# Ventilation



## brendanstl (Jan 29, 2010)

I'm a carpenter that knows nothing about roofing, I just purchased a house with a finished attic and the roof needs to be replaced. The house was built in 1931, I removed all the plaster and lathe in the attic to find that there was no insulation whatsoever. I plan on furring the 2x4 stick built trusses down so I can get 6" insulation between with 4mm poly for a vapor barrier. From sub-floor to the bottom of the ridge board I have 8' and I'll be putting collar ties in at 7'-6" which will leave me with roughly 10" from the ceiling to the peak of my roof. I have a feeling that the shingles are curling up and falling apart due to no ventilation which I'm not sure if I can achieve either. The roof is gable to gable with a full width dormer on front and back which is open to the attic so that only leaves me roughly two feet of soffit on all four corners to put in vented soffit. What are my options for venting this roof, will just a ridge vent work if I fur down the trusses to leave a void between the bottom of the sheeting and the insulation?


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## Kekeever (Nov 17, 2009)

You should have a ridge vent installed. You can drill holes for cross ventilation along the upper part of the rafter. You'll need to leave an air gap between the insulation and the roof sheeting. Then install roof vents every other rafter bay or so. Just need to be careful that your cross ventilation holes don't compromise the rafters load capacity. Small gable end vents would help as well but I'm sure you've thought of that. 

Good luck


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## Grumpy (Oct 8, 2003)

Ridge vent sounds like it will work if you can close the gables and create intake through the soffits. Make sure when installing your new insulation not to block the intake. Make sure there is intake, or you will have to cut it in. Make sure the siding guys didn't install vented soffit panels, but forget to cut the openings. HOWEVER from what you are describing it really really doesn't sound like you have nearly enough area for intake. You'll need to provide pics. 

One thing about the cross vent holes, there is a rule of thumb about the size of the rafter vs the size of the holes, and if i remember correctly it is a maximum of 1/3 the size of the board. Also the holes must be exactly centered in the rafter or they will comprise the load baearing capacity. But if the holes aren't too big and are centered on the rafter, you're ok.


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## brendanstl (Jan 29, 2010)

Kekeever said:


> You should have a ridge vent installed. You can drill holes for cross ventilation along the upper part of the rafter. You'll need to leave an air gap between the insulation and the roof sheeting. Then install roof vents every other rafter bay or so. Just need to be careful that your cross ventilation holes don't compromise the rafters load capacity. Small gable end vents would help as well but I'm sure you've thought of that.
> 
> Good luck


If I make cross ventilation holes and add a ridge vent will I still need to install roof vents every other truss and what size of holes and how often do I need them? Seems to me that it's going to be quite a few roof vents although they will be in the back of the house. Also I have heard that if you don't have vented soffit and the right kind of air circulation I could have a problem with snow blowing into the vents, is this true?


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## brendanstl (Jan 29, 2010)

Grumpy said:


> Ridge vent sounds like it will work if you can close the gables and create intake through the soffits. Make sure when installing your new insulation not to block the intake. Make sure there is intake, or you will have to cut it in. Make sure the siding guys didn't install vented soffit panels, but forget to cut the openings. HOWEVER from what you are describing it really really doesn't sound like you have nearly enough area for intake. You'll need to provide pics.
> 
> One thing about the cross vent holes, there is a rule of thumb about the size of the rafter vs the size of the holes, and if i remember correctly it is a maximum of 1/3 the size of the board. Also the holes must be exactly centered in the rafter or they will comprise the load baearing capacity. But if the holes aren't too big and are centered on the rafter, you're ok.


The rafters are framed with 2x4's (old school lumber) and I was going to fur the rafters down 3 more inches which would leave an inch void between the sheeting and insulation, if I drilled the holes in the center of the rafters after I furred them down that would put my holes in the center of the insulation and common sense tells me that there would be no circulation of air, am I correct?


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## jmiller (May 14, 2010)

For insulation I'd probably want spray foam with no ventilation at all. 

The other option would be continuous baffles attached to the decking, every rafter bay (don't drill cross vents), from the soffit vents to the collar ties, with kraft faced insulation rather than a vapor barrier.

edit: I'd look here for venting and moisture barrier info specific to your area: www (dot) buildingscience.com/doctypes/designs-that-work


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## neolitic (Apr 20, 2006)

Show us some pix.
Attic and exterior.
Any thing any of us says here
is just guessing at what you are 
trying to describe.


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## brendanstl (Jan 29, 2010)

neolitic said:


> Show us some pix.
> Attic and exterior.
> Any thing any of us says here
> is just guessing at what you are
> trying to describe.


 
I'll do that, I have to run into town, I'll stop by there, snap a few pics then upload them when I get home.


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## brendanstl (Jan 29, 2010)

jmiller said:


> For insulation I'd probably want spray foam with no ventilation at all.
> 
> The other option would be continuous baffles attached to the decking, every rafter bay (don't drill cross vents), from the soffit vents to the collar ties, with kraft faced insulation rather than a vapor barrier.
> 
> edit: I'd look here for venting and moisture barrier info specific to your area: www (dot) buildingscience.com/doctypes/designs-that-work


Why Kraft faced rather than vapor barrier?


