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We live in a Victorian farmhouse with an unfinished attic, about 12 feet to the peak of the hip roof. Although our summers aren’t bad, we do get a few 100+ days every year, as well as many in the 90’s, so keeping the attic cool is an important issue. It’s currently not ventilated, and I’m preparing to do that. Two questions:
1. My best memory is that the rule of thumb is one square foot of ventilation for every 300 square feet of attic. Is that correct? Is there any reason that more ventilation is either bad or good? Given how high the attic is (and thus that volume of air it contains), I’m thinking that erring on the side of more venting is probably wise.
2. Since we’re expecting that the hot days of summer will also be the rolling blackout days (once again, California leads the nation!), we want to avoid powered ventilators. However, since it’s a hip roof, we can’t install a ridge vent (two feet of ridge vent wouldn’t do much). We’re thinking to install one or more turbine ventilators. Opinions? Are there indicators of quality, or recommended brands? I seem to remember reading somewhere that plastic bearings are superior to metal-on-metal bearings for these (quieter, I believe).
Replies
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Bill-
Not an expert, but I will offer some thoughts-
I have added additional ventilation to my home (a 1 level with unfinished attic), the 1/300 sq. ft. of attic floor space is correct, however I used the 1/150 sq. ft. figure since my ceilings have no vapor barrier. That 1 square foot should be split equally between the ridge and soffit vents. Have had no problems with this approach- I also erred to the more ventilation theory.
I would strongly discourage you from using turbines- they are noisy over time, and mine had sucked water into the attic in a strong storm with straight line winds (I yanked em' about week after we bought the house). Pretty extreme circumstance but why mess with them? Although expensive (but hey you own a Victorian!) there are solar powered exhaust fans that fit in were conventional roof vents go. I think they are in the $300 neighborhood, but you probably wouldn't need that many to help out. My opinion is that a well planned out venting system shoud work fairly well on convective currents alone- no fans necessary. Make sure you have a clear channel inside the attic to the soffit vents (not blocked by insulation), use attic chutes if necessary.
Good luck!
Brian
*Brian,The solar powered roof vents run about $350 locally, which is a bit steep. Do I understand you to say that you don't have powered top vents? Did you just put some roof jacks in and put a cap on them or something along that line?Given the likelihood that the days on which we need this relief the most will also be the days voted most likely to have a blackout, I don't see any point in installing utility-company-powered vents, which are much cheaper ($50)I've considered a cupola, but am not sure how to make one on a hip roof that didn't look dorky as heck, and wanted to avoid spending all summer on the top vent part anyhow (lotsa other projects, including -- fantasy -- some shop time that's NOT working on the house).If I double the vented area in line with your approach (which is about what I had in mind), then I'd need five square feet of top venting, a pretty hefty area.
*Bill,You'll do more to keep the house comfortable by putting some insulation in the attic that is also a radiant barrier (not fiberglass). Is the attic floor boarded over or is it open to the ceiling below?Steve
*Bill-I do not have the solar powered vents. I also agree with the no-go on the electric ventilation fan.(After you pay to run the fan is the savings really that great over running the air conditioner?)I know what you mean about the lack of area for vents up there- I have a regular roof and am fortunate in that regard. We used the regular square plastic vents that slip in under the shingles- seem to work fine. Not sure what to tell you from here...I -think- the standard square vents are like .68 sq. ft. or so, and if you needed 5 sq. ft. up there, I guess you would need 7 of them...Hmmm...-B
*Steve,We've got some insulation -- faced glass with the shiny side down, stuffed between the joists -- in most of the attic. It's a little thin (R13 or so), but better than nothing. The low-roof part, which is awkward to reach, will be a little trickier, since the framing is sketchy and not on any particular centers that I can identify, so using glass batts will be difficult. Maybe a flat radiant/vapor barrier with blown cellulose.
*Bill,The Cells alone will do a pretty good job of blocking radiant transfer. IF you aren't using the attic at all, I would just blow about 18" of cells in on top of what's there. Is winter a factor wher you are? If so, before burying everything in cells I would lift the FG and do a complete air-seal job...pipes, wires, electrical boxes, partition walls, chimneys, recessed lights, etc. Then blow the cells as deep as you like.Steve
*B-b-b-bingo again.
