Attic vs. roof ventilation, whats right?
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There is something about attic and roof venting I don’t quite get. Some of what I’ve read, states that sofitt vents allow airflow thru the 1.5 in gap between the rafter insulation and the roof decking, out thru the ridge vent. This helps to regulate roof and shingle temperatures. Ok, other information states that the air flows thru the soffit vents into the attic space between the rafters and ceiling joist, and out thru either the ridge vent or another type of roof vent such as a powered dome type roof vent. I want to achieve a balanced system, and I don’t mind if I have overkill. I’ll spend a little extra. My attic floor is 720 s.f. and I am building in Alabama. Needless to say it gets hot and humid here.
1. So which is right, soffit vents mostly for roof or for attic or both ?
2. How should the insulation be spaced at the roof/wall intersection ?
3.Right now I am thinking that I will let the soffit vents flow to the ridge vent, and use a powered roof vent and gable inlets to vent the attic space. Does this make sense ?
Replies
This has been beat to death here. Search the archives. But lets see if anybody will rally and carry the flag forward for you.
<Takes a long pull from coffee flask>
Alright. I am waking up. The most common way to vent your attic (which is unfinished, right?) is to install continuous soffit and ridge vents, which will better set up a flow of air out the building than gable vents.
Edited 3/16/2005 7:12 am ET by homer
1) it ventilates both... If I understand what you mean - it's a little hazy.
2) The insulation is spaced away from the roof deck using insulation baffles. IMO, the cardboard ones are the best as they give the most air passage. Of the thin styrofoam ones, there are a few different brands, at least 1 of which gives more square inches of air flow. Raised heel trusses or stick framing specifically installed to insure full thickness insulation coverage over the wall cap plate, and adequate air flow is optimum.
3) By mixing ventilation strategies, you will end up with an attic that is NOT ventilated evenly, and setup as you described, little to no air may enter the soffit vents. Just go with the soffit/ridge vents combination unless you have a roof config that hampers a proper install such as a hip roof. Whatever you do, loose the gable end vent idea as these will possibly admit wind driven rain, and you will never really know if air is going in or out of these, based on wind conditions, etc.
Our NC code requires that at least 50% but not more than 80% of the roof ventilation be located in the upper third of the roof. The balance being in the soffits. The idea is that the "stack effect" is utilized to get the air flowing from the soffits to the ridge, and out. Certainteed used to have a really good web site for helping one understand the principals of attic ventilation. I think it is still there, but have trouble finding it now - I think Mike S knows where it is.
Edited 3/16/2005 7:40 am ET by DIRISHINME
Take a look at Building Science for a comprehensive approach to insulation. As best as I can tell, their research indicates that a "cold roof" approach is not necessary... just blow a impermeable insulation into the rafter bays and make the attic a part of the conditioned space. Your AC system will thank you (if any trunk lines are installed up there).
Plus, you gain the space in the attic, which is also nice. The only potential downside is that the roofing may experience higher deck temperatures (like +15°F) and therefore not last as long... however, those roof tests were done in Las Vegas, I doubt I would see a major difference in most areas. Anyway, surf the site, read their builders handbooks, and you should be good to go.
IMO, although Building Science offers some great building techniques, I have my doubts if their sealed attic approach is a tired and true method for attics in the southeastern US. Who wants to build "on the bleeding edge". This is a house, not some $30 gadget that you can throw away if it doesn't work right. Also, I see no reason what so ever to condition the extra cubic feet in the attic unless you do actually have HVAC ducts/equipment up there that cannot be covered by the ceiling insulation blanket. To the original poster: check with your local code enforcement officials before planning on a sealed attic. Matt
Well, if you disagree with their approach, there are a number of people that would like to know why. Let's keep feelings out of this... so I simply ask for any scientific data, studies, etc. that you can point me to that shows the sealed attic approach is detrimental to roof life, comfort, etc.
Next, consider the energy savings that buildingscience was able to show as a result of making the roof deck the edge of the conditioned space instead of the ceiling below. Granted, very careful sealing of the ceiling deck may prevent the kind of heat loss/gain foudn in most homes, you may even be able to insulate a AC system or furnace up there... However, how much easier is it to properly insulate a roof deck than a floor full of penetrations, a AC duct, etc.? One surface is reasonably flat, etc. the other has bends all over.
