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Old 10-09-2008, 02:12 AM   #1
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Roof venting

We are rehabbing an home built in 1850, and there are small soffit vents, but no upper vent. What is the best way to determine how much ventilation we will need.

I don't think a ridge vent will be installed since the roof is quite steep.

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Old 10-09-2008, 07:09 AM   #2
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Air Flow,
Assuming you have proper intake and air flowage allready,
ridge vent is the best and most cost sufficent way to go.
I have installed ridge vents on 12/12 pitched roofs, and done so with out problems.

You want to double check your interior air flow at the transistion point, where the roof goe's from mansard to lower sloped,
if air flow is good there than a continues ridge vent up top will work.

I'm sure Ed will post the techinal calculations once he reads this,
but from a lamens terms you need,
one pot vent or two liniel foot of ridge vent per every two square of roof to meet CI requirements.
So if roof size is 16 square 'both mansard and lower slope' than you would need 8 pot vents or 16 liniel feet of ridge vent.


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Old 10-09-2008, 07:12 AM   #3
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Cool house...but I have to ask: what problems do you have now that make you ask how many vents you need? I think the pros here would want to know that, plus what insulation you have there already. Do you see any mould signs or paint weaknesses up there? do you know if there was allowance made for air to rise above any existing insulation - or is the cavity between roof and ceiling filled with it?

There are calculations for the exact amount needed but some more input might be required.
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Old 10-09-2008, 11:23 AM   #4
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Right now there is a small soffit gap that allows air in, however, to my knowledge there is no way for air to escape at the peak. Also at one point when the 3rd floor was drywalled they put the drywall right on the 4x4 roof beams, and put insulation between them, so there is no airflow between the upper and lower portions.

The top portion is just trapped, where the bottom portion is just a cavity, with a soffit gap. There is very minimal insualtion if any anywhere in the lower portion, and the top portion of the roof.

The current ceiling height is around 6.5 ft, so we raised the ceiling 1.5 ft, and now there will be about 2 ft from the peak of the roof to the ceiling (not including insulation. We were planning on re drywalling the entire 3rd floor so that we can put soffit baffles in the sidewalls so air can flow from low to high. We were also going to install 4in x 12 in vents every two feet in the soffit to provide additional venting, as well as an electric thermostat vent so it will pull air at hotter temps. Would this be sufficient.

One concern I have is that i will not be able to put much insulation in the slopes where the baffles are since there is only about 4in of space and the baffle will take up about 2inches of that.

THoughts suggestions, input?

Let me know if something needs further explanation.
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