I’ve been reading Bruce Harley’s book from Taunton, “Insulate and Weatherize” from the build like a pro series. In the discussion of attic insulation he seems to indicate that after you’ve done as good a job as you can sealing air leaks into the attic, you can fill the ATTIC FLOOR with dense cellulose to stop more air leakage and insulate. He also seems to regard attic venting as secondary and not terribly crucial.
This seems counter to everything else I’ve read. I’ve never seen anything about an unvented attic with insulation on the attic floor – its always been unvented with an insulated roof. Is Harley’s method actually acceptable?
Our New England house’s attic floor is covered with loose cellulose and currently only has a gable vent. Adding soffit vents would be hard if not impossible. So if Harley’s method works it would seem to be a good match to my situation.
Any thoughts?
Replies
I'd be interested on people's thoughts on this matter, too. Here in the Buffalo area, I've got a gable vent on either end of the house, and one layer of insulation on the attic floor.
Bob,
I am also interested, because it is hard to use a healthy amount of blown cellulose without covering soffit vents, especially on the 4:12 hip roofs so common here. I have read so much conflicting opinion that I am more confused than informed.
Bill
Well how as the present system been performing? Do you have any mold or ice daming problems?
We've only been in the house 6 months and I think most of the insulation has been there only a couple of years, so I don't have much long term info. There are 3 layers of asphalt shingles on the house which is 45 years old, and they are beginning to curl. We did have a small ice damn this winter and I'm concerned about having more. The ice damn was at a sloped portion of the ceiling near the eave where the celing follows the roofline. This part of the house is packed with cellulose and so there's no ventilation getting in there. This area is my main concern.I think 3 layers of shingles in 45 years doesn't point ot a track record of overwhelming success though.There's no mold as far as I can tell, and the attic, in general, seems pretty dry. But it gets pretty hot in the summer.
You might want to have a home inspector look at your roof and roof structural members. usually 2 layers of shingles is the max you can have before you start to exceed the structural capacity of the roof. most homes were just not designed to carry the dead load of 3 layers...add a heavy snow fall to that and you could be in for trouble.
Just a FYI.
Trev
I have a dumb question: How thick is every layer of shingles? I ask, simply because I wasn't able to get a picture of the guys pulling the shingles off our roof, but there was about 1.5" of asphalt shingles up there. My gut feeling says 3 layers...The roof held that up fine, probably because it was built originally for slate. Now it has but a layer of fake slate, so it should be sighing a big sigh of relief...
I'm not quite sure on the thickness, if I had to guess I'd say 1/8-3/8" thick, depending on if they are regular 3 tab or architectural shingles.
Aren't most fake slate tiles made out of concrete? You may be saving a little weight with these fake tiles vs. real slate.
Found this on another forum:
Roof structure matters, too, so take the time to have your roof examined before deciding on a material. Roof rafters or trusses must support the weight of the roofing, and that varies greatly from product to product. Metal roofing is by far the lightest at 40 to 60 pounds per square for aluminum, 80 to 150 pounds per square for steel. Asphalt shingles are quite a bit heavier, and weigh anywhere from 240 to 400 pounds per square. Concrete and clay tiles tip the scales at 500 to 800 pounds for the lightweight versions, and 900 to 1200 pounds per square for regular tiles. Standard thickness slate weighs 700 to 800 pounds per square, and more for thicker varieties. Roof framing that can support an asphalt or wood roof may not bear up under the weight of tile or slate, so if you want to roof with slate or tile, you'll need the framing to match.
Hi Trev,Thanks for the interesting info. Fake Slate can be had in three materials, IIRC: plastic/rubber, metal, and concrete. Of the three, the heaviest and probably the most durable is the concrete version. Plastic/Rubber fake slate is probably the least fire/UV/etc.-resistant. On the other hand, these are not issues I have to contend with a lot in Boston, MA.Thus, I went with Duraslate which is based on PVC, IIRC. It's quite a bit lighter than real slate and flexible. So far, I'm very happy with it.
Just to stir things up more. In Denver (snow not unusual) the old building code allowed for 3 layers of roofing. I've done roofs but usually with a gun at my head.
I believe most cities around here have adopted the International Residential Code. I figured I would pick up a copy for some late night reading. Couldn't believe the price. Also right next to it was a similar sized book called the International Building Code. Anybody know the difference. Or where can these books be had for something less than a 2nd mortgage? OK, I'm outta here. Tyr
The IRC is a complete building code for 1 and 2 family houses. It inlcudes not only the strucutral codes, but also plumbing, electrical, hvac.The IBC is only structual and includes all types of construction, so you might have to wade through things like firewalls in 20 story hotels, in addtion to looking up the size of a header in 2 story home.The IRC is only about $60 at amazon.