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## brendanstl (Jan 29, 2010)

Here's the pics.


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## brendanstl (Jan 29, 2010)

and more


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## brendanstl (Jan 29, 2010)

A few more


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## brendanstl (Jan 29, 2010)

This is it


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## brendanstl (Jan 29, 2010)

In all four corners I'm going to build rake walls under the valley board and remove all the bulkhead walls to give more living space for kids.


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## jmiller (May 14, 2010)

brendanstl said:


> Why Kraft faced rather than vapor barrier?


I'd be leary of a vapor _barrier_ there because in the summer it's on the cool side of the assembly. I edited that post to add, though, that you should check out the building science site to see what they recommend for your temp zone.



brendanstl said:


> Here's the pics.


That's a tough one (not at all what I was picturing). It may have had gable vents that were lost when they first finished the attic, unless it's always been that way.

You're never going to get enough intake through soffit vents to equal the exhaust you'd create by installing ridge vent. I doubt anything catastrophic would happen, but it wouldn't be effective. Snow wouldn't come in the vents because they're still an exhaust, just not a good one. Usually when I've seen that it's because a ridge lower than the main peak is ridge vented, and the lower one acts as snow intake. 

Personally, I'd go with a non ventilated/conditioned attic using spray foam in this situation. For a standard gable roof I'd prefer the underside of the decking be ventilated with continuous baffles, but here I'm at a loss for how that could be done effectively.

Not sure what effect that would have on asphalt shingles. Usually when I've seen it done it's metal or tile or something less prone to heat damage (not to mention less likely to leak which could go unnoticed for a long time). A lot of inspectors have been calling out blistered shingles lately, and I can't help but think a 'hot roof' ventilation system would contribute to that possibility.


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## neolitic (Apr 20, 2006)

Insulate the floor/1st floor ceiling
in the side bays, and turn those little
square side windows into louvered 
intake vents.
Install ridge vents and fur out the 
center bay for cathedral insulation.


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## Grumpy (Oct 8, 2003)

brendanstl said:


> The rafters are framed with 2x4's (old school lumber) and I was going to fur the rafters down 3 more inches which would leave an inch void between the sheeting and insulation, if I drilled the holes in the center of the rafters after I furred them down that would put my holes in the center of the insulation and common sense tells me that there would be no circulation of air, am I correct?


What the engineers say to do and what works in reality aren't always the same. I do know if you don't center the holes you will weaken the rafters. I do not also think that the furring will add any structural support, infact I do think the furring would increase the load the rafters carry... so the decision is yours on what to do. It's a tricky situation you have.

Don't forget the rafter baffles if you are going to vault the ceiling and insulate between the rafters. This will ensure there is an air flow and you don't accidentially press the insulation against the bottom of the roof sheathing.


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## Grumpy (Oct 8, 2003)

After review of the xterior pics, I doubt you will be able to create a soffit intake. If you were to remove the exterior aluminum you will likely see that the wall goes all the way up to the bottom of the sheathing and that the overhang has exposed rafters and sheathing. If you can't create an intake at the soffits you will have to look into another product like the smart vent or edge vent placed on the roof, past the wall. 

from the interior pics, i can definetly see that you can't create an intake at the soffit. You'll have to reply on the above mentioned products. 


My advice is to forget the ridge vent, and to install gable vents and fans to create a wind tunnel. Consider solar fans which will operate year round. Or consider traditional wired fans with humidistat. Mixing ride vent with gable vent is not usually recommended, however those little side windows may be low enough to create a proper intake. I'd still be very hesitant to try it. 



LOL one thing's for sure. Most roofers would slap a ridge vent up on it and call it done. LOL.


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## Grumpy (Oct 8, 2003)

About spray foam, this may limit your shingle warranty from 30 years to 15. It's not necessarily new to spray insulate the rafter bays, however shingle manufacturers are still on the fence about how it will affect their product. They are taking a wait and see approach, and seeing how it affects their product in real life. One thing is for sure, once one shingle manufacturer accepts it the rest will be forced to jump aboard.


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## brendanstl (Jan 29, 2010)

There is no real way to create air circulation due to the fact that I will be building rake walls under the valley boards and getting rid of the little cubby holes in the corners. What I may do is fur the trusses down enough to have an inch or two gap between the sheeting and the insulation slap on a ridge vent and see how long it takes to bake some Timberline T30's, if I get 15-20 out of them I will be happy. What do you think I'll get?

What I may consider doing is build one wall on each side of the valley board, start at two foot of center at the exterior wall and flare them in towards the interior wall, put a gable vent in at each corner, add baffles, insulate and put a ridge vent in.


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## SLSTech (Sep 13, 2008)

First off - 2" inches is also known as 1 lift which can actually range from 1" to 3" - make sure you get in writing that it is 2" minimum across the whole bay, not just average

Second - it depends on the R Value of the product & your goal, but you also need to remember that it will loose some RValue in the first year (at best this will give you an R12)

4" or 2 lifts is how much they use in walk in coolers & should be adequate for your area / my minimum - further north & then you need 6" or 3 lifts per a contractor from Canada


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