Good evening, I am trying to find out the correct way to vent an attic in a raised -ranch. Presently have almost a full ridge vent, gable end vents and vented soffits but heat is still unbearable.. Was going to install power vent on roof but couple of roofers told me those will just suck in moisture thru the vents and create possibly more problems…Any suggestions??
Discussion Forum
Discussion Forum
Up Next
Video Shorts
Featured Story
Discover a drywall-free approach to refinishing a partially demoed wall.
Featured Video
Highlights
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Fine Homebuilding Magazine
Fine Home Building
Follow
-
Fine Homebuilding
Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.
Start Free Trial Now -
GBA Prime
Get instant access to the latest developments in green building, research, and reports from the field.
Start Free Trial Now
Replies
I don't have a lot of technical knowledge, but my common sense approach sure likes the idea of Solar Powered vent fans near the ridge at the top of roof, hopefully facing South. They only run when the Sun is heating your roof, and cost nothing to operate. You should realize though that an attic is never going to comfortable on a warm day, it gets worse if you have dark color of roofing. If the interior of the house is also to hot, maybe more insulation is in order. Just my thoughts.
Dan The meek will inherit the earth, right after we are done with it!
Are the vents actually working? It is common for the soffit vents to be blocked by insulation or otherwise non-operative.
Some people argue that where you have good soofit and ridge venting, gable vents are counter-productive by disturbing the air flow.
Rules of thumb for venting: 1/300 of the floor area, 1/2 at the top and 1/2 at the botom of free vent space if there's a vapor barrier, 1/150 if not.
FWIW, As a home inspector, I don't see any relationship between that rule of thumb and vent perforrmance.
Ditto what the other guys said:
1. Make sure the soffit vents aren't blocked.
2. Same with the ridge vent
3. Heck with the gable vents - Might as well block them off
4. More insulation
Sometimes you go fishing to catch fish; Sometimes you go fishing just to fish.
Paul - Consider mounting a 1,000 CFM thermostatically-operated attic fan behind one of the gable vents. You need to force air movement whether or not the air is humid. Make sure you can kill it with a switch on rainy days.
"Heck with the gable vents - Might as well block them off" No, no, no .....
T. Jeffery Clarke
Edited 6/3/2002 9:27:33 AM ET by Jeff Clarke
Paul,
I'll offer a different perspective:
I'm guessing from your post that the attic is still unfinished...rafters and exposed insulation. Where is the insulation? On the attic floor between the joists, or on the attic ceiling between the rafters?
I'm hoping that the rafters are still exposed.
Anyhow...the sun is heating your shingles. That heat is conducting through the roof, from the granular surface of your shingles to the underside of your roof sheathing. The hot underside of your roof sheathing is then radiating heat into the attic space. If you have FG batt insulation between the rafters, FG is essentially transparent to radiant...it simply passes right through. FG creates air pockets to reduce conduction and convective air movement, but FG does essentially nothing to counteract radiant energy.
So, even if your attic ceiling has FG batt insulation and the bottom of the rafters are covered with drywall (which covers the insulation), the radiant energy is still passing through the FG insulation, striking the drywall, the heat cionducts through the drywall and then the bottom or show surface of the drywall becomes a radiant panel.
Adding air movement through the attic itself with a gable fan will remove some of the heated air, but it can't move the radiant energy. It's simply there.
My suggestions:
If your rafters are exposed: ensure that you have baffles in each bay from soffit to ridge to promote air movement/venting within the rafter bay. If you already have FG insulation in these rafter bays, that's okay. I'd add a layer of foil-faced polyisocyanate insulaton to the face to the rafters. It comes in 4' by 8' sheets and is quite easy to install. The foil face acts as radiant reflector as long as it is clean...which may not be for too long...but still, the polyiso itself will absorb the radiant energy and prevent that heat from conducting through the polyiso and then entering your attic. The polyiso itself is R7.2 per inch. You'd then likely want to cover the polyiso with drywall for fire protection...I do furring strips, then drywall. The added benefit is that the foil-faced polyiso works to your advantage in the winter...it keeps heat in your house, keeping your attic "warm" and minimizing heat loss through the roof. If you have any forced air (heating or cooling) ducting in the attic, the tempered attic temp shold improve the performance of both your heating and cooling systems.