For what it's worth, I sealed my attic. Then again, I'm in the NE of the US where the heating season predominates by a large margin.
That's the beauty of BT - everyone gets their opinion! Yea, I've read what Building Science had to offer - and I even seem to remember reading - don't build houses in the south, based on recommendations from people from the north, or something along that line. And I got their book for my climate. Still ~98% of people in the US are building ventilated attics. I guess the 2% are right and the 98% are wrong? Here in NC, I've never even seen a sealed attic. Glad you have a sealed attic and like it. Build a hundred houses with sealed attics, and then we will talk some more.
You said: >> so I simply ask for any scientific data, studies, etc. that you can point me to that shows the sealed attic approach is detrimental to roof life... <<
I'm not gonna waste a lot of time discussing or researching this but a Google turned this link up as the first hit: http://realtytimes.com/rtcpages/20010809_roofs.htm And a quote from that link:
>> The unvented attic concept also led to peak roof decking temperatures that were 20 degrees higher than a ventilated attic. As a result, the researchers concluded the unvented attic concept should only be used with light colored roofing or tile roofs to avoid endangering the life of the roofing system. <<
BTW: You probably don't know it but light colored roofs are not normally not installed in the SE due to problems with algae.
This is the way this game works for me - I gather info from a number of sources, and then make up my own mind. I see no one source as THE source.
Generally, this topic has been beat to death here at BT - If you want to learn some more, do a search.
Matt
> BTW: You probably don't know it but light colored roofs are not normally not installed in the SE due to problems with algae.
Sounds like copper would be a good choice if you want to go sealed -- it can handle the heat, and kill the algae.
-- J.S.
Well, I guess there are several ways to look at it.
At one point, the US had 98% balloon framing, then platform-framing took off. Were the folks building platform framed houses off their rocker?
93% of the US housing stock has air-based heating systems.. so are the hydronic folk crazy for insisting on water-based heat distribution? For some reason people are willing to pay big premiums for this sort of heat.
Mac users only make up 2-3% of the entire computer world. Yet, people insist on using them instead of windows computers...
When large numbers are involved, percentages mean nothing. The real question is: what is right for me?
I totally agree that some building practices that work in the North do not work in the South. For example, the vapor barrier location (where fitted) comes to mind with the different heating/cooling climates across the US. What I like about buildingscience is that they actually theorize and test their theories. I haven't built 100 homes, but they seem to have built and instrumented plenty. Perhaps even more than you?
Lastly, there are several roofing materials that can handle the additional heat that a sealed roof can generate, just not the cheapest ones... I'm thinking of tile, cement-composites, metals, etc. The 9% energy savings mentioned in the article that you linked to is pretty attractive for the homeowner considering that the other options (like light-colored roof tiles) seem to be out of the question in your climate.
whoo hoo!
there's still some fight left in us about attic venting! Shh, I think thats the Rocky theme in the background...
I was on the Regal Industries website and here they have gotten Elk to offer a shingle warranty over a non vented roof. I read the same test about the houses in Vegas, and when I saw that they were only getting 20 deg. higher readings on the unvented roofs, I thought to myself, big deal. I think the code writers thought it would be something like 50 degrees higher!
The more relevant question is (if cost isnt a factor) do you use radiant sheathing/radiant barrier to get the temps down whether you vent or not.
>> The more relevant question is (if cost isnt a factor) do you use radiant sheathing/radiant barrier to get the temps down whether you vent or not. <<
Here is a Q for you... Does radiant sheathing raise the surface temperature of the roof? Seems like it would since it's purpose is to reflect the heat back up that way... Matt
Think of radiant sheathing like tin foil in your microwave. It reflects the heat energy back before it has a chance to build up.
>> Think of radiant sheathing like tin foil in your microwave. It reflects the heat energy back before it has a chance to build up. <<
Tin foil in the microwave... Is that a good thing? Have you ever tried it? Everybody needs to try it just once - you know, it's like something your mother told you to never do... A short time period is all you need to "get ti" :-)
Matt
Also try a burned out lite bulb, capacitors, transistors, etc. ;-)
-- J.S.
