What should attic temperature be?

[Mark C]

I live in NC and I am having a problem getting a bonus room HVAC unit to keep the space cooled down properly. It is insulated to R30 and has had the can lights air sealed. HVAC guy says the unit is properly sized.

I am wondering since one side of the house has a hip roof, is there enough ventilation? I was thinking about adding some box vents to suppliment the ridge. The house doesn't have the rigid plastic vent. It has the COBRA vent. Made like a heavy duty Scotch Pad. I wonder if it is as effective as the rigid plastic SORV?

Does anyone know about what the max. attic temp ought to be in my area. I am about out of ideas. Any thoughts on whether the additional ventilation would help?

Thanks for any advice.

[MarkMc]

Re: What should attic temperature be?

Sized right, but is it functioning right? Sized right by whom? What's the indoor humidity? What is the measured attic/[outdoor]ambient temp differential in say, mid afternoon? How much ventilation is there - total area; manu's will tell you the free air rating if you know who made it? What size is the roof area?

[David Meiland]

Re: What should attic temperature be?

Is there ductwork in the attic?

[Mark C]

Re: What should attic temperature be?

Yes, there is ductwork. It seems to be pretty tight at the joints, but I haven't had a duct blaster test done.

I could be the ductwork taking on heat. The house is in a T shape. The air handler is at the base of the T and the thermostat is in a hallway down the left top of the T.

The room closest to the air handler stays pretty well on target. The side where the themostat is never gets cool enough to satisfy the setting when it is 80+ and sunny.

Is it just too far to run ductwork in unconditioned space? That's why I was wondering if additional ventilation would be helpful?

I did have the unit checked out. It is working fine.

Maybe this is a problem that is normal and can't be fixed in a space with batt insulation? I am sure I could fix the problem with zoning, but I want to make sure that I am not just covering up a fundamental problem. I hate to have to go to a zoning system on the same level.

[dgbldr]

Re: What should attic temperature be?

What kind of duct is running in unconditioned space? Metal? How well is the duct insulated?

[Mark C]

Re: What should attic temperature be?

It's mostly flex. I am not sure of the R value. I would suspect it is just to the code requirement.

[Allan Edwards]

Re: What should attic temperature be?

While possibly a factor I doubt a hot attic is responsible for your problem, sounds like it might be under size unit, leaky ducts, under insulated ducts (see any sweating?), duct run too long, too much east-west window glass, return air in wrong place, even dirty filters!

Your AC contractor should be able to tell you the problem.

[AndyGump]

Re: What should attic temperature be?

Your attic venting is really just for allowing moisture to escape and not for cooling down the attic space.
Unless your attic gets to about 200 deg or more I doubt that it will be affecting the ability of the room to cool if you have the proper R value of insulation between the two.

Could there be another cause?

Andy.

[Mark C]

Re: What should attic temperature be?

It is a very long run. I may try to dampen some of the air closer to the Air Handler and see if I can push some more cold air on the longer runs.

[Don_in_Idaho]

Re: What should attic temperature be?

Am I the only one thinking r30 isn't all that much?

I look for a good air return as well as supply - some areas commonly omit returns in every room and just have a whole house return, which I think is about the biggest mistake for anything other than a cheap rental. Relying on return air flow under doors is leaving the hottest air in the room and taking out the coolest!

I'm assuming your air temperature is cool coming out of the duct there's just not enough of it? That's the first thing to check.

[jimAKAblue]

Re: What should attic temperature be?

You can't mix cans with ridge vent.

Check the soffit vents. Maybe they are blocked. If you don't have enough soffit vents, the ridge venting is useless.

Also, I run into a lit of hip roofs that aren't suitable for ridge vents If you are using GAF. With GAF, you can't run the ridge vent down the hips and often, there isn't enough ridge to satisfy the formula.

I'd check the attic venting first to verify that the soffits have vent, and that there is a clear path from the soffit to the ridge and that the ridge vent is of sufficient length and cut in and installed properly. Often, the ridge blocks the flow of air. Also, it's quite common for the insulation to block the free flow of air too.

Lots of questions.

[David Meiland]

Re: What should attic temperature be?

Roof venting isn't going to cool the attic enough to make a difference with HVAC duct performance--in a hot climate, your attic is going to get very hot unless you insulate the rafters very well and bring the attic inside. You need to check airflow and temperature at each of your supply registers, and at the air handler. You may not have enough flow at the coil itself, and you may not have enough--or cold enough air--at the room with the problem. Attic temperature definitely affects temperature in the ducts. Like Allan said, your AC guy should be able to check this out and see what the performance issue is.

I did have the unit checked out. It is working fine.

What specifically was checked out?

[Ted S.]

Re: What should attic temperature be?

What David said.

Keep in mind bonus rooms are a real problem, a ton of the wall, floor and ceiling area is the thermal envelope and they have the hardest details of the envelope. They need more conditioning than the other bedrooms too.

See if there was proper load calcs done, then check what is coming out of the register to see if it is near what was designed. You would need to do a manual J and D for this.

[ThingOfBeauty]

Re: What should attic temperature be?

Bonus rooms are often very leaky: below base board, under floor (through floor system above insulation), at plugs and any other electrical penetrations including lights etc.

The vertical insulation on the side walls doesn't work well unless wrapped on all six sides, and often isn't wrapped on the back side. Hot air convects through it and warms up the room.
Air delivery, leaky ducts, ducts heating up in hot attic, and control problems (namely that the a/c turns off because the thermostat is cool over there near the unit) are also common.
Your HVAC person may not have all the tools needed to figure this one out. If you know a "home performance" or blower-door retrofit type company, they might be more likely to get you where you need to be.

See the pair of photos below for an example of cold air rushing under baseboard, through plugs, and the poor performance of the insulation.

Bonus room IR.JPGBonus room visible.JPG

[Bill Robinson]

Re: What should attic temperature be?

It seems a simple test of the temp and volume at the supply register will tell a lot.

And a room to room pressure test.