I have this situation in my house which is a Dutch Colonial Gambrel Roof. I have large gable end louvered vents. I recently sealed as well as possible and put in 12" of cellulose. I have never had any problems with moisture, ice dams etc. Being a 1920s Gambrel there is insulation stuffed in the bays which are essentially a continuation of the steep pitched portion of the roof into the second floor walls. Hence there is no soffit venting of any sort. I've been in this house for 13 years. I'm in Ohio so we get our fair share of severe winter weather.
Before I can comment on this I would like to make a list of disclaimers.
1- I haven't read Harley's book so I can't comment on it. It is possible that there were details that you overlooked that would explain what appears to be a poor strategy.
2- It is dangerous to give advice on a particular building method question related to a specific structure without seeing the structure. There are lots of variables that can add up to success or failure of identical methods.
3- Building science is just coming out of it's infancy and some of the doctorine that is cutting edge today will be discarded down the line.
Having said all that, this is what I believe about the building science of attic ventilation.
Attic ventilation was developed primarily to prevent interior moisture from accumulating as frost on the underside of the roof deck in residential construction. Other motives included minimizing ice dams and removing heat, but the main goal was to improve the durability of roof structures in cold climates.
When moisture is produced in a home, whether by cooking, perspiring, respiration, bathing, or transpiration of house plants, it will, by the laws of thermodynamics, move to colder dryer conditions. In a cold climate, the moisture will move outward and upward to form condensation or frost on the first surface that it encounters that is below dewpoint.
If the interior environment is 68 degrees F and 30% RH then the dewpoint of that air is 36 degrees. That means that if the air in that house finds a surface colder than 36 degrees then the moisture will collect on that surface. Actually moisture will collect on surfaces much warmer than that but not in liquid form. Anyone who has slept in a small tent may have experienced the collection of their respiration on the inside of the tent.
In old leaky houses, this moisture quickly found it's way to the very dry exterior air and moisture collection was not an issue. As we tried harder to keep the warm air in, we began to experience moisture problems. In the late 30s we did some simple studies in Minnesota to determine how leaky an attic would need to be to avoid collection of this interior moisture. The figure of 1:300 was agreed upon and in the early 40s it became a national building code that attics would be vented at this rate. Henceforth, we vented our attics in the south so the roof wouldn't rot in Minnesota.
Because the factors that cause interior moisture to collect are so variable, we should understand those factors before drawing conclusions about the potential success of any method. In our tent the moisture load compared to the volume of air is high, so the potential for moisture collection is great. A 1000 sq ft home with eight people living in it will have more potential for problem than the same house next door with the bachelor who never cooks. If the bachelor vents his clothes dryer into the attic he may have just as many problems. It is important to consider the moisture load in the overall strategy, with care given to potential for this loading to change throughout the life of the building.
The climate is also determinate of the appropriate building strategy. We look at average low temperatures rather than extremes due to the nature of wood structures and their ability to store a certain amount of moisture before it starts to collect on the surface and cause problems. Generally, we look at the average low temperature for the month of January as the factor to determine dewpoint. (this may be giving too much credit to the ability of wood to store moisture so err on the side of caution.)
Individual structures can have component parts that make an otherwise viable system fail in some conditions. Imagine the home that has a large well vented attic with a small laundry room attached with a shed roof that is not contiguous to the rest of the attic. This small section of attic could have problems. The more complex the frame is, the more potential there is for problems. The saving grace of big houses is the larger volume of air to disperse the moisture load.
None of this directly answers the question, which is good, because the answer can be shades of gray. If you are more confused than before, good. Keep reading. The more you learn the worse it gets until you understand all the mechanisms for moisture movement in a structure. Applying this knowledge is best done on a case by case basis, which is why codes are so hard to write. A prescriptive, one size fits all approach is difficult to mandate without causing problems in certain specific instances.
Now for the short answer. Keeping in mind everything I've rattled on about above this is what I believe about attic ventilation.
In cold climates, good is to ventilate well, with insulation at the ceiling plane and no air leaking to the attic. Better is to spray foam insulation in the rafters with no attic ventilation. Best is Polyisocyanurate panels above the roof deck with no attic ventilation.
In hot humid climates with the AC in the attic, only consider the second two methods.
In closing, the long post I've just typed, only scratches the surface. I gave a simple answer in the end for those that don't have the time to dig deeper, but following a prescriptive method without understanding it's basis and the mechanics of moisture movement can lead to poor results in some situations. If you read this far you may be a geek or a masochist or both.
Thanks
Edited 7/23/2005 4:43 pm ET by RayMoore
This thread would be more appropriate in the energy section.
Well delivered info. Florda has made changes to the law to clear up the code for not venting attics when foam is installed. In the past when foam was installed some building dept.would require venting.
