Calculating Roof Ventilation for Vaulted Ceilings

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Old 04-30-15, 08:20 PM
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Calculating Roof Ventilation for Vaulted Ceilings

I've done a lot of reading on how to calculate proper roof/attic ventilation. All of the calculations start by figuring out the attic floor area required to vent by measuring the attic floor length and width, and multiply to get the area. I've got a 80 year old house with partially vaulted ceilings in two bedrooms upstairs, which means I've got a very small attic floor area above those.

My question is, should I be using the full length and width of the roof (i.e. the upstairs floor space) to calculate my ventilation requirements or just my small attic space? I figure that I need the entire floor space.

I have open soffits at the base of the roof, and have good space between the roof deck and insulation to get good airflow from the soffits, up the roof deck to the attic. I will be adding ridge-vents across the ridge of the roof, and installing round louvre vents between the rafters at the open soffits. I just need to know the proper attic area to use, so I can figure out how much net free area I need.
 
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Old 04-30-15, 08:41 PM
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Hi cappy,
You are not alone with your confusion. General guidelines should apply to the footprint below the roof and not just the small attic floor. However, the reasoning for using the length times the width approach when the entire ceiling is flat is to account for moisture and heat transfer through that ceiling. When the ceiling is vaulted that area increases, so, IMO, you should be generous with your vent area.
If your reading has included the research Bill Rose did in trying to establish the background for the 1/300 approach, you know it was a stab in the dark, and now it is code. I'm not sure what Canada has for requirements, but probably 1/300 for a well air sealed and insulated ceiling and 1/150 if not well sealed.

As for those round pop-in vents, be careful. last I searched they were still rating them at 19% NFA. Since you want your low venting to be the better of the two, as some advise, you might consider one of the continuous strip vents for the soffits.

Bud
 
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Old 05-01-15, 03:54 PM
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Thanks Bud. This is great information. It makes all sorts of sense now that I know the attic floor space calculation is actually to estimate the heat transfer from the ceiling below. Knowing this I can estimate what my total ceiling area is, and go from there.

The Canadian code runs on the same 1/300 and 1/150 principle. 1/150 for cathedral or low sloped roofs.

Thanks for your input on the louvers. I was making sure to look at the NFA, which you're right, is really low on those vent types. I just want to get the best looking vent on my soffits, and thought that would look the best. I'll take a look at some other options after I figure out how much soffit venting I need.

Anyone have experience with the Smart Vent product that runs on the roof deck just above eaves as an intake vent? I live in the Pacific Northwest where we get over 1 meter (3.5ft) of rain a year, and essentially no snow. Curious if the Smart Vent would be suitable for a very very wet climate.
 
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Old 05-01-15, 04:41 PM
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I live in snow country so I would avoid a vent right on the edge, but you say virtually no snow. But I have zero experience with them.

Bud
 
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Old 05-03-15, 06:57 AM
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Just to add my 2 cents, add roof vents on both side and not just on one side of roof that will prevent snow accumalation in the attic through roof vent if the wind is high and direction specific.Also it will allow air to move through attic faster than creating pressurization in the attic.
 
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