# IRC on venting



## Pie in the Sky (Dec 21, 2011)

What are your guys thoughts on the 1/150 requirement for ventilation. I would assume most contractors take the reduction to 1/300 just because of costs. Seems like 1/150 is difficult to achieve on most houses.
-What are your experiences with this?
-Have you seen any issues with ventilation when it appeared to be properly designed?


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## shazapple (Dec 3, 2010)

A 20x30 house requires 576square inches of ventilation at 1/150. Assuming 50/50 split between ridge and soffit a typical ridge vent easily meets the required 288sqin (assuming 15sqin NFVA per foot). Any perforated soffit, and most other continuous soffit materials, should also easily meet the required 288sqin.

The problem comes when trying to retrofit a roof that doesn't have enough (or any) ventilation. A lot of products don't provide enough vent area or too few are installed. A good example would be the round soffit vents. The 4" variety gives 2.4sqin of vent area, so you would need approximately 4 of those vents per foot in order to satisfy the vent requirements of my above example. Talk about swiss cheese soffit!

Edit: In my local code I believe the 1/150 only applies to roofs below a certain pitch (3:12?) or buildings without a proper vapour barrier in the ceiling.


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## LCG (May 30, 2011)

If we have the option and are installing the soffit, fascia and roofing we typically install center vented soffit. We don't install solid, solid, solid, vented, etc... Just vented soffit. We don't even buy solid soffit. Never have, never will. We don't want our guys interpreting code.

As Shaz stated. Retrofitting per IRC/ICC code can be a Bit%$. Typically we will evaluate the attic space on our job walk. If a roof has been in place for 30yrs with no signs of moisture issues than I will typically duplicate the existing (If it's not broke. Don't fix it). If rettrofitting is necessary than so be it.

As far as ventilation I think it's more important to follow code on new construction rather than re roofs. I have seen too many roofs with new venting systems that send the old lumber into a frenzy. It has been under the weight of an old roof for 50yrs. The old 1X sheathing has been in a fairly constant climate for all that time. Now all of the sudden you take eveything off it, install new OSB over the top of it, new ventilation and a metal roof? That old wood can go every which direction but right.

I think ventilation is a very touchy subject for a lot of contractors. They simply do not have the education/knowledge to properly evaluate a roof and install the PROPER ventilation. Let alone troubleshoot it. I think most have the best intentions but use new products as if they are stand alone solutions, one size fits all.


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## shazapple (Dec 3, 2010)

I think it's always beter to over ventilate, which isn't hard to do with current products. The difficult thing is getting contractors to understand an intake (soffits) AND an exhaust (ridge, gable) is required.


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## Pie in the Sky (Dec 21, 2011)

Well put LCG 

- Isnt it the responsibility for the contractor to bring the roofing up to code if it is not? I mean if he doesnt touch it and there is an issue, then I would revert bact to IRC and or the shingle manufacturers installation instructions.


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## LCG (May 30, 2011)

shazapple said:


> I think it's always beter to over ventilate, which isn't hard to do with current products. The difficult thing is getting contractors to understand an intake (soffits) AND an exhaust (ridge, gable) is required.


We are dealing with a home right now that had a new metal roof installed two years ago. It had a fair amount of turtle vents on it origionaly. The contractor covered the turtle vents and installed a continous ridge vent and didn't allow for any intakes. Of course now it has condensation so bad there is mold starting to grown and a half inch of frost on the under side of the roof.

Part of the issue is the ridge manufacturers say things like "equals 8 turlte vents" on their labeling. Without intake they are pretty much useless.

Your right Shaz. Most just install one or the other. They would be better off installing gable vents. Shoot we install at least 10 sets of gable vents a year. Sometime's it really is the best solution. Then again here in Wyoming the wind always blows so cross ventilation is not a problem.

I always hate retrofitting hip roofs. BY design a hip roof does not have the right amount of air volume to begin with. Like home's built in the 70's and 80's. 2-3/12 over a 40' span and two little gable vents. Mold and Heat warp here we come!

We just sat through an all day 2012 ICC compliance meeting. Don't get me wrong I believe in building up to code. It just seem's at this point we are becoming a bit redundant. Build it tight, spay foam the hell out of it, vent the hell out of it (somehow), oh yeah.... Now that we have sealed the house so tight with no more than 3 air changes an hour we have to induce fresh air into the home through the air intake side of the HVAC. 

Now this theory stems from the commercial side of things such as hotels where fresh air induction is an absolute must. But, C'mon... this is just getting to the point of no return. It seems as if the government is getting a bit too involved by way of energy efficency.


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## shazapple (Dec 3, 2010)

You mentioning that metal roof install reminded me of something. I guess the whole point of venting is to eliminate any 'double vapour barrier'. Here is an interesting article that finds that asphalt shingles as an assembly can actually act as a vapour barrier, so any moisture in the attic cannot escape unless there is ventilation. Obviously a metal roof would act much the same way, or a roof completely covered in synthetic underlay or ice and water shield.


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## Pie in the Sky (Dec 21, 2011)

Im not really sure about the Whole double vapor Barrier thing is something to be afraid of... There is still alot about vapor movement thats mis understood. When I was a roofer I ripped off tons of roofs with double vapor barriers.. They were Built ups.. 2-ply VR at the bottom and BUR on top. 

Dont you also vent to reduce heat build up? 

I wish I had unlimited funds.. There is alot off cool testing you could do.


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## shazapple (Dec 3, 2010)

Pie in the Sky said:


> Im not really sure about the Whole double vapor Barrier thing is something to be afraid of... There is still alot about vapor movement thats mis understood. When I was a roofer I ripped off tons of roofs with double vapor barriers.. They were Built ups.. 2-ply VR at the bottom and BUR on top.


That is our typical setup around here. 2-ply mopped vapour barrier and 2-ply modbit (or 4-ply mopped felt) membrane. It's a whole other animal than ventilated roofs, simply because of how it is built. If moisture gets into a flat BUR or modbit roof it will likely never escape, but the damage it can cause is limited to the components between the vapour barrier and membrane (aka the insulation). It will reduce the R value and affect the stability of the materials, but the vapour barrier typically protects the structure from any rot/rust. 

Roof systems with air spaces typically have the structure between the vapour barrier and membrane. Moisture intrusion is more likely to effect structural components in this case, and If there is no opportunity for drying then you'll get rot/rust.


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## Pie in the Sky (Dec 21, 2011)

Great point!!!!


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