Here in Denver, someone’s turned up the heat lately. We’re running betwixt 95 & 100 over the past several weeks. Reminded me that I had the same issue last year and wanted to get more insulation upstairs cause even the 14 SEER ac I had put in a couple of years ago has a tuff time fighting off the PM heat buildup.
So’s the insulation sales shark comes by today. His sales pitch (after examining the house, roof, & attic) focuses upon the roof ventilation of this 12 year old former Parade of Homes place. Gets me to crawl up in the attic with him (all trusses) and examine all of the soffit and up high roof vents. Says his calculations indicates I need 6.5 sq ft of ventilation space both for the up highs and for the soffits. (2400 sq ft house)
There were 4 mushroom roof vents visible from outside. And a 6 x 12 soffit vent every 6 to 8 feet. From the inside of the attic, we discover 2 of the 4 mushrooms are not even cut open from the inside and oneof the other’s has 1/2 of it’s 6″ x 6″ square opening covered by a framing 2×4. So’s I really has 1.5 times 6″ square or 3/8’s of a sq ft of up high ventilation ( I need 6.5 sq ft). And looking around at the soffit vents from the inside show only two have light coming thru. So’s they’re blocked or not cut open either.
At 9 am, it was hot up there!
Anyhow’s, he says they don’t do the vent work. But he’ll be happy to come back and blow in another 12 inches of white fiberglas on top of the 12 I’ve already got (that isn’t packed down).
Winter protection is not the most important issue with the insulation in this climate. It can be -15 one day and 70 the next in February. So I’m more focusing upon the heat issue.
The sales shark tells me he would recommend a powered roof vent to replace one or more of those mushrooms as well a cutting open the soffit vents.
The question being, of these low profile roof models I see when googling the topic, which manufacturer have you’se guys had the best success with? And how do I calculate the CFM need? I’m assuming I should avoid the ones offered by Home Desperate. Are there consumer reports articles that survey roof vents?
Replies
I put in a powered roof vent, because I have a square hip roof and couldn't get a ridge vent in and I hate the cake pans. I got mine from a roofing supply house and it seems quite robust and does the job. It was humming away all day, I have it set to come on at 95 degrees.
Several problems with power vents. One is that they tend to have a limited lifetime -- maybe 3 years -- before the motor goes out.
I'd suggest you get a roofer to install ridge vent (assuming you don't have a hip roof with very little ridge), and get someone to cut in several new eave vents.
If you get insulation added, you need to make sure that the eave vents aren't blocked. Some insulation guys are careful about this, some aren't.
Hip roof with ceement roof tiles. I'm not aware of a ridge vent that works with those.
The lifespan of those motors are that short, huh?
Yeah, the lifespan is short. Make sure the unit is easy to access.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
The lifespan of those motors are that short, huh?
Yeah. What the product literature does not tell you is that those hours of run time add up fast. 3000 hours sounds a long time--but 10-12 hours a day is only 272 days, or about 9 months. That's only three years if your attic only gets to 95º for three months the year. (Not mine, mine will hit 95º at least once a day from about March through October; since 5 June, it's not gone lower than 86º.)
So, in my climate, you get maybe one season before the bearings seize up or the insulation on the windings melts (can happen at 130º)--all for the cost of running that motor 18-20 hours a day, call it 1.8KWH every day for a long stretch.Occupational hazard of my occupation not being around (sorry Bubba)
The Forida Solar Institute has done dones of research on heat through roofs.
And they have had several reports in JLC.
Everytime I go to FSI website they move things around so I can't point out the research.
But you can't ventalate heat out of an attic unless you have so much venting that you have a screened porch, not a roof.
Radiant barriers and the emissivity of different types of roofing had a big affect.
Don't know how difficult it would be to add a radiant barrier to you roof.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
> Don't know how difficult it would be to add a radiant barrier to you roof.It's basically just a big tinfoil had. Everyone here knows how to do those.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
me... i'd ignore the additional venting.. especially with cement tiles and a hip roof.. the venting is going to do nothing for your roof ( cement tiles ) and will not lowe3r the attic temp. enough to notice
i'd add 12" of cellulose on top of your white fluff.. keep looking until you find a cellulose guyMike Smith Rhode Island : Design / Build / Repair / Restore
I asked about cells. This firm does it also.
The shark said, "never to put cells on top of fiberglas. You can put fiber on top of cells, but the weight difference of cells on top causes excess compaction and inhibits the performance of the underlying fiberglas."
So tell me why the ceement tiles (besides being dark grey and a heat sink) will make a powered vent useless?
It seems to me that ANY form of removing this superheated air out of the oven over my living area will provide positive benefits.
Are you suggesting that these tiles will continue to gain more heat than the vents are able to remove?
As far as radiant barriers, wouldn't these have to be applied on top of the roof deck prior to laying the tiles?
As far as another approach to this issue, would spraying foam on the underside of this roof surface provide relief?
