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What is the best way to vent an attic?

Southern_Sawdust's picture

Good Evening To Everyone,


     I have a 1970 Ranch that I am planning on re-roofing in the next few weeks. I want to add some more insulation and improve the ventilation in the attic as well. First off, will using the fiberglass batts be better or will the blow in cellulose be better? Second, will putting on a ridge vent and continuous soffit vent be better than using the soffit vent and gable vents with a exhaust fan placed at one gable vent be better? I am planning on shooting for a R value in the upper 40 range.

(post #113029, reply #1 of 62)

Where are you located? Would you consider a conditioned, unvented attic if you could gain better air quality, lower electric bills and less dusting required in the home?

(post #113029, reply #2 of 62)

Generally cellulose is better, but be sure it doesn't block eave vents.

Ridge + soffit vents is far superior to exhaust fan, and is maintenance-free. The soffit vent can be either continuous or a cut-in 8x16" vent about every 4 feet.


If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #113029, reply #3 of 62)

The best way to vent an attic is to reduce the need for venting!!! We supposedly vent attics to (1) REMOVE MOISTURE and (2) REMOVE HEAT TO LOWER AIR CONDITIONING COSTS AND INCREASE ROOF SHINGLE LIFE.


(1) If we airseal the attic/ceiling interface well, very little moist house air (and heat) will get into the attic to cause condensation/mould growth on the roof sheathing. So do this first before adding the retrofit blown cellulose on top of the existing insulation. Now the existing ventilation is probably more than enough. High 40's is a good R value to shoot for.


(2) A high level of attic insulation is better than attic ventilation at reducing air conditioning costs.


(2) Attic ventilation does very little to nothing at increasing shingle life. The biggest influences are: shingle quality, shingle colour and shingle orientation/direction (and the last we have no control over)  If attic ventilation severely decreased shingle life, we would not be pushing all the foam sprayed unvented attic rafter cavities to make the attic a conditioned space. This technique allows no cooling of the roof during the hottest part of the day......period. They roof can cool to the exterior at night only. So if roof venting is needed for increased shingle life.......there are going to be a lot of prematurely failing roofs based on this "on the street" theory.

(post #113029, reply #4 of 62)

The best way to vent an attic is to reduce the need for venting!!! We supposedly vent attics to (1) REMOVE MOISTURE and (2) REMOVE HEAT TO LOWER AIR CONDITIONING COSTS AND INCREASE ROOF SHINGLE LIFE.


I don't know who "we" is but "you" suppose wrong.  In your neck of the woods the primary reason to vent is to prevent ice damming.  


DG/Builder

(post #113029, reply #5 of 62)

In my neck of the woods in jan/feb/mar, we get still sunny days of 0 to -3,4 celius (25-32 F) with no/little winds. Exposed parts of dark roofs warm enough to cause snow melting/running water until it gets to the cold snow covered eaves when ice dams still occur even with code venting installed. Some days you would have to have full ridge, soffit and gable end venting (a little impractical) to prevent the attic from getting over 32F and causing the melting leading to ice dams.


You can draw all the diagrams you want with arrows telling the air where to go, but in real life this will not always happen or happen with enough air volume so as to be effective to work as designed. Since it is virtually impossible to stop ice dams with code required passive venting, the best way to prevent damages from ice dams is to have proper eave protection installed on all slopes under 9 in 12. The code recognizes this here and requires the eave protection.


Read this recent publication from our national housing agency and you will see some of the misconceptions about venting floating around the streets and our building supply centers:


http://www.cmhc.ca/en/co/maho/gemare/gemare_001.cfm

(post #113029, reply #7 of 62)

Well, around here most winters (though not this last one) you can pretty well tell which homes have proper venting/insulation (they work together) and which ones don't.

If you're not going to vent the attic you should go to a full "hot roof" strategy. Anything in-between is asking for trouble.


If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #113029, reply #8 of 62)

I'm not sure where you are, but in Connecticut, I see "properly vented" vented roofs sporting spectacular ice dams on a regular basis. In most cases, air sealing and insulation are better solutions, with going to a full hot roof as you suggest often the best approach. Part of what I do for a living is home energy consultations. The things people think it's ok to do in attics continues to astound me.


