# Attic rafter vent size



## Jimmybrp (Dec 5, 2011)

Am insulating the attic roof rafters so I a n use the attic as a warm space, and I was given conflicting advice about how much of a bent space I need above the insulation in order to keep the underside of the roof cold. The rafters are 20 ft long and the roof is a 7/12 pitch. The guy that just Rez ingles the roof says use a 3" vent space but others have said as little as 1 1/2".

If I use a 3" space I have to screw a couple of 2 x 2's onto the edge of the 2 x 6 rafters in order to allow for enough space for the 3" air sp e plus a 5 1/2" batt of R22 Roxul. Then I can put hih density foam panels horizontally to ad another R10 or so, and also cover up the rafter surfaces thus reducing thermal bridging. 

Any advice here would be much appreciated. And by the wAy I do not want to use spray foam for a number of reasons.


----------



## Windows on Wash (Aug 30, 2011)

3" is way overkill. You really only need about 1" as long as it is consistent all the way up the roof line. You can create that space by ripping down some 1" foam spacers of screwing in 1" wood into the rafters. I would start with rigid board right up against the spaces so that you don't get the wind wash stripping of R-value (although roxul is far less susceptible) but use something with a decent perm rating (EPS).

If you use a thick enough board and have it supported, you can dense pack cellulose against it of continue with batts. Fill to the edge of the rafter and put rigid over the rafters.

Make sure you tape and seal all the seams in your rigid layers for a proper air barrier.

You will need a thermal barrier over the foam if you are using the space.


----------



## Jimmybrp (Dec 5, 2011)

Windows on Wash said:


> 3" is way overkill. You really only need about 1" as long as it is consistent all the way up the roof line. You can create that space by ripping down some 1" foam spacers of screwing in 1" wood into the rafters. I would start with rigid board right up against the spaces so that you don't get the wind wash stripping of R-value (although roxul is far less susceptible) but use something with a decent perm rating (EPS).
> 
> If you use a thick enough board and have it supported, you can dense pack cellulose against it of continue with batts. Fill to the edge of the rafter and put rigid over the rafters.
> 
> ...


Hello, W.O.W., and thanks for the reply. There is a cardboard product you can use to fill the vent space, which will of course cover the top of the Roxul quite nicely. I also read on this forum that you should leave a space between each section of cardboard so that water droplets won't join together and "waterfall" all the way down the 20' length of the vent space, but then these water droplets will instead soak into the Roxul, won't they?

Also, isn't there some building code spec for the amount of air space you need above the insulation?

Thanks again!

JimmyBRP


----------



## Windows on Wash (Aug 30, 2011)

Ventilation are should follow the same 1:150 rule for finished square feet.

Using something like EPS will ensure that the roxul won't get wet.


----------



## Jimmybrp (Dec 5, 2011)

Windows on Wash said:


> Ventilation are should follow the same 1:150 rule for finished square feet.
> 
> Using something like EPS will ensure that the roxul won't get wet.


So, I have a 1350 sf cottage with an attic that is the same size, and I want to raise the roof to us that space in the future, meaning I will have a 2-storey structure with 2700 sf., which according to your formula requires 2700/150 = 18 s.f. of vent. If I use 3" vent spaces in each rafter cavity and the cottage has 36 ft x 2 = 72 lineal ft. of vent (excluding the 1.5 " rafters themselves), we get 72 ft. at 3" = 72/4 = 18 s.f.. This is just barely adequate, considering we lose 32 X 1.5" = 48 lineal inches of venting where the rafters go. Sound about right?

Jimmy


----------



## Windows on Wash (Aug 30, 2011)

1:150 if for the floor dimensions, not total finished square feet.


----------



## DexterII (Jul 14, 2010)

Jimmybrp said:


> Am insulating the attic roof rafters so I a n use the attic as a warm space


A bit unrelated to your questions, but if you are planning to use this space as some manner of living area, you may have other concerns, such as whether the joists are adequate for the intended use.


----------



## Gary in WA (Mar 11, 2009)

Where are you located?

This covers most of your questions; http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems

This just out, from Joe- 2" air space, foam the baffle joints (I probably sited the end baffle gaps if within the last 1-1/2 years here); http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting

Code on clear vent space (like getting a "D" grade on a school test- pass, but barely);http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_8_sec006_par002.htm

What is the end use of attic?

Gary


----------



## Jimmybrp (Dec 5, 2011)

Windows on Wash said:


> 1:150 if for the floor dimensions, not total finished square feet.


Yes, thanks for pointing that out. I also read that the 1:150 is spread evenly between the intake vents and the (in my case) ridge vent, so I have only half the floor space I mentioned previously, and twice the venting (since both my soffit vents and my ridge vents at the top are also 3" on each side), does that mean I actually have 4 times the venting I need? And how does this change as a function of the fact that I will be putting a couple of bedrooms up in the attic and heating that area as well?

J.


----------



## Windows on Wash (Aug 30, 2011)

You always want more intake than exhaust.

Joe Lstiburek did recently state that he like 2" of venting but that is still probably overkill if the roof is a 7:12 pitch.

No such thing as too much venting as long as it is done properly.

Added ventilation will not really change the heating/cooling characteristics as long as that added venting is not at the expense of proper insulation depth.


----------



## Jimmybrp (Dec 5, 2011)

Windows on Wash said:


> You always want more intake than exhaust.
> 
> Joe Lstiburek did recently state that he like 2" of venting but that is still probably overkill if the roof is a 7:12 pitch.
> 
> ...


Thanks, WOW - my roofer was pretty insistent on putting in a 3" vent space, and he has been at it for some 30 years or more. Also, it gets down to 20 below at this cottage at times, (as it is 2 hrs. North of Toronto), so that may be part of the equation too.

James


----------



## Windows on Wash (Aug 30, 2011)

Heat drives convection...so the colder it gets the more the air is going to move on its own.

I just don't like laying down 2" more sacrificial inches of ventilation that need to be insulation and especially in a froze tundra like that.

If you roofer is squared away, go with what he is comfortable with.


----------



## Jimmybrp (Dec 5, 2011)

DexterII said:


> A bit unrelated to your questions, but if you are planning to use this space as some manner of living area, you may have other concerns, such as whether the joists are adequate for the intended use.


Good point, Dexrer, and in fact the existing joists above the bedrooms were only 2 x 4 at 2' o.c.(!) so needless to say we removed them and will be replacing them with 2 x 10 joists spanning 12' for 2 of the bedrooms and 13'8" for the third bedroom, all of which meet the Ontario Building Code specs if you use braces in between the joists. Another issue regards the walls that the joists will be hanging on. The outer walls are no problem since the main floor perimeter beam is right below, but the inner walls now become load bearing and so a beam underneath the cottage must be within 2' theses walls. (Luckily, we are less than 2' away from a triple 2 x 8 beam, so we are OK.)

J.


----------



## Gary in WA (Mar 11, 2009)

Ahh, a local roofer, page 6: http://www.nrc-cnrc.gc.ca/obj/irc/doc/pubs/bpn/57_e.pdf

Gary


----------

