Best way to vent the attic of a hip roof? Or who cares?
I’m going to be building a small house in Phoenix AZ that must have a hip roof. This is small enough that the roof is almost a pyramid. In the super hot-dry climate of Phoenix, it’s much more important to deal with the roof/attic than to deal with the walls, so I want to get this right (potentially net-zero at some point). Creating an unvented attic using spray foam would do the trick and that’s certainly an option… but it’s an expensive option and so I’m trying to learn as much about efficient vented attics as possible.
My gut feeling is that there needs to be significant ventilation to be effective. My existing house has a hip roof with terrible ventilation and it’s stupefyingly hot in the attic. Case in point: I went up there last October when the high temp outside was 75F and measured temps in the attic no lower than 130F on the top of the insulation and over 160F at the roof-line. It’s like a sauna up there. It feels like having adequate ventilation would result in an attic that was much closer to ambient temperature and so would massively ease the burden on the insulation.
The problem is that most of the articles I’ve read here and on BSC and energy.gov (and more) assume gable-end roofs when describing proven methods. Traditional ridge vents and the like are barely effective on hip roofs, I’m told.
So… what is the most effective way to vent a hip roof?
Alternatively… articles by Martin such as https://www.greenbuildingadvisor.com/article/all-about-attic-venting seem to suggest that it doesn’t really matter because venting does very little to cool an attic. Is that still true when the temperature differential between attic and house is 60-80F?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
While you can use things like GAF Cobra Hip Vent, Lstiburek says that where air exits doesn't matter: https://www.greenbuildingadvisor.com/article/lstibureks-rules-for-venting-roofs
When it comes to seasonal energy use, you should only consider average attic temperatures - so a lot less effect than you might think. But it's certainly measurable.
http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf
If you want to consider your example anyway, say ventilation reduces top of insulation temperature from 130F to 100F (consistent with above link). That's a ~57% reduction in ceiling heat flux. But with enough insulation, it's 57% of a reasonably small number.
Kurt,
The problems with venting of hipped-roofs are confined to those with cathedral ceilings below. A hipped-roof with an attic can be vented the same way a trussed gable-roof is most typically done, with soffit vents, and near the peak sufficient roof vents for the area of the attic.
https://www.ihlcanada.com/building-materials/roofing/roof-vents/
Kurt,
It is true that installing vent openings isn't very effective at lowering attic temperatures. It is also true that your attic temperature is almost entirely irrelevant if you have insulation on the attic floor that meets or exceeds minimum code requirements. (Since you read GBA, I'm assuming that you are smart enough to keep ducts out of your attic.)
For new construction in your climate zone:
1. Specify radiant-barrier roof sheathing. Install it "shiny side down." This will lower your attic temperatures.
2. Make sure that you specify raised-heel trusses (or use a framing method that raises the heels of your rafters).
3. Perform air sealing work at the ceiling plane before installing insulation.
4. Install insulation on your attic floor with an R-value of at least R-40. More is better.
If you need to install attic ducts, ignore all of the above advice, and instead move the insulation to the sloped roofline (creating an unvented conditioned attic).
Thanks Martin!
Whether or not there are ducts in the attic is a little up in the air, hence why I'm trying to research the different styles of insulating attics to cover my bases. I'm not (yet!) sure how I could effectively cool that space without something like a ducted mini-split. Even with that, though, it may be possible to just bury the ducts and air handler underneath the insulation, so that doesn't completely preclude a vented attic...
Well unfortunately you built a large solar collector with that hip roof. Multiple vents entering the attic space will only allow more hot air to come in. What you want to do is limit the infiltration while maximizing outflow. If the whole attic is one temp., the air won't flow. You need a differential in temp to get air moving. Have only one exit as high as possible in the attic and find a way to draw air from inside the house at a lower level, which should be cooler in order to allow the hotter air to rise out. EG. close all windows in the house except for one in the basement, if you have one, and allow flow from basement to attic. Or small fan blowing into attic to pressurize and create a flow.