Some roofs, in fact, are impossible to vent. How do you balance the soffit and ridge vents on a hip or a reverse gable? You can't.


Andy


"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein


"Get off your dead #### and on your dying feet." Mom

Andy

Senior Editor, Fine Homebuilding

(post #113029, reply #11 of 62)

Yeah, I didn't mention air sealing, not wanting to get into gory details, but it's a "complete package" -- air sealing, insulation, ventillation.

True, hip roofs are hard to "balance", but there are ridge vent products that can be used on the corners, so it's not impossible. And we don't have that many hip roofs around here anyway.

Basically, I have nothing against a "hot roof" approach, but you have to pick one or the other -- a "cold" roof with poor ventillation is asking for all sorts of problems. And retrofitting a "hot" roof to an existing home is challenging at best.


If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #113029, reply #6 of 62)

I've got to agree with Experienced. Roof venting is perhaps the single most poorly understood aspect of building, and our long-standing traditions are based on a marginal solution developed 60 years ago. We can do better.

Andy


"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein


"Get off your dead #### and on your dying feet." Mom

Andy

Senior Editor, Fine Homebuilding

(post #113029, reply #9 of 62)

"I've got to agree with Experienced."


OK, then help me out here.  How would you explain this statement of Experienced's? "If attic ventilation severely decreased shingle life, we would not be pushing all the foam sprayed unvented attic rafter cavities to make the attic a conditioned space. This technique allows no cooling of the roof during the hottest part of the day......period. They roof can cool to the exterior at night only. So if roof venting is needed for increased shingle life.......there are going to be a lot of prematurely failing roofs based on this 'on the street' theory."


I can't make heads or tails of it!  Is anyone saying attic ventilation decreases shingle life?  What does it mean that spraying unvented-attic rafter bays with foam insulation does not allow cooling of the roof during the hottest part of the day?  Is he recommending foam in the rafter bays in all attics, or only unvented ones, or not at all?


- Confused in California


"he...never charged nothing for his preaching, and it was worth it, too" - Mark Twain

(post #113029, reply #10 of 62)

He's saying, and I've spoken with researchers whose work bears this out, that attic ventilation has little to no effect on shingle life.


More to the point, shingle color has a far greater effect on shingle temperature than does ventilation. If heat were really the problem, manufacturers would only sell white shingles that don't get as warm as the black ones that seem so popular.


Andy


"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein


"Get off your dead #### and on your dying feet." Mom

Andy

Senior Editor, Fine Homebuilding

(post #113029, reply #13 of 62)

I haven't done any studies, but I can think of at least one roof I tore off and replaced that had inadequate venting ( L-shaped house, one small gable vent at each extremity, no soffit venting) where the shingles near the vents were in pretty good shape, but their condition got progressively worse the higher up and the further they were from the gables.

(post #113029, reply #14 of 62)

That's one. How old was the roof? Were there other possible contibuting factors? The researcher I know has multiple roofs in service, with controls and known variables.


It's dangerous to draw general conclusions from small samples. For example, I've torn off cedar siding to expose deteriorated housewrap. Hmm, seems like the cedar did in fact contribute to the housewrap failure, or at least that was the conclusion my reading had pointed to. But still, what did I know? It's possible that housewrap stood exposed to the sun for 6 months before it was sided. There are lot's of unknowns in any tearoff.


Andy


"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein


"Get off your dead #### and on your dying feet." Mom

Andy

Senior Editor, Fine Homebuilding

(post #113029, reply #16 of 62)

Let us all rejoice in the return of the vent wars. It's been a loooong time!

;)

 

 


Welcome to the
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Knowledge FHB Campus at Breaktime.
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Excellence is its own reward!

 

 

Oh Well,

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(post #113029, reply #17 of 62)

It has indeed. When asked your political affiliation, the proper answers are: Pro-vent and Con-vent.

