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Old 04-18-2013, 12:40 PM   #1
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Attic ventilation


I have a cape cod with a converted attic as the second floor (knee walls, pitched ceiling). It was not properly vented/insulated when the previous owners converted it to living space, and it gets very hot upstairs once the weather gets warm. The AC is working overtime to cool the upstairs, while the downstairs is freezing.

What's the best way to keep it cool up there? Gable vents, soffit vents, whole house fan? Looking for some info on what will work best and be most cost effective.

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Old 04-18-2013, 01:46 PM   #2
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Attic ventilation


so the problem is that the conversion was not done well??? in terms of insulation? ducts??? air infiltration???

so each of these has their own fixes, but some overlap...

First off let me start with the fact that shade is your best bang for the buck. Trees are good!!!

air sealing a room or house for less air infiltration is also a great idea.

now to hvac... A room must have the proper amount of air flow in to the room and out of the room. if the supply is not sized correctly or does not have the right CFM delivery, your room will be hot. additionaly, the room must have the right amount of air exiting it, back to the hvac system.

one of the things you might want to think about is what is called a mini split. Its sort of like a window ac system that is split in two. you hang the fan part on the wall and the noisy bits are outside on the ground... very good at keeping a space like an attic cool (or hot with a mini split heatpump).

so it comes down to lots of things to check into and decide what to do first.. my order would be
1. measure air flow in and out of room.
2. find and seal air leaks
3. correct insulation
4 vent un-used attic space
5. fix the airflow/ducts...

you might want to look into manual j to figure out what you need as far as air flow etc in the room.

bob in phx.

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Old 04-18-2013, 03:08 PM   #3
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Attic ventilation


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Originally Posted by bobinphx View Post
so the problem is that the conversion was not done well??? in terms of insulation? ducts??? air infiltration???

so each of these has their own fixes, but some overlap...

First off let me start with the fact that shade is your best bang for the buck. Trees are good!!!

air sealing a room or house for less air infiltration is also a great idea.

now to hvac... A room must have the proper amount of air flow in to the room and out of the room. if the supply is not sized correctly or does not have the right CFM delivery, your room will be hot. additionaly, the room must have the right amount of air exiting it, back to the hvac system.

one of the things you might want to think about is what is called a mini split. Its sort of like a window ac system that is split in two. you hang the fan part on the wall and the noisy bits are outside on the ground... very good at keeping a space like an attic cool (or hot with a mini split heatpump).

so it comes down to lots of things to check into and decide what to do first.. my order would be
1. measure air flow in and out of room.
2. find and seal air leaks
3. correct insulation
4 vent un-used attic space
5. fix the airflow/ducts...

you might want to look into manual j to figure out what you need as far as air flow etc in the room.

bob in phx.
Also I'd try to check & see if they upgraded the furnace & A/C when adding on. If they didn't do the insulation right I'd bet they didn't resize the furnace or A/C for the extra space either.
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Old 04-19-2013, 09:59 AM   #4
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Attic ventilation


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Originally Posted by jmorc2003 View Post
What's the best way to keep it cool up there? Gable vents, soffit vents, whole house fan?
Gable vents can be effective but only when the wind is blowing, and in the right direction. They can also do more harm than good if coupled with a soffit-to-ridge system.

A Whole House Fan is designed to bring cool outside air into the living space (typically during the night) to cool the occupants instead of using more expensive A/C. But, I'll assume you meant "attic fan" which brings air into the attic from the soffit vents and then exhausts it outside -- the interior air is not involved. Attic fans are surprisingly ineffective and typically draw more electricity than they save in A/C costs.

Soffit vents are great, but useless by themselves. They need to be coupled with ridge vents (either the older "turtle" or "mushroom" units or better, a modern continuous ridge vent). This allows a chimney effect to draw cooler air in through the soffits and exhaust it from the ridge and is the best way to go.

I'm not sure soffit-to-ridge venting is possible in your situation though -- you'd need to have a space between the attic insulation and the roof decking to act as a channel for airflow. If your insulation fills the void between the ceiling drywall and the roof decking, you're out of luck as far as attic ventilation goes and you'll need to look into other solutions like rebalancing your HVAC airflow or getting a larger HVAC unit.
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