Tom,
Thanks for the reply. Are you maybe thinking I'm referring to livable space in the attic, though? This attic is not intended to have any usable space at all. In fact, there is an attic in the first place only because my city planners won't allow me to have a flat roof (same reason it has to be a hip roof).
Well you just explained the whole problem, city planners. A nice flat, possibly green roof would solve your problem. I would tell them to f off and build what you want since I am sure none of them are qualified to do anything which is why they chose a position of authority rather than one that actually does something.
Living space or not, the same principles apply. I've had to add a fans to such spaces since the soffet and ridge vents weren't doing their job especially since the attic space had different areas and levels that would trap air and inhibit flow even though the "calculated" vent sizing was applied.
Kurt,
Ignore Tom's advice. You don't need powered attic ventilation. You need an airtight ceiling and an adequate layer of insulation above your ceiling. Running an attic fan is a waste of electricity.
Soffit vents and a ridge vent (or a few mushroom vents) won't hurt, and are probably required by your local building code. But don't be tempted to install powered attic ventilation.
I suggested natural flow from differential temperatures as a first solution, just as nature has done since the beginning of time. I suggested a "small" fan, just enough to get the air moving. (solar fan w/t-stat...free power) What is an airtight ceiling going to do about temperatures in an attic space? Heat will flow into the rooms below the attic due to differential temperatures differences regardless of "tightness". (Re-study your heat transfer.) Soffet and ridge vents only allow hot outside air to penetrate into the attic space and if not properly sized, installed or blocked, heat will build up. If a window/s is/are located and open in the attic, they, the vents, are moot.
Tom,
No doubt you have the best of intentions, but you are giving awful advice. The whole reason for having both soffit and roof-top vents is that they promote air flow without needing any assistance. The air in the attic is much warmer than the surrounding ambient outdoor air temperature so the stack-effect exhausts air from the ridge while replacing it with cooler air from the soffits. The current best practices suggest about 2/3rds of the ventilation area should be at the soffits, and 1/3rd at the ridge to slightly pressurize the attic - the exact opposite of your suggestion.
What an airtight ceiling has to do with attic temperatures is pretty straightforward. What really matters is how warm the house gets. The attic temperature only matters if it contributes to warming the house, s0 cooling the attic with air from the house makes absolutely no sense.
I believe you have it backwards about the soffit to ridge vent ratios. Think about it, hotter air expands so therefore you would need a larger area/volume out than what area/volume comes in order to have proper flow. 100 ft3 of air cannot flow out unless 100 ft3 or air comes in to replace it. ( PV=nRT) If T increase, P and/or V must increase. Adding a restriction to the output will hinder flow. (eg.Turn you tub on full blast and block half the drain, water (heat) should build up)
If the area of soffit to ridge area is not the same then mechanical means must be implemented to drive the flow. Simple fluid dynamics.
I am not suggesting cooling the attic space, I am suggesting making the hotter air leave the attic space by being displaced by cooler air which in a sense will cool the attic naturally. I've been using this method in every house I've ever lived in and never had a need for an ac. Occasionally I momentarily put a fan in the highest window in the house, exhausting the hottest air while pulling cooler air in through the lowest point.
Look up earth tubes, a means of cooling used in ancient civilizations.
We do not know or are told if this hip roof attic has windows. If a window is open then as mentioned, the vents are moot. If two windows are open then they are totally useless.
Tom,
Edit: I think we may be talking at cross purposes. The OP is proposing an unconditioned "attic" with insulation on the floor. Like the truss-spaces commonly used in almost all North American houses. Your solutions seem to assume a traditional storage attic as found in old houses, which is a semi-conditioned part of the house.
Tom,
Malcolm is right and you've got it backwards. Once again, I'm going to urge you to read my article (summarizing building science research on the topic), "All About Attic Venting."
