Added Load

[Rich Goeken]

While I was in Florida recently, I inspected a home under construction in the Kissimmee area. The contractor was trying a new concept for him in insulating the homes. He was spraying foam on the roof in lieu of having conventional insulation above the ceiling. I think he was also spraying the attic walls to finish sealing the attic, but I'm not sure.

To me, this brings up some issues. Now this attic area actually becomes part of the conditioned space of the house, with added heating and cooling loads. Does anyone have experience with this situation and does the added volume area have any major effect on the loads? Does it have any effect on the rule of thumb of quickly estimating if the AC is sized correctly for the home?

[Bob Spermo]

Rich,

I do not advocate a rule of thumb for HVAC system sizing. I believe in a properly conducted Manual J V8 load calculation using a recognized program such as Elite Software. RESNET defines conditioned space as being serviced by a heating or A/C system. The attic may or may not be part of the conditioned space. That being said Elite will use the foam insulation on the attic decking in its load calculation and will reduce the size of the HVAC system accordingly.

[Rod Butler]

I use rules of thumb often in my work. But always follow up with calculations to determine the proper equipment size.

Office/Commercial: 350 to 400 SF/ton
Residential with average window exposure: 500 SF/ton
Industrial processing: 250 SF/ton

Great for use in conversation and preliminary equipment selections or evaluations.

[Rich Goeken]

Rich,

I do not advocate a rule of thumb for HVAC system sizing. I believe in a properly conducted Manual J V8 load calculation using a recognized program such as Elite Software. RESNET defines conditioned space as being serviced by a heating or A/C system. The attic may or may not be part of the conditioned space. That being said Elite will use the foam insulation on the attic decking in its load calculation and will reduce the size of the HVAC system accordingly.

It's nice to rely on software. But, before software the determination was done manually by observation and experience. In the situation that I described, heat or cooling will be transmitted through the uninsulated wall into the attic space (unconditioned), the same way it is transmitted through a single glass window pane, until it hits the foam. Maybe, not much, but I think it would affect the environmentals of the building for sizing the systems. In extreme cases it might make a room too hot or too cold. To ignore this and allow the software to put the insulation on the ceiling (attic floor) may not be a good practice.

You could do the same calculations with a good spreadsheet application. You would have to write it yourself, but you would know exactly how and why it does things, and also allow you to easily make subtle changes should the need arise. It would cost less $$$ too!

I did not imply that the "rule of thumb" was to be used in sizing a system, I only asked if there was a easy and quick evaluation method based upon the situation that I described, and suggested "rule of thumb". I don't think anyone would question the proper method in sizing a system.

[Jerry Peck]

While I was in Florida recently, I inspected a home under construction in the Kissimmee area. The contractor was trying a new concept for him in insulating the homes. He was spraying foam on the roof in lieu of having conventional insulation above the ceiling. I think he was also spraying the attic walls to finish sealing the attic, but I'm not sure.

To me, this brings up some issues. Now this attic area actually becomes part of the conditioned space of the house, with added heating and cooling loads. Does anyone have experience with this situation and does the added volume area have any major effect on the loads? Does it have any effect on the rule of thumb of quickly estimating if the AC is sized correctly for the home?

To my knowledge, no, it does not have an effect on sizing of the a/c systems. That insulation method, known as sealed attics, has been in use in Florida since around 1995 (give or take a few years).