The shark said, "never to put cells on top of fiberglas. You can put fiber on top of cells, but the weight difference of cells on top causes excess compaction and inhibits the performance of the underlying fiberglas."Like choped FG has any performance worth talking about.As far as radiant barriers, wouldn't these have to be applied on top of the roof deck prior to laying the tiles?No and in fact if you apply it to the under side of your roofing you should have an air gap of around 1". Not for ventilation but because the barrier will just let the radiant energy pass thru if it is in contact with the roof. Imagine you have an old radiator wraped in tin foil. It will still heat the room, the heat energy passes right thru the tin foil. But if you had a sheet of tin foil held an inch or two away from the radiator it would reflect alot of that heat energy back.
"No and in fact if you apply it to the under side of your roofing you should have an air gap of around 1". Not for ventilation but because the barrier will just let the radiant energy pass thru if it is in contact with the roof. Imagine you have an old radiator wraped in tin foil. It will still heat the room, the heat energy passes right thru the tin foil. But if you had a sheet of tin foil held an inch or two away from the radiator it would reflect alot of that heat energy back."Not exactly.The radaint barrier can be in direct contact with the tile or sheathing.It just needs 1" air gap on ONE SIDE. And the down side is better as the dust won't settle on it.The confusing part is that is just not the radiant reflectivity that comes into play.Materials that are highly reflectively are also low emssisive. That is that don't radiate radiant energy when heated.So in the case of an under roof radiant barrier with the air gap away from the roof it is the emisivity that is coming into play.The FSI used show some school science experiments and one of them showed exactly this..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
OK not disagreeing with you but his roof is already in place.
And I got a nest of trusses on the inside.
Again, is this a possible application for foam?
couple things.. 1st .. compacted to what ?
that fluffy shid has so many air passages thru it , it's pretty useless.... so compactiong it is only going to help
when we were still using fiberglass, we' d only buy the high density for our walls
typical standard was R-11 for a 2x4 wall... you can also get R-13 friction fit and now you can get R-15 ... so ... what does the fluff do for you .....he's blowing smoke
2d....
"super heated air" is not that superheated..... if you exhaust 120 deg air, what is it replaced with ? 100 deg ?....... 90 deg ?
how much energy does it cost you to run a power vent ? and what does it do ?
well, first off.. it costs a lot of KW... and where is it getting it's air from ?
is it sucking the air in from the soffits, in from the mushroom vents ?... or is it sucking air conditioned cool air out of the house right thru that fluff you have lying in your attic ?
truth is ...... it's moving air along the path of least resistance ... what ever that is
whole house fans... power vents.... for the most part are just crap... can you set up a system that sucks cool air out of an earth tempered tube under the house.. moves it thru the house to cool .... then exhausts it thru the attic to further cool the attic ?
no, huh ?...... thought so
you have a tract house... from your own inspections they took short cuts..left many paths for air to move thru
your best bet is separating the hot attic from the cool conditioned living space ... vents and power vents don't do that... only insulation does
cellulose is a radiant barrier.. fiberglass is not... most radiant barriers are snake oil..
" staple this to the underside of your rafters " ..... "lay this over your attic insulation"
forgeddaboutit..
best bet.... get an energy audit to come out and do a blower door test... air seal.... then add add 12" of cells to your fluffy base
roofs are vented because the asphalt shingle mfr's thought they could blame someone else for reduced life
current thinking is vented roofs are overrated... not worth the effort and expense
we do FULL soffit vents and continuous ridge vent on all our houses....once by conviction that it was helping... now only because code requires venting .. and if we're going to vent, then we want to do it right... right means balanced and controlled
your cement tiles don't care how hot they are.. they are going to live the same life-span no matter..
and yes... the power vent is not going to appreciably lower their temp...
and most of the reduction in attic temp is going top be because your are sucking airconditioned air out of your house into the attic
but, hey, whadda i no ?
Mike Smith Rhode Island : Design / Build / Repair / Restore
Cellulose on top of the FG will likely actually enhance the performance of the FG (which otherwise is doing virtually no good). Besides, what difference would it make to impact the FG performance, given that the existing FG is inadequate anyway?
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Within the scope of this project, the wife wants me to have the air ducts cleaned.
Would it be worth considering to have the duct vacuum truck suck out the fiberglas and have the whole thing reblown with cells?
I'm still wondering about foaming the roof deck as a radiant barrier...
It would probably cost more than it's worth to suck out the FG. Just blow cells over the top of it.I really don't know enough about foam to say whether it would be a good idea in your situation. I'm guessing, though, that since you have a nominally open attic, insulating the roof deck isn't a big winner.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Would it be worth considering to have the duct vacuum truck suck out the fiberglas and have the whole thing reblown with cells?IMHO, no. The mingling of the two might up the performance of the FG. FG is not evil. It just doesn't work as well as the pink panther would have you think.http://grantlogan.net/
Sometimes, when I lie in bed at night and look up at the stars, I think to myself, "Man! I really need to fix that roof."
