Venting rafter spaces

[pushkins]

When an old attic is closed in and drywalled for a new room, given that there are vents in the roof and that the new insulation between the rafters is kept off the roof sheathing via insulation spacer panels, how will each run of rafter vent ?
(and no there is no ridge vent):mrgreen:

[N5XL]

Damn good question!!!

How about a power ventilator on the gable end. If you can't do that, a power ventilator where a turbine might go?? Can you put a roof ridge vent in after the fact?? Now I'm curious too.

Dave

[pushkins]

Yes you can always add ridge vents at anytime.
A power gable vent wouldn't work as it would vent the entire attic, but seeing as the said attic is drywalled over it would vent nada.

[Wgoodrich]

You say you have roof vents and you have shutes to ensure air gap below roof decking. Check to see if you have vents or slots in teh soffit.

If you do you have proper venting. Heat rises. The attic is hot and tries to act as a chimney rising that hot air up through the roof vents. At the same time cool air is drawn in from the soffit vents into teh attic to replace that exhausted air out the roof vents. Inlet is soffit vents and outlet is roof vents using the same technique a chimney is disigned using the fact hot air rises.

You should have 1 vent per 150 square foot of attic space.

Good Luck

Wg

[pushkins]

WG I fully understand how venting works.

My example question was, how does it work with vents in the roof (say for example 4) and full soffit vents on the complete exterior.
Now the ceiling in the attic is drywalled over, but prior to this insulation was added with insulation spacers used in each run of rafter prior to being drywalled over, naturally.
My question is, if the vents fall for example in four rafter runs how are the rest of the rafter runs ventilated correctly ?

[deejoe]

Wg; your statement of 1 sq ft of vent to 150 sq ft of attic floor area applies to an storage type attic (no ceiling at roof.)

Puskins. for the type of attic ceiling your referring to, the insulation has to bekept away at least an 1" from the roof sheathing so there is a "passageway" for the soffit air to exit out of the high vents.
in this case, the continuous RIDGE vent works best, otherwise you would need several of the maushroom type vents at every rafter location.

[pushkins]

ah ha !! finally an answer to the question.

Basically on a roof that had say 20 rafter spacers )10 each side) the only way to make the ventilation work correctly would be to have 20 can vents ? (This is assuming that the owners didn't want a ridge vent)

[deejoe]

If there is no continuous ridge vent, and naturally you don't want to put in 10 mushroom type vents, you could cut a 1" x 22" slot in the upper part of the rafter up near the ridge(peak)in every second and/or third rafter,etc, to allow air from these rafter spaces to get into the rafter space that contains the mushroom vent.
do ya get what I mean?

[pushkins]

Yes I understand what you mean.

I don't physically have this problem the point of the post was that a friend asked me how unfinished attics without ridge vents that are drywalled over are correctly insulated. I told him for the most that without a ridge vent it probably isn't insulated correctly, This then prompted him to ask about his own home without ridge vent and it's cathedral ceilings (drywalled over not exposed beams). I've built two cathedral ceiling homes but these all had ridge vents, so it got me to thinking about all the homes out there that people have finished the attics and no ridge vents.

Is the 1" x 22" slot a code size (I cannot find it) or is it a practical working size ?

[Wgoodrich]

I didn't catch a cathedral type ceiling.

If you have closed rafters all the way to the ridge each rafter space needs vented at the top. This is a lot of can vents or one ridge vent.

However many cathedral ceilings have a flat spot at the top such as collar ties. If there is an attic space there above the collar tie type ceiling then you are back to the regular roof vents using that small attic as outlets of each rafter into that small attic area and you are back to the 1 square in per 150 square foot including the roof area of the closed rafters.

Wg

[deejoe]

[QUOTE=Wgoodrich]I didn't catch a cathedral type ceiling.

. If there is an attic space there above the collar tie type ceiling then you are back to the regular roof vents using that small attic as outlets of each rafter into that small attic area and you are back to the 1 square in per 150 square foot including the roof area of the closed rafters.
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I suggest you have your "formulas" for venting area a little bit mixed up.
as you stated..."1 sq in per 150 sq ft" would be absolutely of no benefit what-so-ever.

Note; Most bldg codes dictate 1 sq ft of free unobstructed ventilating area for each 300 sq ft of insulated ceiling area for roofs with a slope exceeding 2 in 12,
or, 1 sq ft of free unobstructed ventilating area for each 150 sq ft of insulated ceiling area for roofs with a slope of 2 in 12, or less.
the totals arrived at using the above formulas are the split 50/50 between soffit and peak.

Using individual high 'mushroom' type vents instead of a continuous ridge vent is also a very good substitute for this roof venting problem as long as the "math" is done correctly. More is better when it comes to venting a roof, as long as adequate R values of insulation are in place.

[Guido]

You're right - Wgoodrich's statement should say 1 sq ft instead of 1 sq in. I think it was a typo.

You're also right about the 1/150 and 1/300 requirements, but my research found nothing to do with roof slope. instead I found:

"The building code and most roof manufacturers recommend venting at a rate of one square foot per 150 square feet of ceiling space.

The code allows for venting at a rate of one square foot per 300 square feet of ceiling space where a complete vapor barrier exists at the bottom of the insulation."

And I think it's important to realize the square footage comes from attic floor, and not roof area.

But I wouldn't be surprised if the requirements are different for different areas.