Last time I had our roof replaced, we went with ridge vents instead of roof vents.  Unfortunately, I did not measure the summer attic temperature before the change, however, I do remember quite a bit of air flowing into the attic on a hot day when I opened the attic access.

We went with ridge vents, but not the lower continuous intake.  Remembering the flow mentioned above, we added 4 one foot intake vents at the bottom on each corner of our home under the overhang.    Last summer when I was experimenting with ways to cool our home, I placed the outdoor part of an indoor outdoor thermometer in the attic.    I'm not sure how much I trust the thermometer, but to my surprise it was quite hot during hot summer days, sometimes reaching 155 degrees.

My first thought was that we needed more intake vents, so as an experiment, I left the access door open to the attic for a week and it made no difference at all.

Next, I took a 20 inch box fan, mounted it to one of the intake vents outside (under the overhang on the north side) and blew air into the house.  Maybe 5 degree temperature lower at best and maybe cooled off a little sooner at night.

In Denver, the attic always cools off at night and with my non-contact thermometer, I measured the ceiling temperature on hot days.  I have quite a bit of insulation in the attic and the inside ceiling was always room temperature and I don't thing heat was building into the house from the attic.

Still seems like it might be to hot in the attic to me and would mind experimenting.   One idea was to install a different brand of ridge vent and I was wondering if anyone has experience with Shingle Vent II video ?   I would like to cool the attic some, but definitely don't want rain or snow blowing in.

I gather adding more other vents can be tricky because outgoing air may be sucked from the nearest other vents.

Ideas and experience?

A similar topic was discussed not that long ago here on this forum.

Attic venting is mainly for keeping the wood trusses and OSB dry. Specialized attic vents and fans have proven to be mostly a waste of time and money. As long as your attic has the proper code vents to allow for drying of any wood members that get wet, leave the attic vents alone. Spend your money elsewhere.

Attics will get hot in places like Denver. 155F sound about right in peak summer. Here in Phoenix they have recorded temps of 185F+ in the attic.

Depending on your location (city vs rural), the more attic vents you have, the more places to worry about embers and fires burning down your home. In rural spots this is a concern, in addition the attic vents provide more places for critters like rodents to get in an make a home and wreak havoc in your walls and ceilings. Nothing says home to a rodent like a warm, dry attic filled with nice insulation to sleep in and raise a family of rodents.

First, attic venting is never a strategy for cooling the house (except for whole-house fans and massive nighttime ventilation.) Roofs & shingles cool primary by radiation and convection on the exterior, and unless you reject the heat at the exterior (cool-roof shingles or radiant barriers under the roof deck) the attic is going to run hot. Whether that's an issue for keeping the house cool is a function of how much (and to some degree the type of) insulation you have at the attic floor. If you have enough insulation that the ceilings below are within a degree or so of room temp who cares if the attic runs hot?

But keeping the attic dry is another matter.

In general it's better to have more cross sectional area to the soffit venting than the ridge to limit the stack effect pressure to the whole house. By having the major opening at the soffits/eaves the pressure difference at the upper floor ceiling and the attic is small but if the major opening is at the ridge it becomes significant.

Gable vents and ridge vents don't play well together, since it short-circuits the flow from soffit to ridge, reducing the drying function. If you're going to vent the attic, VENT THE ATTIC, and massively, with a very large SOFFIT cross section (at least some soffit area for each raft bay) and a ~30-50% smaller ridge vent cross section. The stack effect pressure between soffit and ridge will still move air, but it won't depressurize the attic relative to the house.

From an energy use and comfort point of view you don't want to cool the attic by drawing cooler conditioned space air in through leakage at the attic floor/conditioned space ceiling plane, which is what will happen if you actively depressurize the attic with a fan. If you're pressurizing the attic with fan (as it sounds like you did) that would drive air flow in the other direction- cooling off the attic but driving superheated air into the conditioned space. It's much better if you rigorously air seal between the attic and condioned space, and put at least R50 in IR-opaque dense insulation on the attic floor (either cellulose, high-density rock wool). The goal is to cool the house, not the attic.

