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bill's quest 07-30-2009 08:34 AM

test for ventilation or new soffit vents?
I posted recently on a new roof installation, but thought to break this out as a separate topic...

How do you test to see if the attic is adequately venting? The contractor's response is,"You have ridge vents, heat rises, don't worry." Well, we know how short sighted this is...and he eventually gets that one has to consider intake, too. So,his next response is, "You have continuous soffit vents, we used a blower (from the outside) to blow out any obstruction, don't worry." The problem with this is that the lourve vents have been compromised by paint, not too bad but still covered with paint and some places clogged, and that this is 40 yr old 3-12 pitch. Then I did what his crew was to do but kept avoiding... I crawled into the attic and sampled a few places along the eaves. The insulation is mounded near the eaves but not over the vents and there is about 3-4 inches of clearance. So, this is encouraging, but should I consider that I am getting good ventilation? I don't want just minimum ventilation....the last roofing job left me with a lot of warped planks.

So is there is a simple smoke test or is there a standard for temperature differences in the attic vs outside temp?

The soffits are 3 feet wide. Should I just rip out the old vents and put in new ones?....A few contractors look puzzled when asked to bid on this. They talk about taking all the plywood to do the job...since the vents are secured from the inside.

glennjanie 07-30-2009 04:59 PM

Welcome Bill:
I recently had vinyl siding added to my house, including the soffits with vents. I can stand in my attic with the lights off and see light all along the soffits and along the ridge vent. I think this is a perfect way to ventilate an attic and will preserve the shingles by cooling them.

It sounds like you have a good system too. You may want to use a wire brush to open some of the eve vents if they are too clogged.


Nestor_Kelebay 07-30-2009 06:32 PM

I don't think there's any widely accepted method to test the amount of attic ventilation you have. That's because attic ventilation depends on either convection of air out through a ridge vent, or wind ventilation of the attic through soffit and gable vents.

Contrary to the popular misconception of everyone who lives in the south half of the United States, attic ventilation is not all about keeping your attic cool in summer so you can sit up there. It's really something you need in winter. That's because having an unbearably hot attic is uncomfortable, but it won't cause any damage to your house. An excessively hot roof would bake your roof shingles, but not cause any structural damage to your house. Also, I'm thinking that white roofing paints would help keep your shingles cooler than attic ventilation.

Having insufficient ventilation in your attic where I live (Winnipeg, Manitoba) can cause really serious damage to your house.

You see, regardless of how well contractors try to build houses, heat rises and there is inevitably heat loss as the warm air in the insulated part of the house escapes through ceiling light fixtures, ceiling fan housings, electrical boxes and everything else to escape into your attic. There, the warm moist air rises and ends up forming frost on the underside of a cold roof. Where I live, over the course of a winter, a house with very poor attic ventilation can accumulate up to 2 inches of frost on the underside of it's roof. (I've seen a picture of such a situation myself.)

That thick layer of frost on the underside of the roof doesn't do any damage at all... until spring, when it starts to melt. When the warmer temperatures come in spring, the frost starts to melt and you have melt water dripping onto the cellulose or fiberglass insulation between the ceiling joists. Insulation is a wonderful thing. It works by keeping air stagnant, and that means there are no air currents within the insulation to carry heat from the warm side of the insulation. But, that also means that if your insulation gets wet, there are no air currents in the insulation the help it dry out. In fact you could die of old age watching insulation dry.

And, having wood ceiling joists in contact with that soaking wet insulation is a recipe for wood rot. Wood won't rot unless it stays wet enough for long enough that the wood rot fungus (genus name: "Serpula Lacrymans") starts to grow on the wood, but being in contact with wet insulation will keep the wood wet for much longer than it would if there were no insulation cuz the moisture just stays in the insulation... there are no air currents in there to help it dry out.

Also, if it's a well built house, it'll have a layer of polyethylene vapour barrier between the ceiling joists and the drywall screwed to those joists. That vapour barrier will prevent the ceiling drywall from getting wet from the wet insulation and hide the problem from you. It won't be until you start to notice something wrong with the ceiling that you'll learn the extend of the damage and by that time it will be too late to prevent it.

In milder climates, proper attic insulation also helps with something called "ice damming".