I did an attic that had exposed FG insulation in the rafter bays. The temp at the peak of the attic was 127 degrees. As soon as the polyiso was installed, the temp at the peak of the attic dropped to 77 degrees...at 2:30 in the afternoon.
More details if you want them.
Radiantly, Mongo
Edited: For what it's worth...I also agree that gable and soffit-to-ridge vents can be counterproductive.
Edited 6/3/2002 10:26:00 AM ET by Mongo
Ditto Mongo. Its good to find someone who uses my method (or perhaps, I their's).
Just one small change to Mongo's approach. I rip 1X1 nailers and tack them in place where the rafter meets the roof deck. I then tack 3/4" polyisocyanate, foil facing the roof deck, up against these nailers, with 8" bats (depending on depth of roof rafters) filling in the rest. This channel I create runs from the soffit to the ridge. I've done this on two houses, the most extreme was in Maryland where in the summer it could reach 100 and in the winter, 10-15 degrees in January. I never had an ice dam and the attic was livable during the summer....and that hot-wood smell of the attic was gone!
BruceM
Thanks for your detailed answer.. here's a little more info: the soffits appear to be clear, the under side of the roof deck shows no sign of rot, the ceiling joists are 2"x 6" with 10" of insulation between them, the house is close to ten years old and there isn't much room to move around up there..remember the name "BigPaul".
Another buddy suggested I install one of those whole house fans that lay over the rafters and blow up into the attic and "push" all the heat out whatever vents are up there.. (which also pull heat out of the living space of the home.)..But wife says those are just too damn noisey... Do you know of some quieter units on the market?
Paul
I just installed a 30" belt drive variable speed "Cool Attic" brand whole house fan 2 days ago in my home. I don't like AC myself, so I will get a lot of use from the fan. I would not recommend it for cooling a attic If you use AC because you would not want to run it on hot days.
It makes noise for sure, but I hear that there are quieter models available. All in all it's not unpleasantly loud. But a major beef is the very poor quality of the louvers. They are so leaky that they will have to be covered with an insulated panel during winter.
I suggest that you start a new thread about fans if you want more information.
Paul,
Sorry...I don't know much at all about attic fans because I never use them.
If you're set on using an attic fan someone here will likely chime in with a quiet fan. Still, you're putting a bandaid on something that needs stitches. Instead of letting the heat into your attic, then using a not-so-perfect method to remove it, why not just stop the heat from entering in the first place?
My opinion.
Still, the best of luck to you in that I hope you find a solution that satisfies your needs.
A couple of questions regarding "...the ceiling joists are 2"x 6" with 10" of insulation between them...". Are the rafters 12" on center, thus about 10.5" apart...or is there 10" thick insulation crammed into the 6" depth of the rafter bay? With 2x6 rafters, is the roof trussed or stick framed?
No need to worry..."Mongo" understands "BigPaul's" difficulty in working in cramped spaces.<g>
Ditto Mongo's experience with the PIC insulation. Half my attic has it, half doesn't, and that's the only difference. There's a door and a wall between the two and the temperature difference is remarkable. Andy Engel, Forum moderator
you can't stop heat from entering the attic, it's there because it radiates from the roof, not from the living spaces.
the hot air in the attic makes the house hot, regardless of how much insulation you have, because the heat from the living space can't rise. You are, in effect, creating a mini-high pressure area right over you living space.
A powered attic vent evacuates the hot air from the attic and allows the heat from the living spaces to radiate through the ceiling into the attic, thereby relieving the house of energy.
If you can control the radiation effect from the roof itself through the use of insulation in the rafters, then you have tackled that problem, but hot air will still accumulate in the attic as the living quarters relieves itself of the energy that is created normally. You still have to evacuate the attic air. Most homes I've seen, at least in this area, don't have insulation in the rafters, it's either blown in or laid in the attic, the radiation from the roof is unabated.
