to move through the wall. The offset and constricted cross webs delay the heat flow. The first layer of insulation (R-5.3) greatly assists in this delay. The middle face shell (R-2.2) acts as a heat sink and may also realize some heat gain. The interior insulated cell (R-10) stops any remaining heat flow. In addition to these delays, the sun (solar heat source) has sufficiently risen and moved off the wall by around noon. There is no longer sufficient energy to drive heat to the interior surface.

This is where logic takes over; since the heat has been effectively stopped, how much more does it need to be stopped? This is all that anyone should really care about. Now the focus centers on the effective thermal mass that sits on the surface of the block.

The heat will still enter the building via other sources (windows, doors, ceiling intrusions, and lighting, etc.) no matter what the building’s wall is constructed with. In lightweight non-thermal mass wall constructed buildings this heat is effectively trapped. The wall R-value of other systems (wood frame, ICF, block furred, insulated and sheet-rocked, straw bale, hemp and others) prevents the heat from moving back out of the wall. The result that the thermostat registers the heat gain and kicks on the air conditioner to cool the hot air that has built up to uncomfortable levels.

Omni Block interiors are left exposed (even if you cover it with a thin coat of drywall compound to match the other interior drywall; see this link) and will absorb a tremendous amount of heat passively, without the inhabitant knowing that the transfer is happening. This adds to the comfortability of the interior spaces.

The thermal mass effect also allows for a residential owner to downsize the required HVAC (heating, ventilation, and air conditioning) equipment. Instead of using the commonly suggested ratio of 1 ton of HVAC for every 400 square feet of livable space (a ratio that dates back to 1950) Omni Block is routinely engineered to use a ratio of 1 ton for every 800 square feet. All HVAC equipment manufacturers (Trane, Carrier, etc) are all allowed to run their equipment for 20 minutes before the SEER rating is taken per ASTM testing criteria. The manufacturers are telling the industry (nobody is listening) that the equipment is meant to run and to run for long periods of time. Smaller equipment draws less BTU’s and is significantly more efficient and the interiors are more comfortable. Another logic point, illustrated by the analogy that automobiles get better miles per gallon on the highway than in stop and go situations. Time your own HVAC system. It is probably around 6 to 8 minutes meaning that your system does not get close to the advertised or expected SEER (seasonal energy efficiency ratio) rating.

How Omni Block “Works” in the Winter

During the winter months the block’s thermal mass will absorb a thin layer of heat (ambient air temperature does not have the energy to drive any heat beyond the surface) from the heat source (furnace, fire place, wood burning stove, etc). This thin layer of heat is still heat that will leave the surface if the interior of the building begins to cool or there is a draft situation.

Heat always chases cool, but the mind can play tricks. In the winter, when the interior of the building is nice and warm and the outdoor temperature is frigid cold, and a window or door is opened, what do you feel? You no doubt feel a blast of cold making you think that the frigid cold is dying to get into the house when what is actually happening is that same amount air exchange is the heat being mandated to escape. Understanding heat is paramount to understanding how Omni Block works.

The heat exchange off the surface of the thermal mass is something that is subtle, but adds to the comfortability of the interior space.

Final Thoughts

Omni Block System 8 has an R-19.6 rating (see Table). There are many systems that have higher R-values. If you are looking at the highest R-value and you stop there, Omni Block will probably not be your selection. R-value is an important piece of the overall energy puzzle, but does have a law of diminishing returns. There are a lot of information sources that puts that value at around an R-19.  Sure, you can spend a great deal more and add more R-value but will you be getting any value add?

A good illustration is sun tan lotion’s SPF (sun protection factor) rating. Most people use SPF 30, maybe SPF 45 and there is a baby product out there that is SPF 70 and who knows maybe one that is higher. The point is that most use SPF 30 to 45 and none of use SPF 500. Why? Because SPF 45 works very well and protects our skin from harmful UV rays. The question then becomes if you effectively stop the harmful UV’s with SPF 45, how much more SPF do you need? The same is true with R-value. Once you effectively stop the heat from moving through the wall, how much more do you need to stop it?

The company Omni Block is about getting accurate, reliable information into the marketplace. Therefore, we have developed an interactive “correction factor” web page. Obviously, where there is rebar and grout called for in the cells of the block there is less insulation (R-value). The Correction Factor prompts you to enter your specific data to see how the rebar and grout frequency affects the overall R-value. Omni Block is the only wall system that accurately provides this correction. Wood frame builders always promote the center of cavity R-value (best case scenario) and do not take into account the amount of wood studs in the wall assembly.