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Not enough exhaust and intake

2204 Views 5 Replies 4 Participants Last post by  Gary in WA
My one story house is probably 1600-1700sq ft. I have 11soffit vents(8x16) total but don't know for sure how many is blocked by insulation or not. The only exhaust I have is about 20ft max ridge vent. Which is not enough in both intake and exhaust. I should have 15-16 soffits if I did my calculations right. I have high and low ceilings. So trying to see if my soffit vents are clogged is hard once I am in the attic. I was thinking of taking the soffit vents off but they are stapled under the eaves. I left my thermometer to see what my attic temperature is. At 101 degrees outside temp, my attic is at 130 degrees. With the a/c running 90% of the day, inside my house is 90 degrees. What do I need to do? I was thinking about blocking off the ridge vent and adding two wind turbines.

The little squares around the house are soffit vents. Thick black line is ridge vent.
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I would start by pulling down a little bit of soffit to see what your dealing with. You should have a 1" air space between you insulation and your sheathing. if you don't have that then you will need to install some foam baffling (home improvement store). Add more vented soffits in the areas where you have none. Then provide an exhaust fan (or maybe two - one at each end) close to the ridge. Considering you extreme temperatures in you attic I would make it a powered ventilator with a thermostat/humistat. You can buy them at your local home improvement store.

I am assuming you have fiberglass batt. insulation? If so, you can make it much more efficient by adding an air barrier over the surface exposed to the attic. Moving air through the batting greatly reduces its effectiveness. Tyvek or equal is a great fabric type air barrier.
I think the minimum calculation is 1 sq.ft. of net free area of ventilation for every 300 sq. ft of attic space. I would shoot for 1/150.

Your current nfa of ventilation; 11-soffit [email protected] 65 nfa =715 nfa
20' ridge vent @ 18 nfa per lin. ft.= 360 nfa
Total=1075 or 7.46 sq.ft.

1/300= 5.66 sq.ft. 815 nfa
1/150= 11.33 sq.ft. or 1632 nfa

To get to 1/150


2-4- 8x16 soffit vents @ 65 nfa and make sure existing vents are cleear
8- roof vents @ 60 nfa at the peak for exhaust.

Your soffits are close you just need peak ventilation. This was based on 1700 sqft of attic space. If the equipment you use has a different nfa the numbers will change. Ideally you want to split your ventilation requirements between the intake and exhaust vents. Hope this helps.
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Well I went and pull down the first soffit vent and here is what I found! No air flow because fiber batting is blocking it. I'm assuming is the same for the rest of the soffit vents because when I went to go look for light penetrating the attic I did not see any. Oh as for insulation I have loose fill fiberglass on top of fiberglass batting. Looks like I'm going to have to stop by and pick up some foam baffling at home depot which is like 3mins down the street. Also can the baffling be install from the outside because installing them from the inside would be almost impossible to access the eaves due to the high ceiling.

@AGWhitehouse: Wouldn't I just be sucking air from the ridge vent if I install exhaust fan on each end of the soffit vents? J/W
Once you install the foam vent panels (yes you can put them in from the soffit with care not to crush or tear them). you can install the ventilators near the ridge (not the soffit).
The bigest problem with the 8X16's are the required spacing, 7' apart for 9 NFVA per foot, ideally. But using them, there is 7' between each without individual cavity ventilation, solely relying on the soffit to deliver air equally to all. Not going to happen, depends on the incoming wind direction which bay gets more/less. Optimum performance would be continuous soffit vents, installed near the fascia board, pp.616: airflow with gable vents&f=false

The plastic baffles deliver 40% more air than the foam ones which snag on the protruding roof deck nails, and they have a built-in air/insulation stop at the outside wall line. Using the foam baffles, buy the accessory wind/insulation blockers. From the pics, your vent locations are messed-up. You may need foam board at exterior wall top plates to meet your minimum energy requirements, but at least air seal first:

You probably have f.g. in the exterior walls which any air from holes to the crawl go to the attic, creating convective loops in them:

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