<<best bet.... get an energy audit to come out and do a blower door test... air seal.... then add add 12" of cells to your fluffy base>>I agree with Mike. the blower door test and air sealing are essential. After that is all taken care of go with the cellulose over the fiberglass. I don't think this is a good place for foam. And I'm not sure having the ducts cleaned is such a great idea either but if the wife says do it the upside of arguing with her is non-existent. ------------------
"You cannot work hard enough to make up for a sloppy estimate."
Ok, the energy audit is scheduled for Friday. $350.
Talked with the BI also. His over the phone suggestion was simply add cell over the fiberglas. Agreed that there was insufficient venting, but seemed unconcerned about resolving the issue in favor of the added insulation. Said, "yeah, a power vent could help."
We'll see what the report says.
Well, I'm still waiting for the auditor's report. Maybe tomorrow.
In the meantime, I've been discussing this with my BIL whom is a researcher at one of the top solar energy manufacturers. He in turn has been spreading it around his other researchers.
The topic of, "is there a way to utilize this heat rather than simply blow it off" has creeped into the discussion.
They respond on the basis of the laws of thermodynamics. (I never studied that stuff in school.) But in essence, they point out that even though we get freaked about how high the temperature is in a poorly vented attic out here in Denver, there really isn't that much heat. They comment the trapped heat itself acts as a radiant barrier - to a degree - which limits the further buildup.
A couple of mechanisms have been tossed about. The Ericsson engine principal seems interesting. If there were a way to use that externally heated engine to move air, a powered vent would be created, but not take a power source or use an electric motor.
And then the "drinking bird" phase change principal... could this motion propel some form of bellows?
But the most intriguing thought to improve the venting (remember I'm apparently unable to add much more in high vents due to the hip roof) is to build in a vented cupola.
More to come.
It seems to me that ANY form of removing this superheated air out of the oven over my living area will provide positive benefits
Ah-hah! The nub of it, indeed!
It's mass.
Your roof has mass; your roof, especially so, for being clay. It's gaining somthing like 20-65W per square meter of solar energy per hour every hour there's light on it. That's a lot of energy.
Ok, so lets get the hot air out.
What are we replacing it with? The ambient 90+º air? Mind you, the roof mass, once it warms beyond ambient, will transfer heat to any air you run across it, too.
Materials scientist the the other day said it better than I could (least until I steal his phrase <G>)--venting a hot roof in Texas is like cooling an oven with with a fan white the oven is still on. (Now, if one of those grants we shopped would just come through . . . )Occupational hazard of my occupation not being around (sorry Bubba)
I say forget about the power vent - they use as much energy as they save. If you want to increase your ventilation - which it sounds like you need to do, get some kind of passive vent. Heck - I'd even go for a turbine vent before a power vent.
For that mater, call the original builder - regardless of the age of the house, and tell him to come cut out the 2 vents that aren't. It was probably his roofer that messed it up, but it is still the builder's responsibility to check his sub's work. This coming from a builder...
As far as calculating the amount of ventilation you need, this is from the NC Residential code:
Minimum area.
The total net free ventilating area shall not be less than 1 to 150 of the area of the space ventilated except that the total area is permitted to be reduced to 1 to 300, provided at least 50 percent and not more than 80 percent of the required ventilating area is provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet (914 mm) above eave or cornice vents with the balance of the required ventilation provided by eave or cornice vents. As an alternative, the net free cross-ventilation area may be reduced to 1 to 300 when a vapor barrier having a transmission rate not exceeding 1 perm (57.4 mg/s · m2 · Pa) is installed on the warm side of the ceiling.
Obviously NC code doesn't apply to you, but the concept is still there.
Personally though, I like the idea of adding cells to the attic better, or in addition to... As far as that thing about adding cells on top of fiberglass - that is the first time I've heard that. I'd do it in my house if I had a problem. I can see the guy's point, but the net gain would still be significant especially with the greatly reduced air leakage.
We used to have this guy Freddie around here. He had an energy audit/weatherisation company and was very knowledgeable. He always used to talk about abating (sp?) fiberglass. Not that I agreed with that. Actually he didn't last here because he couldn't deal with people with different opinions than his own :-)
I was at Sherwin Williams the other day and they have a new product: Paint on radiant barrier. It's a silver-ish paint that I guess you apply to the underside of your exposed roof deck. The painters in the ad photo were both smiling, so it must work great...
Anybody else know anything about this?
Radiant barriers work, when the reflective side is exposed to an air space, but they don't work all that well in most cases. Especially when ventillation is poor, air will conduct more heat than will radiant transmission. And it's generally hard to tell how good of a "reflector" the paint is -- simply being silver colored doesn't tell you much.I can't imagine trying to paint that on my roof deck, even before the extra foot of cells was blown in. A 4/12 roof has a significant fraction near the eaves that can only be reached while laying on your back with your nose against the roof sheathing.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Florida Solar Institue has done a lot of research on attics, roofing materials, insualtion, radiatain barriers and similar.They have a number of reports of radiant barriers. The last I looked at their website they radaint barrier paint was just listed in the footnotes as one type of radiant barrier. But gave no brands or how it compared to other materials..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.