I'm new to this. I am wondering and don't know the answer. Could someone explain to me:

If the R-Value, the temperature differential, the time, and the area are all the same for both a vertical wall and a horizontal flat roof is the heat loss thru each of them the same?

I've always been told that heat is usually lost faster thru the surfaces above us versus the horizontal surface next to us, and even less for the surface (floor) under us.

 

A vertical wall and a ceiling will almost never truly see the same temperatures. Heat rises so unless you have a decent, continuous airflow the ceiling will be warmer that the bottom of a wall. In my living room with 12ft ceiling height it is noticeably warmer when I go up on a ladder to change a light bulb.

 

I have two answers to help explain the differences.

Yes, The heat loss is the same through both horizontal and vertical surfaces when you only measure the 'radiated' heat loss. This is because heat radiates in all directions equally and is not affected by the air it passes through.

Yes, The heat loss is greater thru the ceiling, when compared to the walls and floors as you measure the 'convected (convection) heat loss in a room. This is because warm air rises toward the ceiling due to convection properties of air.

The essential difference between the two, is which method you are using to measure heat loss: radiation or convection. There are no practical applications for radiation effects in home construction.

Interestingly, infra red space heaters can heat a nearby wall to the point of catching fire, while convection space heaters will more quickly heat a entire room. That is as good an example as I can give.

 

guys - thanks for the replies. much appreciated.

followup question: is the equation for convective heat loss Rate of Heat Loss (BTU/hr) = [Area of surface*temp difference between the two surfaces (inside and outside)*time] / R-Value

If it is, can you tell me the equation for the radiation heat loss?

best,
Ron

 

I think your equation for conductive heat loss is good but trying to calculate radiant heat loss would be a challenge. Understanding that it is part of the process is probably the best you can do, but actual btus/hr would have too many variables.

The one detail about radiant heat loss I keep in mind is the rate is a function of the temperature difference raised to the 4th power. That 4th power is enormous and emphasizes how much radiation can contribute.

Bud

 

The 3 heat transfer mechanisms are conduction, convection and radiant. Since conductive heat loss dominates the other two during the heating season, insulation is rated accordingly. The rating such as R-19 is a standard. Which stands for 1/19th of a BTU traverses 1 square foot per hour of that material. In order to make this rating universal, the equation takes variables and makes them constant. For example, when rating the material they make the inside temperature 70 degrees and the outside temp 32 degrees. So regardless of the type of insulation, the resistance of heat loss would be the same. What this explicitly implies is that if the temp outside is lower than 32 degrees or the temp inside is higher than 70 degrees, the heat loss is more and vice-versa. So to answer your question, because of the natural buoyancy of warm air causes the ceiling to be warmer than the walls and floor, the ceiling has more heat loss.

 

Thus the reason ceilings are recommended to have R48 vs walls with R19!

 

Large quantities of insulation in a ceiling is also a function of having the space to install it. Walls would also benefit from R-48 but the cost of building 2' thick walls makes that option unattractive.

Bud

 

Guys: thanks for your responses and useful information. If/When I ever build the house I’m designing some important decisions will be determined by your answers.

“The one detail about radiant heat loss I keep in mind is the rate is a function of the temperature difference raised to the 4th power. That 4th power is enormous and emphasizes how much radiation can contribute.” Q: I was unaware that radiant heat loss was related to the delta temp to the 4th power. This is big. Can you give me some real world examples of this so that I can better understand?



“So to answer your question, because of the natural buoyancy of warm air causes the ceiling to be warmer than the walls and floor, the ceiling has more heat loss.” Q: are there any simple ways to quantify these differences between floor, walls, and ceiling so that my heat loss estimates are better?



I think I posted my original question on the forum to get some guidance on: how can I better calculate the different individual heat losses thru floors, walls, and ceilings when the variables are mostly insulation R-values for the floors, walls and ceilings? And the temp delta is the same for all three (inside temp minus the outside temp)?



“Large quantities of insulation in a ceiling is also a function of having the space to install it. Walls would also benefit from R-48 but the cost of building 2' thick walls makes that option unattractive.” Q: Then if one had all the space they might need to install insulation in the walls and floor, should the same R-value of insulation go into both the walls and floor, as the ceiling?

 

Beyond my focus to ever try to calculate the radiant heat transfer but here is a link showing an equation with T to the 4th power.

Radiant heat transfer is one of the 3 main paths that heat is gained or lost and it moves heat at near the speed of light. But the actual transfer involves how fast that energy can be replaced from its source so that rapid exchange isn't all that rapid.

It's a difficult topic and my guess is that is why it is so often avoided.

"Q: Then if one had all the space they might need to install insulation in the walls and floor, should the same R-value of insulation go into both the walls and floor, as the ceiling?"
Basically yes, but we use the term "diminishing returns". The recommended amount of insulation, even in building codes, is determined by the cost of energy. If energy were free codes would specify far less and if energy costs doubled so would insulation guidance.

From your questions it sounds like you want to build an energy efficient home. Three reasons are often stated, reduce energy costs, increase comfort, and reduce ones carbon footprint.

The comfort issue drops out rather quickly once you pass a reasonable level or energy efficiency. The other two are similar but vary in that you might spend more to achieve a low carbon output.

Search "net zero homes" and "building a super insulated home".

Bud