*ditto
*I wound up buying turbine vents -- perhaps our area has less scary rain, but local folks I talked with don't report any problems with water. The bearings are metal-free, so the noise may not be a problem. We'll find out.The ideal solution, a cupola with lots of square footage of vent, just isn't on the agenda right now.Anyhow, along the way, I found a six-page discussion of how to calculate ventilation requirements by the manufacturers of my vents. It's pretty good, and does confirm that I want the 1/150 ratio after all. It's at http://www.certainteed.com/solutions/attic/pdf/about_ventrequ.pdf. Thought I'd post this here for others who might find it useful.
*Bill, Steve is giving you the best advice, but he left out one important question, one I frequently ask here:What problem are you trying to solve?He and I assumed a comfort problem - and it seems that is true. But I ask the question just to be sure.Forget the vents and crap, lets focus on your problem. You started the thread with questions on how to execute a solution, but never gave us the problem.-Rob
*I grew up in Soouthern California, and could never figure out why our house was so hot at night, while it cooled off outside. Our attic was ventilated -- but "passively." So it heated up during the day, and then radiated it back all night long. There is a real time-lag involved. If the attic is hot, it will radiate that back as the house cools. Conversely, if the house and attic are cool, they will soak up heat, until they are as warm as the surroundings. In addition to your turbines (which are fine, but I doubt their adequacy, see above) I would use powered ventilation (pick a quiet model, Panasonic makes the quietest, but there are others), run on a timer at night, to build a "sink" of cool. I wouldn't worry about the rolling blackouts -- they won't be that often (unless the apocalypse...), and you'll have the coolness available to soak up what happens then. There is a big difference between the minimum ventilation requirements from the point of view of the building materials, and which don't consider the particular climate at all -- and what makes for confortable living. -- Christian
*Turbines are going to look great on a victorian. ...not!TIC, Jerry
*Cellulose insulation's property of blocking visible light ("radiant barrier") is only valuable if you have visible sunlight coming through your attic! We hope that isn't the problem!True "radiant barriers" as the the term is normally used in the trade, use the low emmisivity of silver-colored surfaces pointed down. Like under a floor.Be careful if you have knob and tube insulation (not unusual in a Victorian) and are thinking about covering it with any kind of insulation. Wiring can be damaged by walking on it and heat can build up to the point of combustion.By the way, the original little square cupolas of 150 years or so ago, were designed with vertical baffles installed diagonally accross them both ways. The idea was to deflect incoming air down into the space where it would mix well. Rather than just having the air blow in one louver and out the next. Over time the exterior design was often retained, but the original patented baffles were left out. (per a very old encyclopedia)
*Gary, the wavelenths of visible light aren't far off from those of infrared, which I understand to be an important aspect of radiant-heat transfer. Now, I don't know for certain that, because cellulose blocks visible light, it also blocks IR. Do you know for certain that it doesn't? I'm not asking to be inflammatory, but because I think this supposed property of cellulose has been bandied about quite a bit and needs either confirmation or debunking. My inclination is that cellulose does block a considerable amount of IR, based on the fact that I live in a cellulose-insulated, non-air-conditioned house that stays cool in the summer.Anyone know of a credible source for this info?Andy
*I'm not saying cellulose does not block IR. It does block it.Glass ALSO blocks IR, or MOST of it. That's why you can't hardly get a suntan behind a window. Glass windows and glass fibers restrict the transfer of radiant heat energy. Solar panels for instance use glass rather than plastic glazing because it will transfer light energy, but it tends to block "heat" radiation. Glass windows work the same way to trap solar heat.It would be interesting perhaps to try some backyard experiments. With something that is hot like a wood stove, but isn't glowing red visibly like a heat lamp. I don't have one here, and its expected to be 90 degrees this afternoon!I could be wrong, but my professional judgment for the past 25 years or so, has been that glass blocks heat energy. Cellulose attic insulation values are the same as fiberglass values. Although the "cells" industry disagrees.