The relevant section of my article is reproduced below:
“'If you choose to vent the roof deck, then be serious about it and really vent it,' [Building scientist Dr. Joseph] Lstiburek recommends. 'The code calls for a minimum of 1 in. of airspace between the top of the insulation and the back of the roof sheathing. That’s not enough. For best performance, the airspace in the vent chute should be a minimum of 2 in. deep.'
"Lstiburek advises builders to include more soffit ventilation than ridge ventilation. 'Building codes suggest balancing the intake and exhaust ventilation,' Lstiburek wrote. 'The code, however, is wrong, and I’m working hard to get it changed. More ventilation at the eaves than at the ridge will slightly pressurize the attic. A depressurized attic can suck conditioned air out of the living space, and losing that conditioned air wastes money. For best results, provide between 50% and 75% of the ventilation space at the eaves; a 60/40 split is a good sweet spot.'”
Well Joe L. is trying to change the code. Why is that? Is there something wrong or is it just something he disagrees with? Has he done the math? Or is this just a personal preference? What about his sidekick Betsy? I know of Joe and people who have worked for him, note workED. I almost joined his crew after finishing up my Mechanical Eng. degree, but didn't agree with what he was doing which is why others left. Everybody has their opinions, but numbers don't lie. Every house is different and exposed to different conditions, so how can one apply a "one rule fits all" to any building? This is why codes are bogus, just do what's right. Design, test, adjust.
Any attic will fluctuate pressures and temperatures naturally over the course of the day, to say it is always constant and design on that belief is nuts. Let nature do its job.
Tom and Kurt,
Listen to the researchers. Here are a few paragraphs from my article, "All About Attic Venting":
If a house has insulation on the attic floor, there isn’t any evidence to support the idea that attic ventilation will reduce your air conditioning bills.
[Researcher] Jeff Gordon, in his presentation on attic ventilation, wrote, “Cooling season energy savings? Well, we tried to measure energy use, but this did not work very well. … Basically, [any savings are] lost in the noise.” Gordon reported that research has shown that “Attic ventilation is not an effective energy conservation procedure for houses with more than 6.5 inches of attic insulation.”
Summing up, Gordon reported, “Venting will reduce the temperature in an open attic. The difference in attic temperature between a vented an unvented attic, with R-30 at the ceiling, translates into minuscule [cooling energy] savings. No savings have ever been measured. … In mixed climates, savings from delta T must be balanced by losses from delta T in the winter. In cold climates, this is clearly a net loser.”
Tom,
No human activity starts from first principles every time. If we did we would not have the benefit of experience and would probably still be living in trees. We have a fair knowledge as to what common building assemblies work and why - among them vented attics. I've got to confess I'm shocked you are a P.E. with so little basic understanding of how buildings work.
Well rather than believing what code enforcers tell you, maybe you should investigate other alternatives. The code writers and enforcers are not there to help you, the homeowner. If they were, we would all have zero energy homes and it would be the norm throughout the world. A home that doesn't need outside energy is useless to them and they can't have that. Me, second generation construction worker with close to 40 years experience, mech engineer, journeyman's lic in plg, heating and gas, solar and have worked on thousands of commercial and residential construction jobs, so I've seen what works and what doesn't, systems that have come and gone, methods that have changed year after year, products that don't deliver, institutions like LEEDS that change their models again and again, bogus certifications co's, etc. etc. What are we to believe? There are methods that have been proven to work time and time again, but there are those who are in it only for profits and introduce new technologies that fail time and time again, and at whose expense? They go bankrupt, close shop, get out of liabilities and retire a millionaire. And these are the types you put on a pedestal? People have lived and still live in homes comfortably without any type of power long before any building codes were introduced and will continue to live long after the code makers are gone.
After all, isn't this a green building site? Why all the technology? Where are the basics we should be talking about. A lot of building science today is just that BS. Toxic materials, fuels, plastics, styrofoams, chemicals, glues......etc.