HVAC for areas w/ high ceilings
Lots of detail. Read closely.
Background & set up: 2 story house construction completed in Spring 2006 in Dallas/Fort-Worth metro. Radiant barrier (Kool-ply) roof decking installed. 8 inches or so of blown-in, loose-fill fiberglass insulation in attic including over several raised-box ceilings, and blown-in cellulose in walls. Tyvek installed over OSB board shielding. Two 13-seer Rudd heat pumps installed: model UPMD-048JAZ with coil RCHJ-48A2GG24XI, and UBHK-21J14SFA with coil RCHJ-36A2GG21XI. (The heat pumps were originally designed as 12 seer but upgradeable to 13 seer.) The bigger heat pump is for about 2,000 sq/ft downstairs and the smaller unit for about 1,000 sq/ft upstairs. Both units use digital, programmable Honeywell thermostats for dual-zone A/C.
In the floorplan, the ceiling over the entryway, dining/family room area is the same height as the second floor and is open to the landing/loft on the second floor leading to a hallway that connects to three bedrooms and two bathrooms upstairs. Roughly 1,000 sq/ft (downstairs & loft) under that ceiling, and there are several IC-rated recessed lighting fixtures in that ceiling. There are two registers in the ceiling over the dining/family room opposite of the loft and a return near the floor. Over or on the loft, there are also two registers, a return, a thermostat, an insulated side-door to the attic over the downstairs-kitchen, and a pull-down attic stairway. In the winter, the lower return is used, and in the summer the return above the loft is used. Both air handlers are in the attic space above the loft. There is a custom-made fiberglass-batt insulator over the pull-down attic stairway. There is also attic decking around the stairway above the loft particularly underneath the air handlers. Some of this decking is Kool-ply with the RB surface facing down towards the insulation underneath it. Some of the insulation underneath this decking was blown-in cellulose before the ceiling drywall was installed on the 6” or 9”(?) joists. A little bit of the decking was added after drywall and before the loose-fill attic insulation was blown in; so, it MAY not have any insulation at all underneath. Some of the decking was added after all insulation was installed; so the insulation is somewhat compressed and thus have a lower R-value.
The main downstairs area does not heat up as well as I’d like in the winter. It maintains 68 degrees ok, but if the thermostat is set higher than 68, the heat from the 2 main registers seems to stay high nearer to the ceiling towards the loft which gets too warm. Making this loft zone so warm also affects an upstairs bedroom and bathroom in that bed & bath in that zone do not get the benefit of the warmth from the two main registers.
In the summer, the house seems to cool well enough around 78 degrees although the loft tends to be warmer. The warmer loft has the effect of making an upstairs bedroom and bath in its zone significantly cooler because of the zone turning on to cool the loft where the thermostat is.
1. What effect does the Kool-ply used as attic decking have on cooling or heating where the RB surface is facing down? 1.a. Is the effect different with or without loose fiberglass underneath it? 1.b. Whether the fiberglass is touching the RB or not? 1.c. Is it better to use wood without Kool-ply surface as attic decking in this case?
2. What can be done so that the warm air from the main registers in the high ceiling fills in toward the main floor and not warm up the loft as much?
3. I am considering adding fiberglass batt roles over the blown-in insulation in the attic. How can I add more insulation AND still have decking to walk on without compressing the insulation?
4. What is a standard minimum of how many inches of blown-in fiberglass insulation should cover the wood sides and corners of the raised-box ceilings?
I’ve got more questions, but I’ll start with those for now.
Last edited by BoNoHVAC; 03-13-2008 at 08:13 AM.
In my house, the entryway and living room are both double-height (19-20' or so). Probably around 500 sqft.
In the middle of the living room is a ceiling fan. It stays on all the time (on low). As a result, I have very little temperature differences between upstairs and downstairs. Because it's on low and 15' above the floor, I never feel a draft. In the winter, I have it pulling the air up so that I don't feel it.
I have a fan set up that is similar and hangs about 12' feet or so from the main registers. Why do have it pulling the air up though? I would think that would produce less effect than fanning down.
Originally Posted by Waterloo
where do U live?
have U sealed the penetrations?
for excellent insulation: R60= ceiling, R40=walls, R10 under slabs.
if you crush insulation, it looses its R-value --
are your ceiling joists covered with insulation?
harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!
As stated in the opening post: "Dallas/Fort-Worth".
Originally Posted by cem-bsee
"under slabs"? Slabs aren't insulated in N. Texas.
Ceiling joists are not covered. A couple of inches peering out.
I thought only South FL needed to cool in the winter?!!
Originally Posted by BoNoHVAC
It's Not Rocket Science, But It is SCIENCE
with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE
Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities
oops! edited. thanks.
Originally Posted by dan sw fl
In the winter, I have it pulling the air up. This is to keep the slight breeze away from my skin - otherwise, it could be felt as a draft, and be tempted to raise the setpoint. With it pulling the air up, you have to be near a wall to feel the air coming down - and you usually have to turn up the fan to Med-High or High. (We have a wall switch that allows us to control the speed).
Originally Posted by BoNoHVAC
Here's the other part of my reasoning - if you push the air down, then very little airflow is left to pull the air out of the top edges/corners. I would think that the fan would deal with the "stack affect" in a relatively small area around and above the fan, versus the walls and corners. By pulling the air up, I think I would have a better chance in getting that air out of the upper corners.
If someone has a heat-load you can do a comparison of various R-values to find a cost effective level of insulation. There is a point of diminishing returns with insulation.
The radiant barrier on the decking will provide a condensing surface so watch that.
We have installed heat only zones with grills that blow down the outside wall with the most glass.