I contend that a whole house fan is largely unneccesary if you have adequate attic ventilation, by that I mean positive evacuation of the energy through a powered fan and recharging with fresh air through adequate ventilation in the soffets, gables, dormers.
The whole point of a whole house fan is to remove energy from the living quarters to the attic, where hopefully it will exit the house structure. But this happens naturally if you have good attic ventilation and positive air flow through it. If you live in a newer home that is tight as a button with whole house wrapping, tight fitting windows and doors and insulation, then a whole house fan serves another purpose of ventilating the house structure to keep it from getting stale. In most houses, like mine :), there is plenty of natural ventilation through the older windows and doors so you wouldn't need to worry about that aspect.
Bottom line: a powered roof vent will keep your house cooler, and you won't have to deal with a fan the size of a small airplane propeller in your living quarters.
By the way, belt drive fans are much quieter.
Edited 6/4/2002 1:56:24 PM ET by JEFFN7
Um, what if there's a fire in your house or attic when that roof fan is running?Andy Engel, Forum moderator
What's your point Andy? A fire either place is bad news. Are you suggesting the fan would intensify or make the fire less detectable? I don't see a problem other than the obvious one!
BAM
While researching my perfect house I did some reading of a DoE report on attic ventilation. The report was written about 1980 and concluded that barring special circumstances attic fans used more energy than they saved if the attic was properly insulated. Also if there was inadequate make up air or gaps to the living space the fan could suck out conditioned air and increase infiltration of outside air.
While solar power fans would cost nothing to run they could cause the other problem. DoE in after reference to some pretty fancy experiments concluded that a combination of ridge and soffit vents, properly sized and maintained, was the most energy and cost efficient. The solar powered option was not available at that time but it sounds promising if the ridge/ soffit combination couldn't get you there.
My one experience with attic fans was extremely positive. I had a 2,000 sqf house with southern exposure to the front in southern calif. The roof was composite shingles and unnecessarily sharply pitched. Ventilation came from gable vents at the east and west ends. On the south side of the roof where two dormers that with the roof pitch gave the look of a two story house. The roof ridge ran east to west so the large souther half of the roof got a lot of sun exposure. These dormers also had 2'x3' vents in them. This was a problem with any wind driven rain from the south it would go right into the attic. West wall had a sliding glass door that caught the noon to sunset sun unabated all Summer long. In the heat of the summer the poorly installed cheap and undersized air conditioner (another story) was unable to cool the house and would not be able to cycle on and off because it could not get the house temp down low enough unless you set the thermostat to a temp in the 90's. (great for the power bill) I did two things one was build a solid roofed patio cover over that glass door on the west side keeping all but the last rays of sunset off that glass. And then installed a fan on the west facing gable vent to draw air through the attic from the cooler shaded east side. Worked great, the airconditioner could now do its job with a lower power bill. Best thing I ever did for that house.
It is a simple matter to put a smoke detector in the vicinity of the airstream connected to a signal-activated relay that will turn off the fan when the detector is in alarm. I even wired mine myself and it works, even on the test button.T. Jeffery Clarke
Quidvis Recte Factum Quamvis Humile Praeclarum
My point is that an attic fan might intensify a fire by providing a ready draft. I could imagine an un-attended thermostatically controlled fan to be a particular problem.
I'd also point out (or agree with if I'm chiming in late) that an attic fan that doesn't get enough make-up air from the attic vents will take that make-up air from your air-conditioned space. That's one way to cool the attic!
Andy Engel, Forum moderator
Edited 6/5/2002 8:07:46 AM ET by ANDYENGEL
Andy, I see your point, but am not sure it matters.
For one thig, I'm not sure the volume of air they move would all that significant. At least not enough to "fan the flames". And it would only matter if the fire started in the attic or had burned theough the ceiling.
As I understand, the fire service typically cuts holes in the roof to vent smoke out so they can see to fight a fire.
They also occasionally use gas powered fans to help clear smoke out so they can see to fight the fire better. (Although I'm not sure exactly when and why they use them)
If you are in a spaceship that is traveling at the speed of light, and you turn on the headlights, what happens?