Andy


"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein


"Get off your dead #### and on your dying feet." Mom

Andy

Senior Editor, Fine Homebuilding

(post #113029, reply #18 of 62)

Thats a lot of extra money you are talking about I believe.


If I remember right the correct way to do it is wet spray the rafter bays and then you still have to fill with celulose in the joice cavities.


Now is the attic conditioned as well?


So if its 105 degrees out side in the shade what would that attic temp be in the sun with a black roof and a white one ?


tIM




 

 

(post #113029, reply #19 of 62)

I have as well seen premature checking and cracking of shingles with dark colors of unvented  roofs in the soffit area while the ridge was in good shape  and ridge vents were used.


Tim




 

 

(post #113029, reply #24 of 62)

Interesting,  because with a ridge vent, the shingles closest to it should be the hottest on the roof. Think about it - If air moving up the rafter bays is cooling the roof above, that air is getting hotter as it rises. So, if the shingles nearest the ridge vent were in the best shape on the roof, then wouldn't that suggest that heat is good for shingles?

Andy


"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein


"Get off your dead #### and on your dying feet." Mom

Andy

Senior Editor, Fine Homebuilding

(post #113029, reply #26 of 62)

If the soffit was vented I think you would be right . That way it would bring in a cooler supply of air from the bottom up. In the cases Ive seen where I could tell a difference the soffits were not vented but had two or more gable vents . From the description I was taught , the top section gets a cross breeze but no up draft.


From a mechanics view it takes air intake to produce an exaust.


Im more interrsted in the cost with my other questions involved about a conditioned attic space.


Tim




 

 

(post #113029, reply #27 of 62)

Conditioned attics cost more to do initially. Of course - there's more surface area to insulate, and foam costs more than other insulations (although it outperforms anything else on the market). Because foam actually provides its stated R-value, unlike fiberglass, there should be an energy savings there despite the greater exterior surface area. Factor in the losses that occur with air handlers and ducts in a vented attic( Which even if insulated to code at what, R-6? aren't insulated to ceiling and wall standards. Given that the highest Delta-T in a house is going to be between the attic ducts and the unconditioned attic air, this is just plain stupid, in my not so humble opinion) So, in fact, I'd expect both an energy and a comfort dividend with a hot roof that's not approachable with a conventional attic.

Andy


"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein


"Get off your dead #### and on your dying feet." Mom

Andy

Senior Editor, Fine Homebuilding

(post #113029, reply #28 of 62)

What air handlers? What ducts?


If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #113029, reply #29 of 62)

Im not trained in this subject. I had a state hvac inspector that could blow people away on the subject and taught at a votech school . Always made me feel stupid .


I think what hes talking about is the air temp outside the duct service. Which could include an air handler because lots of them are in the attic. Hes also including  a hot roof which could be black [or dark color] or hes talking about the type .  


He needs to get his fingers warmed up for me so I can fully understand it and hopefully take questions.


Its always been a subject of air temps around the duct pipes and plennum. There is little insulation compared around the pipe and such a small area inside the pipe that its hot inside. So we get a blast of air thats hot and still fights it as it runs because it will be warm at best for a minute or two. Then after 5 minutes of run time it shuts off and the cycle starts again.


Another topic has always been the temp of the attic compared to the amount of insulation separating the home area. Here he hasnt given the temps of a conditioned attic in either a white roof or a black one. Also the total costs to get one.


Now more in my area,


The problem I always have when we add new gadgets on a home is will the appraisor pay for them?[figgure of speech]   They dictate the values and are lord of setting our prices off comparibles. The bankers use them to  their own advantage to  often set up their own team they use of AP that work only for them although the public pays them. They nearly always set values low if a bank calls them .


I asked the question in the truckers forum , What will it mean to owner operators when the fuel prices exceed 3.00 per gallon this year as forecast. BTW its already there and will climb higher. They have reports of 3.01 fuel now. They simply said they will charge more and they are not worried,  but the public is the one who decides if the frieght moves and at that which frieght. Since transportation is a sizeable increase to some products it remains to be seen if we will pay the increase or do with out.