You make some good points and I wish I had a crystal ball so that I could see standard building practices 20, 40, & 70 years from now. In the mean time I won't be doing too much experimenting with houses I build, I'll leave that to someone else. Don't get me wrong, I'm all for new materials and methods, but they have got to be handled with a large amount of scrutiny, and in addition to technical endorsements, I like to see a few local builders using the stuff/techniques before I jump on the bandwagon. Cutting edge - maybe a little... bleeding edge - no. Like I said, a house is not a $30 gadget you can throw away if it doesn't work right.
As far as your statement: >> When large numbers are involved, percentages mean nothing. << I could counter that by saying now that now that we have the internet, I can find an article to support any opinion... so, I like to find lots of info to back up a significant conclusion. I do like the second part of your statement: >> The real question is: what is right for me? << and maybe it comes down to personal style. Everybody knows that Sony Betamax was better. ;-) I never had one...
I think we can agree to disagree on this one and I respect you because you have obviously done your homework, as is the case with many of the folks who "hang" here.
I also like your 'north/south' statement but more of this stuff is regional though than many people here realize - for example some of our left coast brothers have probably never heard of pouring concrete footers without forms... and they think their methods of working with PT lumber are the same as ours - but hey, they taught me what "incised" means... Being a building enthusiast, I guess I'm a geek, but when we go on vacation to other parts of the country, I like to visit a construction site or 2 to see how 'they' do it 'there'.Peace,Matt
constantin.. i like sealed attics.. but they are NOT code allowed..
so building science is not practical in promulgating it..
they only confuse the issue
a balanced soffit/ridge vent is the gold standard
introducing gable vents and powered roof vents is a bad idea
musroom vents are pretty bad too..
i've got all of the building science books.. and i certainly do not agree with their findings in a lot of cases..
is your position that the homeowner should not ventilate ?
Mike Smith Rhode Island : Design / Build / Repair / Restore
Actually, the new IRC addresses sealed attics. They're allowed with closed cell foam applied directly to the roof deck. Connecticut adapted that section. Maybe RI didn't?Andy Engel
Senior editor, Fine Woodworking magazine
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
But a poster a while back got local AHJ to approve hot roof based on report by a building scientist....And the scientist recommended open cell foam so the shingles could dry to the interior.... That may have been down in the south....
Also a possibility. I was just speaking to the sea change represented by the IRC accepting hot roofs.Andy Engel
Senior editor, Fine Woodworking magazine
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
andy.. i just read chapters 8 & 9 ..
the only reference i can find is section 806 "Roof Ventilation"
and i see no mention of "hot roofs".. just the usual 1/300 and 1/150 ratios
do you have a citation ?
i also read the RI amendments and could find nothingMike Smith Rhode Island : Design / Build / Repair / Restore
806.4?
huh ?
mine goes... 806.1 thru 806.3
the next section is 807.1 i don't have an 806.4Mike Smith Rhode Island : Design / Build / Repair / Restore
When I was over your place I remember seeing 806.4 in the bottom of your cat's litter box. Which, I must say...needed venting.
Might need to have a chat with Helen.
Seriously, I don't have my copy handy, but I do have a mailing from them with this:
R806.4 Conditioned attic assemblies: Unvented conditioned attic assemblies (spaces between the ceiling joists of the top story and the roof rafters) are permitted under the following conditions:1. No interior vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly.2. An air-impermeable insulation is applied in direct contact to the underside/interior of the structural roof deck. "Air-impermeable" shall be defined by ASTM E 283.3. In the warm humid locations as defined in N1101.2.1:a. For asphalt roofing shingles: A 1 perm or less vapor retarder (determined using Procedure B of ASTM E 96) is placed to the exterior of the structural roof deck; i.e. just above the roof structural sheathing.b. For wood shingles and shakes: a minimum continuous 1/4-inch air space separates the shingles/shakes and the roofing felt placed over the structural sheathing.4. In zones 3 through 8 as defined in N1101.2 sufficient insulation is installed to maintain the monthly average temperature of the condensing surface above 45? F. The condensing surface is defined as either the structural roof deck or the interior surface of an air impermeable insulation applied in direct contact to the underside/interior of the structural roof deck. "Air impermeable" is quantitatively defined by ASTM E 283. For calculation purposes, an interior temperature of 68? F is assumed. The exterior temperature is assumed to be the monthly average outside temperature.