*The reason why you don't get a suntan behind glass is because glass blocks UV light (pretty much) and passes IR light (the reason why its used for solar panels). Two different ends of the spectrum.Cellulouse blocks IR light better than FG because it has less 'holes' in it. Remember that at a campfire, even a single sheet of paper (or better, a thin sheet of shirt cardboard) will block the fire's heat from your face, IR is pretty low energy light waves. IR passes through whatever it can 'see' through (almost the same as what you can see through - I'm simplifying here, but the mechanism still works).Hold a piece of FG up to the light and you'll see much of the light come through. Hold a similarly thick amount of Cells up to that light and you'll see dark.
*"Hold a piece of FG up to the light and you'll see much of the light come through. Hold a similarly thick amount of Cells up to that light and you'll see dark."True, but that is neither UV or IR, that is Visible light! That test doesn't say much about IR. Are you asking me to design out my own test to prove I'm wrong? That should be someone else's job.I do agree IR lights are made of glass, but also note that that glass is also very very thin and gets quite hot. Due to its inefficiency of transferring radiant heat perhaps.Try shining a flashlight through a paper backed fiberglass batt and see how much light comes through. :-)
*If I remember the physics of electromagnetic radiation correctly, from Mr. McHale's chemistry class in 1977, IR is at the lower-energy (longer wavelength, lower frequency) end of the spectrum relative to visible light, and UV is at the higher-energy end (shorter wavelength, higher frequency). What is opaque to either of them, or to visible light, isn't necessarily opaque to any of the others. In support, IR and visible light pass through window glass better than does UV. Low-e coatings pass some wavelenths and reflect others. I don't think that the light-through-FG test is a valid means of comparing the opacity of cellulose and FG.So, does anyone know of any scientific studies that compare the opacity to IR of cellulose and FG?Andy
*Any warm surface radiates heat directly to any surface that it can "see." We think of the sun or a hot stove when we think of radiant heat, but a warm wall, or a hot roof deck are also very good radiators. Five or 10 degrees can make a big difference in our comfort level. In the winter we feel cold when the air is 72 degs. because the wall is colder than we are, and WE radiate heat toward the cooler wall. In the summer, we are comfortable at 72 degs. because the wall is closer to our skin temperature and we don't radiate so much heat.I think cellulose blocks radiant heat better because it is denser than FG -- there's simply more mass there, so it is more opaque.
*Andy - as soon as the data is recovered off my hard drive I'll put up pictures from the infra red camera!Gary - The difference between a pane of glass and a glass fiber is that the pane has a surface for reflection and refraction to take place. The fibers don't They reflect and refract equally in all directions (I assume they are somewhat unidirectional inside a batt). Cellulose neither reflects or refracts any of the spectrum. IT'S OPAQUE!!!My infra red camera says cellulose wins every time.My fingers tell me that the blown fiberglass in an attic in the summertime is hot until I'm in it to my elbow.My fingers also tell me that they are not hot much after I have half my hand inserted into the pack of insulation.-RobP.S. - Bill never told any of us what problem he was trying to solve. But you guys all had solutions, doesn't that seem odd?
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We live in a Victorian farmhouse with an unfinished attic, about 12 feet to the peak of the hip roof. Although our summers aren't bad, we do get a few 100+ days every year, as well as many in the 90's, so keeping the attic cool is an important issue. It's currently not ventilated, and I'm preparing to do that. Two questions:
1. My best memory is that the rule of thumb is one square foot of ventilation for every 300 square feet of attic. Is that correct? Is there any reason that more ventilation is either bad or good? Given how high the attic is (and thus that volume of air it contains), I'm thinking that erring on the side of more venting is probably wise.
2. Since we're expecting that the hot days of summer will also be the rolling blackout days (once again, California leads the nation!), we want to avoid powered ventilators. However, since it's a hip roof, we can't install a ridge vent (two feet of ridge vent wouldn't do much). We're thinking to install one or more turbine ventilators. Opinions? Are there indicators of quality, or recommended brands? I seem to remember reading somewhere that plastic bearings are superior to metal-on-metal bearings for these (quieter, I believe).