A typical house will have it's roof extend beyond it's exterior walls by a few feet. Warm air rising into the attic can keep the roof warmer than the outside air by a few degrees, and that can result in snow melting on the roof. As the melt water drains down the roof and encounters the COLDER part of the roof outside of the exterior walls, that melt water can freeze. The melting of snow on the roof and refreezing of the melt water on the overhang can result in a "dam" forming on the eves that contains that melt water in a puddle on your roof. As that melt water puddle grows, it can get under your shingles and then into the roof where it can cause water damage to the exterior walls (or ceiling) of your house. Having good ventilation in the attic will allow the heat in the attic to be lost so that you have less melting and therefore less ice damming going on on your roof. And, where I live, it's common to have at least one, and commonly two widths of something called "Ice & Water Shield" on the edge of your roof. The purpose of Ice & Water Shield is specifically to prevent water penetration into your roof as a result of ice damming.

So, now you know the basic reasons why you need attic ventilation.

Attic ventilation solves these problems because the whole gameplan is to allow cold outside air into your attic, where it will warm up. As the cold outdoor air warms up, it is able to absorb moisture, and so it evaporates any frost of condensation up in your attic. That warmer air then somehow escapes from your attic, taking that moisture with it so it doesn't do any harm in your attic.

The most popular gameplan for attic ventilation is to allow cold outdoor air in at the soffits and to allow it to escape out a ridge vent at the very top of the roof. People claim that system has the best "sweep efficiency", and is therefore the most efficient way to ventilate an attic.

There's no question that a combination of soffit and ridge vents would THEORETICALLY work well. But, theoretically often doesn't jive well with reality. Where I live, winters are usually pretty windy affairs. When the wind blows against the side of your house, cold dry air gets pushed into the soffit vents on one side of the house, and the result is that the warmer wetter air in the attic gets pushed out the soffit vents on the other side of the attic. So much for the calm convection of air up and out the ridge vent, eh. And, where I live, a calm snow fall ends up covering the ridge vent with snow anyway, and at that point it becomes useless.

My thinking on it is that once the warm wet air has risen above your ceiling drywall, it's not going to do you any good any more. If it does anything, then it's going to do harm. The moisture in it is gonna form either condensation or frost, and the heat in it will warm your roof, and neither of those things is gonna do you any good, only harm. So, my philosophy is, put in gable and soffit vents for windy days, put in a ridge vent or whirlybird vents for calm days, and the more holes in your roof, gables and soffits, the better. Wind is going to make the air flow patterns in your attic anything but organized and predictable, and in a situation like that, more holes makes for better sense than planning for an calm and orderly evacuation of the warm moist air in your attic.

Here's how you determine if your attic ventilation is adequate:

Go up into your attic on the coldest days of winter and get above the highest humidity places like the kitchen and bathroom. Look for the formation of frost (or, in warmer climates), condensation (and/or mildew) on the roof rafters, trusses or underside of the roof. Any frost you see up there should be gone within a coupla days (3 or 4 days) max. If you have really good attic ventilation, you won't see any frost or condensation or mildew up there to begin with, except perhaps rarely. Occasional formation of frost or condensation won't do any harm. The biggest concern is the ACCUMULATION of frost or condensation up there during the winter.

But, it's best to check when conditions for it to form are the highest; on the coldests days of the year after a lot of cooking or a lot of showering.

If it wuz me, I might consider installing a ceiling fan in a gable wall to blow air INTO your attic if ice dams ever become problematic. The air will always find a way out, but even a small ceiling fan running continuously will keep your attic full of cold outdoor air, and that'll stop the melting of snow on your roof. There are other ways of combatting ice damming too.

bill's quest 07-30-2009 08:00 PM


i live in kentucky.....we get both heat, humidity, and cold....but, of course, not anything in the way of winter chill and ice as you get. What i take from your thoughtful commentary is that one size does not fit all. Roofing requires attention to the uniqueness of the home.

From the prior roof installation,years over years of a slow leak and very poor ventilation led to rotten wood and watching my hall walls weep condensation....and, probably a sagging roof and warped planks of wood on the deck. To a large extent, I'm in a reactive mode. I don't want this to happen again. Even with a sound roof, the humidity of the ohio valley is almost as bad as a prior orleans.
I can't expect the attic temp to be the same as external air in the summer time but I'm surprised that there is not some standard as degree of differential.
When it was 86 outside, the floor of my attic was over much more I don't know because the thermometer did not go over that amount.

I've thought of putting in soffit siding but the plywood needs to come out...3 foot overhang...ain't cheap. From what I've read the risk is condensation that does not get aired out.

I wish that there was a continuous soffit vent much wider than the 2.5inch that could be popped into a recut opening and looks good....if that makes sense.

GBR 07-30-2009 09:09 PM

Can't help much with the test, but you can figure your net free venting area: Audel Complete Building Construction - Google Books
Gable ans soffit vents covered.
Use of fan: Ron Hungarter investigates black mold with Air Vent, Inc.