They of course dont have my studies to base their opinion. Across the Southern states where income is low it has remained low exceopt we have a bigger share of the market where as the unions are folding in the North East. We are picking up their work for half the price and are proud to get it . Outsourcing has been a big problem with the workforce and it depends on what work force someone is in which relates to customers. Although our construction figgures are just becomming soft there are still bunches of others going belly up. Our folks have been stangled by Heating costs this past winter and some have reported paying 5 times the normal amount . Gasoline as I mentioned has been another one to watch closely but its certain there isnt a lot of extra money. Of course I speaking about the Southern states only. Taxes have continued to climb for property taxes and bonds. Food costs citing beef prices are the highest they have ever been. Interrest has now began to rise  after a surge in labor and material prices in housing . So whats left for them to give ? Not extra unless it can be financed. Which brings us back full circle.


Tim




 

 

(post #113029, reply #30 of 62)

My point is that in many parts of the country it's unusual at best to have any ductwork or air handlers in the attic.


If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #113029, reply #31 of 62)

That could be. Here in the northeast, there aren't many new attics without ducts, and particularly on bigger homes, it's quite common to find air handlers there - often with hydronic coils.

Andy


"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein


"Get off your dead #### and on your dying feet." Mom

Andy

Senior Editor, Fine Homebuilding

(post #113029, reply #32 of 62)

Tim, the answer to your question depends entirely on the house and the location. In areas without temperature extremes, it wouldn't matter a lot. In the north or the south, it does. Heat loss depends on the difference in temperature, delta-T, expressed in Kelvin. In the north, my attic might get below zero F, and the ducts might have 130 degree F. air in them. That's an extreme delta-T. The reverse happens in the summer with AC and 150 F. attics. Not to mention the typical lack of duct sealing that leaks to and from the attic. Conditioned attics solve these issues.


I am assuming fiberglass in the ceiling v. foam in the roof, btw.


Another issue is workmanship. I'm not saying that I've never seen fiberglass insulation installed correctly in an attic floor, but I could count the number of times that's been my experience on one hand. And even so, with one face exposed, the stuff loses half its r-value at a delta-T of 70F. or so.


Foam installers, in my experience, not only have a better product, they're aware of it's ramifications and do a better installation. Not always, but more often than the glass guys. And if you have an air handler in a vented attic, don't you think it would perform better in an insulated room there? Ever seen that done? You'd have to add that cost to the fg job for a fair comparison of fg and foam.


So, it's nearly impossible to compare apples to apples in most houses because of the typical differences not only in insulation, but in installation. I believe that the advantages of a hot roof and a conditioned attic frequently outweigh the disadvantage, which is only initial cost.


Go to www.buildingscience.com. Their Building America program did a study in Las Vegas that clearly demonstrated substantial energy savings from conditioned attics in that climate. They may have more data now. Also look on PATH's website.


 


But you've got the basics down right.


Andy


"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein


"Get off your dead #### and on your dying feet." Mom

Andy

Senior Editor, Fine Homebuilding

(post #113029, reply #33 of 62)

Oh joy, I wanted to talk about unvented attics again, thankfully someone else has started the thread.

I'm trying to become a foamed roof True Believer, but I can't shake these niggling doubts.

Since I'm in a cold climate (NJ), buildingscience tells me that there needs to be drying to the interior as snow melts. But the only "Houses that work" spec for that scenario (Boston, no details BTW) requires foam insulation with 10 perms. Given permeability is determined by thickness of material as well as its perm rating, can you really get that with several inches of foam? Even EPS becomes vapor impermeable beyond 3". It seems to me that allowing some venting via baffles to dry the roof deck (from external moisture, not from interior moisture, which can be handled in other ways) would be useful insurance.

Even in hot-humid climates, if you didn't put a VB over the roof deck like buildingscience says you should to prevent vapor pressure, then venting might at least dry out the roof deck faster than if you just relied on the foam absorbing and releasing the moisture.

