Roof venting, ridge vent or power fan?

[trader4]

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

1. 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.

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.

[trader4]

From the research I did today, it seems most credible sources agree with this.

Currently both roof sections that have fans also have a gable vent, plus the soffits. So, you have two openings up top for the air to exit. No moisture issues.

When I bought the house 18 years ago, that was one thing I caught and corrected. There were no baffles and the insulation was blocking the soffits. I installed baffles.

No problem with that. All that needs to be done is close it off from inside the attack.

and

Given all that I've learned researching it so far, I'm hjeading in the direction of going with the ridge vents. Next thing is there are all kinds of product, from rigid plastic ones that come in 4 ft sections, to ones that come in rolls. Oddly, the price is almost the same. I'm guessing that the roofers like the roll material because it's probably easier to install. However the rigid ones have more airflow, like 20 sq in per foot versus maybe 14 for the other stuff. And with the roll type, I think it would be easier to not install correctly, ie mash it down, etc so you could wind up with even less venting. So, if I go ridge vent, I'm going to spec which one.

Thanks for the help. And if you have any more tips, I'd be happy to hear them.

[DerbyDad03]

... 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:

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.

[krw]

+1

No, it's not better. It's noisy and doesn't work nearly was well as ridge vent with the appropriate (amount of) soffit vent.

Some are nearly impossible to get to (both bathroom and attic). Go passive.

[bob haller]

my home had just a gable vent at either end, no soffits so no soffit vents and no ridge vent.

inside attic temperature around 140 degrees, used my fluke recording temp meter highly accurate...

got eve vent installed high attic temp dropped to 115 degrees.......

my ridge vent worked great

[Vic Smith]

I've had 2 roof power vents in this house in the 16 years I've owned it. The original was replaced with a new one when I re-roofed about

10 years ago. Attic is about 1200 sq ft and batted. Neither one was loud.or vibrated.

[Stormin Mormon]

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

.

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.

[DerbyDad03]

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.

[Tim Watts]

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?".

[HeyBub]

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.

--- end quote

Evidently, reliance on the UK "code" (not cited) is a recipe for disaster.

[Stormin Mormon]

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

.

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.

[trader4]

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, 2x25x1=100.

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.

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.

[DerbyDad03]

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.

[Tim Watts]

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

which is referred to by Approved Document C (Building Regulations):

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.

[Doug]

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?

[trader4]

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.

[Doug]

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.

Keep us posted when/what you decide .

[Chomper]

Specifying the type and installation method in _writing_ would be a very wise decision. Hope I don't come off as a sales person on the manufacturer & type we always used.

My choice was Shingle Vent II by Certainteed. At the time, it was the _only_ brand & type, which met Dade County's stringent code because of the severe wind driven elements. (BTW, I roofed in the Midwest, not Florida).

I would specify the ridge is to be cut no more than 1" on each side from the peak of the rafter/truss. A line to be chalked to follow for cutting, with the blade of the saw being no more than 1/16" through the plywood. - Reason being is some never set the saw depth, and they attempt to free hand their cuts, cutting to far down from the peak.

I would also specify a bead of Geocel 3300, with a minimum width of 1/4" x

1/4" high running the continuous length of the ridge vent of both sides, so the vent can be imbedded (this will prevent wind driven elements from entering underneath. Mechanical fasteners to be _screw_ type, being 2-1/2" in length being placed at least every 24" O.C. on each side to hold the vent down. Shingle caps to be mechanically installed with barbed nails being at least 2" in length.

I would not let anyone install the rolled vent. At first, people thought the netting/cloth/fabric used in these would deteriorate. They are designed to keep bugs/wasps out. The problem is attics get a lot of dust, after years, the cloth collects the dust, reducing the ability to function properly.

Maybe I come off as being anal on how to install roofing. However, back in the day, I was proud of my work, which consisted of highly specified projects, down to the diameter of a fastener head. In fact, I'd still be doing this type of work, if age didn't catch up with me some years ago.

[Doug]

No disrespect intended here because I think your knowledge is helpful but I wouldn't spec all of this (even if possible). No problem spec'g material because that's easy to confirm but spec'g workmanship is another unless you have an inspector up there during installation.

[Doug]

Just further thought on this.... I guess you can spec workmanship but just hope it's done to your specs if you can't inspect it during construction.