Roof venting, ridge vent or power fan?

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On Thursday 21 February 2013 23:40 HeyBub wrote in alt.home.repair:

Feel free too - but you are disagreeing with established government regulations[1] and not my personal opinion (OK, it's the UK, but the physics and aim of preventing condensation on timbers remains the same).
25mm or 1" is obviously an arbitrary rounding for convenience, but they will not be far out from whatever was worked out as "ideal".
[1] This branch of regulation is actually quite sane and deviating from it would require actually thinking "why do I think i know better?".
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Tim Watts wrote:

From the International Code Council:
---begin quote R806.1 Ventilation required. Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. Ventilation openings shall have a least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Ventilation openings having a least dimension larger than 1/4 inch (6.4 mm) shall be provided with corrosion-resistant wire cloth screening, hardware cloth, or similar material with openings having a least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Openings in roof framing members shall conform to the requirements of Section R802.7. Required ventilation openings shall open directly to the outside air.
R806.2 Minimum vent area. The minimum net free ventilating area shall be 1/150 of the area of the vented space. http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm --- end quote
Evidently, reliance on the UK "code" (not cited) is a recipe for disaster.
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wrote:

Quoting or referring to sources in which you don't understand the reason for them is silly in my opinion. Why not see what the local building code requires since that's the one that counts?
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The source he cited is used as the basis for the code in much of the USA, isn't it? From everything I've seen, that 1:150 ratio is widely used by building science experts as what is recommended today. You can probably get away with 1:300, which was the older standard, if you have to. The reasons for requiring adequate ventilation are well known.
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On Fri, 22 Feb 2013 07:15:20 -0800 (PST), " snipped-for-privacy@optonline.net"

Not sure. I've used the UBC and SBC years ago and there's another called BOCA. Don't recall using that one. Here's a reference for you but I would still call the local jurisdiction to see what they use in your county. I agree with all your research, you probably are correct but as long as you are going to this much trouble might as well do it all the way.
http://www.starrco.com/misc_nbc_codes.php
Keep us posted when/what you decide <grin>.
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Did you see my post? Your math had one substantial error and one whopping 12X error. Applying the 1:150 ratio, you claimed your 25x25ft attic needed 20 sq ft. It only needs 16.6. A bigger mistake was when you came up with 1400 linear FEET. It's not feet, it's INCHES. Doing the math correctly, that attic would need 200 sq in of venting. Further, that is TOTAL venting, ie it's split between intake at the soffits, exhaust at the top. So, to meet the 1:150 ratio, you only need 100 sq in at the soffits. With a 25x25 attic, that would be met by 1" wide openings run along the two sides of the roof, 2x25x10.
What Tim is saying is done in the UK would easily exceed that, if it's done on all 4 sides, with 1". Or using 2" on just the two sides would do it too. Curiously, that is what I have here in the USA.
Also, I see your reference to the current Intrnational Code being 1:150. I agree that's sounds like a desirable number and it's recommended. I think it's a good idea for a number of reasons. However the roofing manufacturers who are giving a very long, if not lifetime warranty on their products, say 1:300 is the acceptable minimum. At least the ones I've looked at in detail.

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I agree erring on the side of having more is better than having less. But going nuts and putting in 10x what is needed doesn't make any sense to me either.
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You might want to recheck that math. At the ratio of 1 sq ft for every 150 sq ft of attic floor, you need 16.7 sq ft, not 20. That is 2400 sq in. With a 2" wide vent, you'd need 1200 linear INCHES, not feet. In feet it's 100 feet.
But there is another thing missing here. That ratio of 1 sq ft of venting for every 150ft of attic floor is for TOTAL venting, which is supposed to be divided between intake and exit. So, for the example, you'd need only 50 ft of soffit venting. Using soffit venting on both halves of the roof, you'd have twice that. I was making this mistake earlier today when I was doing the math too.
And the minimum acceptable venting ratio is 1 sq ft for 300 sq ft of attic floor, so I should have plenty.

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If anything, you probably need more with big fans. One big issue with attic fans, and why most experts are against them is that they create negative pressure in the house. This winds up sucking air conditioned air through any available route, eg ceiling recessed lights, up into the attic. So, you're increasing the cooling load on the house.
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... snip ...