You COULD achieve lower attic temps by putting radiant barrier on the rafters, but unless you have uninsulated air handlers and ducts up there it wouldn't be economic in a Denver climate- increasing the heating season energy use more than is offset in cooling costs. (Rodent nests are less of an issue with cellulose than with rock wool or fiberglass, presumably due to the eye-irritant effects of the fire retardents used.)

Ridge vent do not ventilate when the wind is not blowing, like in the summer. What you want is a static roof vent, those will remove hot air with no wind.

Sounds like not nearly enough soffit ventilation. How large is your house? With a square foot at each corner, that air was probably going straight up the edge of the roof(gable) and out the vent without really removing any heat from the middle of the attic space. The soffit vents, like the ridge vent need to run the entire length of the roof to pull heat from the entire space. The problem with heating or cooling with air is the lower the temp difference, the more massive the quantity required to move the heat.

The house we are building now is 56X40(around 2000 SQ/FT). The 56' dimension will have a 3" soffit vent the entire length on each side of the house. That is 28 SQ/FT minus about 1SQ/FT for truss framing.

As mentioned if you are not really well air sealed between attic and living space, with such little soffit ventilation, your attic stack effect is placing the house under a vacume and causing warm air from outside to be drawn into the home to do battle with any cooling you may be providing. Or if not providing any AC, just raising the inside temp unnecessarilly, which takes longer to cool in the evening(back to that massive quantities of air thing)

The issue you mentioned with soffit vents Occurs(mostly in winter) when bathroom and dryer vents that go thru a wall or out to a soffit provide a source of moisture which is sucked up thru a soffit vent to then condense on the inside of the roof sheathing causing issues. The solution for this is to ensure bath and dryer vents go out beyond the soffit, up thru the roof, or out thru a gable end. If this is not possible then you omit the soffit vents in close proximity the these sources of moisture...

Really- we're responding to 4.5-years old long-dead threads now?

Yes, apparently WE are

Old thread or not, in WI this is probably one of the most ignored code rules we have, so maybe an example would be helpful for any construction nerds.

Attic ventilation products all have a "net free air" measurement. For example Rollex soffit gives 6.48 square INCHES of net free air per lineal foot of soffit (overhang dimension NOT running dimension). Owens Corning VentSure ridge vent gives 12.5 square INCHES per lineal foot of ridge vent (2 products I prefer).

WI code states a minimum of 50% must be on the HIGH side of the roof which is the opposite of what some manufacturers say and reps and whoever else will tell you.

Sample 30'x50' ranch house with a gable roof will fall under a minimum 1/150 or 1/300 square foot ratio. If I fully vent the ridge with VentSure I will get 625 sq in. of ridge venting, technically that needs to be 50% or more of my total attic venting. If I have a 2' overhang with fully vented soffit all the way on both sides I get 12.96 sq in of net free air PER PIECE of soffit with 37.5 pieces of soffit per side equaling 972 square inches of net free air at the eaves and a code violation because I have more venting low than high. 1' overhang fully vented front and back totals 486 square inches which is over my 1/300 minimum, less than my ridge, and code compliant.

To balance the ventilation with a 2' overhang, I want 625 square inches on the low side split into 2 for front and back and I get 24 out of 37 pieces per side should be vented soffit and the remainder should be solid. In that case I am well over my 1/300 ratio which is fine but don't quite make the 1/150 ratio which only is required if my ridge venting is over 75% of my total net free area. Since I'm balanced I'm only bound to the 1/300 minimum which for my 30'x50' house is 5 square feet or 720 square inches TOTAL.

To further muddy the waters, if my ridge venting was over 75% of my total provided net free air, for example 750 sq inches on the ridge and 250 sq inches on the eaves, suddenly my net free air required jumps to the 1/150 level. At that point it ironically becomes difficult to provide enough ventilation on the high side with ridge vent alone. My 30' x 50' house would be looking for 1440 square inches total, 1,080 sq inches up top and 360 sq inches down low. Typically the case with no overhang homes but new products allow for eave venting to be installed under the shingles or in the drip edge to pump up your eave venting and get you out of that 75% bracket.