Clean your vents and more wind washing: -Attic Ventilation Problems

Probably saw this: Air Vent: Continuous Soffit Vents Specifications

The 2-3/4" continuous soffit vent is rated perfectly for supply to each rafter bay, if you do the math splitting 50%, high and low. You could use the 8 x 16" metal add-on soffit vents but they only have 56 sq. in. per vent of net free venting area (NFVA). And a lot of dead air spaces. Use the soffit/fascia board vent (that's where is should be located anyway), need only to make one rip cut- a swath next to the fascia only
Be safe, G Sorry, I double posted all this again, getting older...

Nestor_Kelebay 07-30-2009 11:39 PM


Originally Posted by bill's quest (Post 32795)
From the prior roof installation,years over years of a slow leak and very poor ventilation led to rotten wood and watching my hall walls weep condensation....and, probably a sagging roof and warped planks of wood on the deck. To a large extent, I'm in a reactive mode. I don't want this to happen again. Even with a sound roof, the humidity of the ohio valley is almost as bad as a prior orleans.

I'm hearing a lot of different problems, and I'm wondering if we can alleviate some of what you're worried about by explaining that some things you're mentioning simply can't cause some of the problems you're concerned about.

My understanding is that wood CANNOT rot from high humidity alone, even extremely high humidities and high temperature like New Orleans. There has to be liquid water present for a number of reasons. First of all, the fungi spores won't even start to grow unless they are physically wet. Next, the spore will only continue growing if the wood moisture content is above 28 percent. Between 22 percent and 28 percent, an established wood rot fungus will survive, but not grow. Below a 22 percent moisture content, an established wood rot fungus will become dormant and eventually die. Wood rot spores will die after several years if they don't have the opportunity to grow during that time as well.

Take a look at this web page written by Paul Fisette of the University of Massachusettes at Amhurst Department of Natural Resources Conservation:

UMass Amherst: Building Materials and Wood Technology » Wood Myths: Facts and Fictions About Wood

In it, Mr. Fisette says: " If you install a new stick of lumber against a piece of rotted wood the new piece of lumber will not begin to rot. Rot fungi are “seeded” by the spread of single-celled spores. These spores are everywhere. Essentially all wood is exposed to the seed stock. Only when conditions are right will the infection develop into rot. The moisture content (MC) of wood needs to be above 28% to be initially infected. Since all lumber is above 28% MC at some point in its life, all lumber is infected. When the MC of wood drops below 22% the rot fungi goes dormant. It’s harmless, but is will be reactivated when the MC rises above 22%. The solution is: keep wood dry or poison it with a chemical treatment."

Now, look at this page that lets you calculate what the moisture content in wood will be under various conditions of temperature and relative humidity:

Wood Equilibrium Moisture Content Table And Calculator

You can see from that table, the wood simply will never reach that high a moisture content due to temperature and relative humidity alone. There has to be a source of liquid water ADDING moisture to the wood, such as a roof leak, or the wood being in contact with wet ground from which it can absorb moisture, etc.

Also, if you remain concerned about wood beginning to rot due to high humidity, you can always spray any wood in your house with a borate solution called "Borocol". Borocol contains the element boron which is present in the product as the chemical disodium octoborate tetrahydrate, which we'll call DOT for short. DOT is very effective against all kinds of fungi, including all of the wood rotting fungii, but it's almost harmless to mammals like people and pet dogs and cats. It's highly soluble in water, so it can't be used where there's running water, or water that would be lost such as rain water. However, in the case of condensation forming on wood, that condensation will eventually evaporate, leaving the DOT behind on the surface or inside the wood. Consequently, the ability of DOT to protect wood from wood rot would not be diminished by condensation, as long as that condensation didn''t collect into puddles and drain away (carrying the DOT in solution with it). The use of borates to prevent wood rot is much more common in Europe where they've always been more concerned about the effects of chemicals on the environment. For more info on Borate based liquid wood preservatives, go to this web page:

Technical Info Links


From what I've read the risk is condensation that does not get aired out.
Is this condensation that falls on the floor of your attic, or condensation that falls into the fiberglass or cellulose insulation between the ceiling joists of your attic? Wouldn't simply covering the exposed ceiling joists of the attic with an absorbant cloth, (like old bed sheets) absorb the condensation and prevent it from falling into the insulation?

There are white roof paints on the market specifically to reflect the light of the sun off the roof. If you Google "roof paint", you are sure to find the products available. I have no experience with these, since where I live, keeping warm in winter is very much more expensive than keeping cool in summer. But, I'm thinking that if the paint reflects the Sun's light, it will also keep your new shingle roof cooler too, extending the life of the shingles. It would also make your roof extremely slippery when wet, too.

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