One unasked for thought...
If you have pull down attic stair or even just an access panel, built one of these to insulate the opening:

http://m.youtube.com/#/watch?v=8POZvK9QB6k

I just recently built one and realized I should have done it 25 years ago when I first moved in. When I pull down the attic stairs and put my head up into the box, I can feel the heat that the box is keeping from going up into the attic.
When I had my roof replaced last year, I had full soffit vents cut and a ridge vent installed. I also added baffles to improve the air flow. No icicles on my house when other houses had them. Snow remaining on my roof after other houses had none.
Prior to the new roof, I had no soffit vents and only gable end vents. Icicles every year, ice dams kept at bay with heat wires. This year, no wires, no icicles. I'm pleased.
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So, the vents let out the heat, instead of your heat staying in? That could be a bad thing. You have insullation to keep the heat in while the roof is cold?
Christopher A. Young Learn more about Jesus www.lds.org .
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When I had my roof replaced last year, I had full soffit vents cut and a ridge vent installed. I also added baffles to improve the air flow. No icicles on my house when other houses had them. Snow remaining on my roof after other houses had none.
Prior to the new roof, I had no soffit vents and only gable end vents. Icicles every year, ice dams kept at bay with heat wires. This year, no wires, no icicles. I'm pleased.
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The soffit vents, ridge vents and soffit baffles keep the roof surface cool to prevent the snow from melting down to the cold overhang and freezing, causing ice dams. That's one of the reasons for adequate attic ventilation.
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If you have no insullation in your attic, that could mean more heat used. If you hvae insullation between you and the vents, then the vents are a good thing.
Christopher A. Young Learn more about Jesus www.lds.org .
wrote:

The soffit vents, ridge vents and soffit baffles keep the roof surface cool to prevent the snow from melting down to the cold overhang and freezing, causing ice dams. That's one of the reasons for adequate attic ventilation.
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On Feb 22, 7:39 am, "Stormin Mormon"

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Well, I guess I just assumed that we were discussing a typical house with at least some attic ventilation.
I don't think too many people would pay to have soffit vents, ridge vents and baffles installed if that meant the heat would just fly out through the roof.
On the other hand, keeping the heat in (assuming there is no insulation) - especially in the summer - would mean that you were cooking the roof from underneath.
Bottom line: Insulation and ventilation are both required for energy efficiency, comfort and roof longevity.
Here's what I know: The new venting and baffles have significantly improved how the snow acts on the roof. The attic stair cover I mentioned in another post is limiting the amount of heat lost via the stairway access opening. All in all, I'm happy with the upgrades.
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On Friday 22 February 2013 13:54 DerbyDad03 wrote in alt.home.repair:

The ideal is to seal the cold side against vapour transfer and heat loss and ventilate sufficiently.
The 25mm-continous figure for soffit venting in the UK comes from BS5250
http://www.commercialconnections.co.uk/Content/building_standards/BS5250%20-%20Code%20of%20Practise%20for%20condensation%20in%20buildings.pdf
which is referred to by Approved Document C (Building Regulations):
http://www.planningportal.gov.uk/uploads/br/BR_PDF_AD_C_2010.pdf
The 25mm is conservative - it does say it is not always required, but builders tend to install by default, unless you have a breathable roof membrane under failry air-leaky tiles.
In my case, the roof itself is seal almost completely airtight with PIR foam boards. Once I have an additional vapour barrier under that to reduce moisture movement into the rafters (which bridge the hot and cold sides of the insulation, therefore are subject to the risk of interstitial condensation) I will need very little ventilation into the batten space.
If I still had bitumen felt sarking under the tiles, I would need the 25mm vent in the eaves and ridge vents.
A lot of older houses here insulate the attic floor and a naturally leaky roof allows enough ventilation. However, people have come unstuck when they've increased the insulation and rammed loads of glasswool onto the eaves blocking the gaps. Coupled with very moisture permeable ceilings leads to disasterous amounts of condensation in the attic in colder months as some have found to their cost.
--
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http://www.sensorly.com/ Crowd mapping of 2G/3G/4G mobile signal coverage
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