Couple of other mistakes people make: don't use vented soffit on the gables if the framing is open to the attic; don't use ridge vent or vented soffit on dormers; don't do anything that short circuits the low/high airflow pattern. Those are the situations that can put a pile of snow in your attic. Expect very confused looks from your builder and your exterior soffit installer when requesting these calculations.

I'm only illustrating what my code says, I am not an advanced thermodynamics airflow specialist, nor has an inspector EVER asked for or questioned my net free air calculations or any other builder I know.

Some building scientists (Joe L included) believe that it's better to put 50-75% of the vent area at the eaves , 25-50% at the ridge. Having the larger area at the eaves limits the stack effect infiltration drive pulling air from conditioned space into the attic, since the pressure at the eaves is about the same as at the ceiling plane. There's no harm in going with much bigger ventilation area than code-min, but skimping on code isn't always risky either. There are plenty of unventilated attics in cold climates that have withstood the test of time, and many vented attics that have failed (many due to air leakage at the ceiling plane, or poorly considered bath venting schemes venting directly into the attic, now disallowed by code.) see: https://buildingscience.com/sites/default/files/migrate/pdf/PA_Crash_Course_Roof_Venting_FHB.pdf

The 1/300 rule has been around for decades, but it has no real science behind it- it's basically rule of thumb that works-mostly for heating dominated climates. The local climate, pitch & height of the roof etc can make that either adequate or completely inadequate, but it survives. The primary reason for venting the attic & /or roof is purging moisture, preventing rot, not cooling. In hot humid climates such as the gulf coast states, vented attics in air conditioned homes usually raises rather than lowers the moisture content of the wood in the attic (with many exceptions to prove the rule, of course.)

If you want an attic with temps closer to your conditioned space then encapsulate the attic with closed cell foam and eliminate all ventilation.
I did this in my newest home and what a difference it makes.

Posted By TWhite on 12 Apr 2017 01:49 PM
If you want an attic with temps closer to your conditioned space then encapsulate the attic with closed cell foam and eliminate all ventilation.
I did this in my newest home and what a difference it makes.

That's an expensive solution (and none too green) but it works. Closed cell foam uses ~2x the amount of polymer per R of half-pound open cell and most of it is blown with climate damaging HFC245fa (~1000x CO2 global warming potential, compared to open cell foam's very low impact blowing agent (=water).

There are a few HFO1234ze blown closed cell foams out there with very low global warming potential, notably Demilec Heatlok XT HFO (aka "HFO High Lift"), and Lapolla's Foam-Lok 2000-4G. They're usually more expensive than HFC blown foam, but that may be changing, since at higher thicknesses it can be installed in much higher lifts.

To install HFC blown 2lb foam at high quality & low fire risk during curing required lifts of 2" (~R12-ish) at a time, but most HFO blown foams can be installed in 4-6" lifts (read the instructions) without incurring quality issues or fire hazard.

It's possible to do unvented roofs with a foam/fiber hybrid at lower cost and lower damage than all closed cell solutions, but it has to be designed for the local climate (or at the very least follow the IRC 2015 Chapter 7 prescriptives.)

It's also possible to do vented cathedralized ceilings (that are air-tight to the interior) using no foam at all.

it's better to put 50-75% of the vent area at the eaves

Conceptually, passive attic ventilation and powered attic ventilation do the same thing (use pressure differences to remove attic air) and suffer from the same potential problem - if the system doesn't keep attic floor pressure the same as the interior ceiling (note, this is rarely the same as atmospheric), it can* cause problems. The differences are things like initial and operational cost (power costs more), flow patterns and control-ability (can't put a dew point switch or pressure controller on a passive vent).

* - as in very small amounts in the right direction can be helpful but large amounts can waste energy and deposit moisture