Firefighters use those fans to clear the air when they're there to spray water on the fire.
My thought was simply to, um, add fuel to the fire by listing another reason to avoid attic fans. I see it this way: Fans aren't the solution to a hot attic. The right insulation is what makes the difference, because attic heat is mainly radiant. No fan in the world is powerful enough to suck out IR radiation.
Andy Engel, Forum moderator
Edited 6/5/2002 9:18:20 AM ET by ANDYENGEL
As a volenteer fire fighter I have not seen any real effect on a burning house from a power roof vent. Did have a fire in a meat market where the fire started below the compressior room and burned up threw the floor. The heat from the fire kicked on the very large power vents in the compressior room and sucked all the flames up threw the venting and out the roof. Looked like something from "Gost Busters"
Jeffn7,
"...you can't stop heat from entering the attic, it's there because it radiates from the roof, not from the living spaces."
Sure you can. The polyiso does exactly that. It minimizes the amount of heat that enters the attic via radiating from the roof.
The foil face on the polyiso that's installed on the underside of the attic rafters acts as a radiant barrier, keeping the radiant heat from entering the attic. Even if the foil face gets dirty and its effectiveness as a radiant reflector/barrier is reduced, the polyiso still doesn't allow radiant energy to pass through it. Conduction? With the top side of the polyiso running at 125-135 degrees, the R-value of the two inches of polyiso (7.2 per inch, or R-14.4) prevents, or minimizes, the amount of heat that enters the attic by conduction through the RFBI. Convective gain? Not when you gap the sheets during installation, then foam the gaps. The roof is as tight as can be.
Radiant gain through the polyiso? Essentially zero.
Conductive gain through the polyiso? With R14.4 (or even half that) and a delta-T of 50-60 degrees, there's some...but it's minimal.
Convective gain through the polyiso? Essentially zero.
Polyiso will virtually stop the gain. Cellulose will absord the radiant, then it will use its R-value to minimize the amount of heat passing through the insulation via conduction. Packed well, cellulose will virtually stop convective air movement as well.
FG? It's transparent to radiant, meaning radiant energy passes right through it. Unless the FG is encapsulated in a six-sided bay, air movement (air wash) in the attic reduces the FG's R-value as well as its ability to stop convective air movement. Blow on a sheet of polyiso, your breath will not pass through. Blow on cells, your breath will not pass through. Blow on FG and not only will your breath pass through the batt but you'll get a glass fiber facial.
I suppose I'm just not a fan of actively depressurizing a building's envelope when properly installed passive methods work better.
Regards, Mongo
Mongo,
View Image
Edited 6/5/2002 9:43:41 AM ET by J Fusco
Edited 6/5/2002 9:44:22 AM ET by J Fusco
Mongo,
Does your system require a ridge vent or air movement in the rafterbays? My house has only soffit and gable vents. I had thought of doing as you wrote, but was not sure about the roof temp not having the ridge vent.
I also agree that putting a power exhaust fan would create a negative pressure in the attic and suck conditioned air from the living space.
Will
Mongo
I agree with what you are saying (I use the same system) but I don't think that's what JeffN7 is talking about.
The only problem I had with the soffit-to-ridge ventillation and attic rafter insulation system in my finished attics was that during the summer, hot air rising from inside our house would go up to the attic and 'stick'.... nice in the winter & bad in the summer. I doubt any of the heat was coming through my roof. So what to do with the rising heat? I wound up installing a window A/C in the attic, and it made a noticable difference in the 2nd story bedrooms, by simply giving the rising hot air a place to go
BTW...did you really like Sheriff Bart?
BruceM
As I stated in my post (the second one, I believe) insulation in the rafters will work to minimize the radiant heat from the roof itself. Of course, that still doesn't do anything about the heat rising from the living space. That energy is still trapped in the attic.
There's a number of strategies, it's not my position that any single one is really better or worse, and ideally you will employ a range of strategies to combat. A powered roof vent is an inexpensive, easy to implement solution that really works.