From what I've seen of roof leaks in my house, sometimes they come with water getting underneath the materials and into the sheathing and rotting it out, not a steady stream of water onto the attic floor. This form of "leak" will never show up on a foamed roof, but at least venting would dry out the roof deck a lot faster than if it is covered in foam. So maybe the roof deck might survive until the next tear-off.

In all the talk of R-values of foam, I'm surprised no-one mentions thermal bridging through the rafters. That will surely reduce energy savings by anywhere up to 20%. Putting rigid foam over the rafters will obviously alleviate that, although again you won't see roof leaks. Just out of curiosity, has anyone actually seen a roof leak manifesting through open cell foam? I've heard it repeated as motivation for open cell many times, but I'm starting to get skeptical.

And since I'm ranting here, let me complain about rafter baffles that provide a 1" air space, when buildingscience says it should be 2".

I'm wearing asbestos, flame away.....

(post #113029, reply #34 of 62)

All interesting questions. I'd use Building Science's info for Boston in NJ. I think Boston is in the same agricultural zone as Sussex County, and that's considered a mixed climate. Living in that same zone myself, I can tell you that it's not a lot different than Warren County, where I grew up. If you're in Cape May, I might look to Baltimore as a model.


Otherwise, I'd research more. Look to PATH (Partnership for Advanced Technology in Housing) for more info.


If you want 2 in. vent channels, just make 'em out of rigid foam.


Andy


"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein


"Get off your dead #### and on your dying feet." Mom

Andy

Senior Editor, Fine Homebuilding

(post #113029, reply #35 of 62)

I look at both Cold and Mixed-Humid climate for planning, since I expect big climate changes in the next 10-20 years. The Designs That Work page for BS, for both Cold and Mixed-Humid, both emphasize drying to the exterior, with an XPS vapor retarder under the insulated roof deck. With XPS on the inside, I wouldn't put XPS baffles on the outside over the vents, let alone the plastic provent baffles.

Given the lack of documentation, suggesting that this option has not been fully investigated, I'm increasingly nervous about following the Boston model. Also the Designs That Work have the feel of having been designed by a committee, which is good because they may reflect several viewpoints.

My current plan is:

  • Baffles constructed from 1x2s and 1/4" plywood (rafters are 2x6 @ 22oc, want to keep load down).
  • Fur out with 6" Marinoware studrite steel studs (smaller steel studs are just harder to install)
  • Fill the 9" space with open cell foam
  • 1-1/2" XPS with 1x3 furring strips (Owens-Corning Wallmate)

My worry about closed cell foam: first that over time gaps will open up between the rigid foam and the sheathing and the air impermeability will be compromised; second, the double VB between SPF and XPS thermal break.

It is very very tempting to say, oh screw it, and just have open cell foam sprayed in the 2x6 rafter bays, as Icynene recommends.

Thanks for the PATH ref, I will look it up.

(post #113029, reply #36 of 62)

I'd just go with the Icynene, myself. If the AHJ allows it, that is, or if he's not looking. The Connecticut code allows hot roofs if they're insulated with closed cell foam. Lots of the local BIs look the other way and allow Icynene. Air sealing is the key, and open cell foam acheives that well enough while allowing some vapor transmission.


 


Andy


"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein


"Get off your dead #### and on your dying feet." Mom

Andy

Senior Editor, Fine Homebuilding

(post #113029, reply #37 of 62)

Thanks for taking the time to respond, I very much value your opinion.

I've spoken to AHJ, as long as I've done my homework he is open to argument.

Ray Moore suggests at least 8" open cell foam on roof deck. One time he made a remark that Icynene recommends 5-6" because that is as far as they are fire-rated, I didn't understand that.

Do the math: 5" of icynene gives about R-18. Lose at least 20% to thermal bridging, brings it down to R-15 or less. Adding 1-1/2" XPS thermal break brings that back up to about R-22 optimistically. Still not great. Ideally for the roof deck I'd like something like R-38 (assuming in foam terms that is equivalent of R-50 in other insulation schemes). I can get there with 3-5/8" steel studs furring out, or do the baffles and fur out 6".