Do you have the 2004 supplement? I'm pretty sure this was approved around springtime...Apr/May '04...and was part of the '04 supplement.
Not "sure" sure, but that's my recollection.
no... i'll ask our Building Inspector what the latest Rhode Island stuff is
thanksMike Smith Rhode Island : Design / Build / Repair / Restore
What Mongo said.Andy Engel
Senior editor, Fine Woodworking magazine
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
To me, the mere homeowner, the code as written makes sense. Give the wooden shingles some air to dry out, keep the moisture out of the home, etc. We installed Grace Ice & Water shield throughout before putting the Duraslate down. Plus, we installed a pretty wide copper drip around the edges, etc. Time will tell if this was sufficient to keep the water out. Not having vents up there eliminates several flashings and other things that my limited experience to date has shown to be a source of moisture into the home.
It's really a much more complex issue than it appears on the face. For more info, try http://www.buildingscience.com.
Andy Engel
Senior editor, Fine Woodworking magazine
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
Thanks Andy... I have been pointing out the reams of research published at buildingscience to the naysayers of sealed roofs for a while. Yet, a single source of information doesn't seem to satisfy many of them. Plus, there is some DoE research published elsewhere re: the issues that sealed roofs can have in some regions of the country.For what it's worth, our roof looks happy and the house is warm. What more can I ask for?
For what it's worth, our roof looks happy and the house is warm. What more can I ask for?
Long life, good family, and a satisfying life? <G> Andy Engel
Senior editor, Fine Woodworking magazine
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
DJMerc did that.I think that the is in NC.The AHJ fought him tooth and nail.He he his engineers specify it and the AHJ still tried to not allow it because of claims that it would short the shingle life.
Un vented roofs are allowed in Mass. And I have been told in several other New England states as well but I cant confirm that.
Thank you for your long and informative posting! I note with interest the requirement for roof venting in some locales... we certainly didn't seem to have any issues with getting approval in MA with a non-vented roof. However, I could see why people would go it safe and rely on the approved way of building things rather than a new approach that may or may not stand the test of time.
For what it's worth, I think it will. The arguments made for a vent-less attic make a lot of sense to me, and the only downsides I see are the higher deck temperatures and the higher cost of insulating more surface area. I'm happy to deal with both. Thus, in our house, the exterior building envelope and the rafter bays are filled with a fairly impermeable insulation - Corbond. It's one way to control infiltration and exfiltration.
Our "attic" is basically the 25'x25' pyramid under a Mansard roof. We reused the old sheathing, rafters, etc. as the old roof was built for slate and was basically in good shape... However, we also took out all vents, etc. as we've prevented moisture from entering via the basement (a new, insulated, and waterproofed foundation will do that). We also have 2 HRVs to control air quality and a Nortec to control humidity...
Yeah, I now realize that there is more to insulating a roof than just blowing some Corbond up there. The house has to be able to handle a non-vented roof as well. I hope that between buildingscience, our insulation contractor, and our GC that we did the right thing.
It sounds like you tried to take the proper approach.
I have absolutely nothing negative to say against unvented, as long as the details are addressed.
I reread my post ands there are a few brain farts in there...hope it's not too confusing. Too little coffee, too many keystrokes.
> 55615.15 in reply to 55615.14
> Thank you for your long and informative posting!
Don'tcha just hate it when the long and informative posting gets deleted? What happened?
Here in Southern CA, it's been very common for a long time to have no attic. A bearing wall supports the ridge, and the top floor has high slanted ceilings, dw on the bottoms of the rafters. Economical to build, less economical to heat and cool.
-- J.S.
My bad.
It was a long post, and I emphasize "long". (sorry MadDog)
I went back to edit it to add a link and while only sorta/kinda looking I accidently hit "delete" instead of "edit".
Without recognizing that I was on the "delete" page, I made things worse. I looked up to see the "delete" and "cancel" buttons, and without really thinking about it clicked on "delete".
Doh!
I just got back from a vacation to the West Indies...you'd think I'd be clear-minded and refreshed instread of plumb worn out and bleary-eyed.
Not a big deal...the topic will come up again.<g>
Ah, OK. At least it wasn't so controversial as to be deleted by the management..... ;-)
-- J.S.