My house had a real problem with heat buildup during the day. It features a large southern exposure along the entire back of the house, which also has very large windows in every room (one of the reason we love this house, expansive views of a beautiful valley below us). I don't have air conditioning, don't like it or need it in our climate zone (moderate temperatures year round, some heat in the summer, very low humidity).
The house has good insulation in the attic, but it's blown in, not in the rafters. The roof is black composite shingles, so you can imagine how much energy radiates through it. The venting is adequate in the soffets and gables, along with 5 dormer vents at the ridge. The problem is that there wasn't anything to evacuate the energy from the attic, other than natural airflow through the vents. In the summer, the inside termperature would easily get in the high 80's during the day.
I installed a powered roof vent a couple of months ago and the change was instant and dramatic. The very next day, a warm spring day, I came home in the late afternoon and the inside house temperature was pleasantly moderate, probably about 76 degrees. Through today, including some days in the 90's, the inside temperature has yet to break 79 degrees. Nobody can tell me that the roof vent is not responsible for this difference, the evidence is overwhelmingly obvious.
Any solution that cools the attic and the homeowner or his contactor can implement is just that; a solution. There are downsides to all of the systems mentioned: some don't work well or at all, some are difficult and or expensive and some only cool parts of the attic. But any method that cools the attic is better than none. I would like to know more about Mongo's system.Old Pro, not quite old, not quite pro, but closer every day on both...
Dad,
View Image
Joe; Interesting point. I always assumed that the venting was done to keep it cool in the summer and cooler in the winter (so ice dams are not a problem). The humidity is the humidity (at least here in the deep south) or so i thought. Yes venting the humidity that escapes the envelope must be wicked away. But no matter the method is it not just an attempt to releave the high temp and save the shingles and reduce the ac costs?Old Pro, not quite old, not quite pro, but closer every day on both...
Dad,
View Image
Sorry let me clarify: 2"x 6" rafters, spaced 16" centers with 10" Kraft paper insulation, roof is conventionally framed (stick built)... many thanks for all your info and the same to all the others as well....
suck in the moisture through the gable vents... i guess it's possible if you didn't have enough venting to 'recharge' the attic air with a powered roof vent. But let's think about this for a moment... you have a powered roof vent to evacuate the hot air from the attic and it's sized for the amount of space you are working with. Too big a fan in the space and the negative pressure rises to an unacceptable level, with the possibility that you overpower the venting... okay, I could see that potentially sucking in moisture, but it's pretty unlikely.
but if the experts you spoke with are talking about humidity, it's not only there already (1 of the major objectives of a roof fan, to equilize the humity level), but it increases without adequate ventilation.
sucking in moisure through the powered vent? Impossible, when wire correctly it blow out of the roof, not in.
prescription for a happy attic: proper venting (cleared of insulation blockages, as was previously noted), a good amount of insulation and a powered roof vent. I don't have any opinion on the effectiveness of gable vents or dormer vents.
I have a powered roof vent in my house, along with good ventilation and insulation. Not only does my house stay cooler in the hot summer months, but the attic 'feels' dry. I keep the roof vent thermostat set to 105 degrees, so it's not what I would call pleasant up there!
In performing leak repairs in the summertime, I have come upon countless attic power vents that are no longer working (bad motors) or the thermostats are not adjusted properly. The homeowners are usually oblivious to the fact their vents are not working. In my experience the soffit vent/ridge vent combination works best, needs no maintenance, and doesn't use electricity. I am not familiar with the solar powered vents mentioned previously, but I would suspect they are expensive. Hip roofs call for static vents if enough ridge is not available. When installing ridge vent or a power vent when gable vents exist, it is best to block the gable vents. In some cases the flow short circuits from the gable vents to the ridge vent or power vent rather than pulling from the soffit vents. This is not terrible, nor is it optimum. I have seen some cases where power vents are the best solution (my own house is an example - I have a tile roof - so I have installed a powered gable vent. I can barely hear it run, so I know if its working - the motor has been replaced once), but the less moving parts, the less there is to break.
gl
My powers can only be used for good.