RayI appreciate the long and detailed response as well as those from others. Harley's book does emphasis controlling the interior environment -i.e. moisture which, I assume is why he feels vents are secondary when insulating the floor - and stopping air leaks is primary.This all comes from the fact that we have sloped ceilings at our eaves and small soffits so we essentially have a choice - put insulation in the eaves or put soffit vents - there isn't room for both.But I think the most interesting point you raised is to consider what goes on in the house - right now its just me and the wife so maybe we aren't a worse case scenario. This is a point I've not seen brought up much in the literature.I've read a lot of the building science corp's info. It seems to me that the insulated roof deck is probably the way to go in my case - it just sounds very expensive and Harley's method is obviously cheaper. Maybe we can reroof now and reinsulate in a year.Also the challenge is finding a roofer in MA who will do what I want - I've interviewed 7 and they all want to do it their way, even if that means using just 1 gable vent.Thanks for the great info. I'll post on Energy next time as you suggest.
If you decide to try to go through a winter before you reinsulate, there are some things you can do.
1 Use bath and kitchen vents religiously.
2 Air tighten fastidiously.
3 Maintain a dry indoor environment. 25-30 RH (less than 25% is bad due to viruses)
4 Minimize houseplants.
5 Squeegee shower walls and floors when finished or use the bathtub.
6 Monitor the moisture content of the roof deck, especially at the top of the north facing roof. Moisture rises. Roofing under trees stays warmer at night than roof that faces clear sky.
7 Come back here and report your results.
Well, I just got off the phone with the building inspector in my town and the whole point might just be moot for me. He says the MA building code requires venting, so I'd have to vent with baffles and them apply insulation over the baffles under the roof deck - which seems to defeat the purpose, in my mind. I find this strange since Builing Science Corp is in MA so I know there are some houses around here built unvented. How does this work - does the interpretation of the code vary from building inspector to inspector or is it statewide? Has anyone successfully reroofed in MA using an unvented method? Maybe I just picked the wrong town to live in.BTW, he didn't sound crazy about the PERSIST idea either.
Well, you must be living in a part of MA with a different building code than I do. Our building inspector had no issues with the lack of vantilation and a sealed attic. Was the inspector quoting from the latest codebook, or is recolection at work here?
Sounded like it was off the top of his head. It didn't sound like he was quoting. I live in the metrowest. Maybe I approached it wrong - I just called up and asked if this sort of thing was okay. He said this was not kosher according to the Massachusetts building code, not the town. Maybe the interpretations are different but I'm not sure how I can change his interpretation.Did you have a roofer or insulator do yours for you, Constantin? Or was it DIY?
Anderson Insulation put all our insulation in. It is my understanding that the code has been changed to allow insulated, sealed attics, but I'm happy to be wrong. I would simply approach a contractor like Anderson Insulation that works in your area to see what they can show the inspector to change his/her mind w/o being too adverserial about it.
I'll ask Joe Lstiburek about it.
Edited 8/3/2005 3:42 pm ET by RayMoore
does the interpretation of the code vary from building inspector to inspector or is it statewide?
It can, that's one of the 'interesting' things about working with BI.
Codes can (usually) vary by location; each township, city, etc., can enact their own. The enacted codes are not supposed to conflict with other codes within a jurisdiction (like township is not supposed to conflict with, say, State). The key word there is "supposed."
You may have seen "AHJ" on this board, that's "Authority Having Jurisdiction." If you store used motor oil on your site, US EPA and State EPA have juridiction over that. The county might require one thing; the township yet another, and so on. The BI is kind of at the "point" of all that needlework, too. A jaded BI can stitch up a person if he cares to (or is jaded enough, etc.) That's tough sledding by ny definition.Occupational hazard of my occupation not being around (sorry Bubba)
OK Bob-
I think we can solve this pretty easily. Can you get me the name of the building inspector? If so, I think we can get you a green light without too much trouble. I had dinner with the man that wrote a large chunk of the code and the amendments that allow for unvented attics in the MA building code. He and Joe may know the guy you are having trouble with and a call can probably resolve it. You could line up the appropriate sections to refer to but a call from the code writer may be more surefire. Let me know. I'll be flying out tomorrow to head back to TX but I'll check here in the morning or when I get home.
Edited 8/4/2005 12:45 am ET by RayMoore
Now I sleep better at night knowing that this sort of install is legal in MA. Thanks for clearing that up! I hope you had a good visit up here. Good luck on your return trip to TX.
Hi Ray,I'll email you the guy's name - he might not like me posting it here.
Your help already has been more than I could have asked for.
Check out this link on polyisocyanurate insulation panels with nailbase applied under composite roofing shingles.
http://www.pima.org/technical_bulletins/tb106.pdf
It's a lot easier to insulate the floor than the roof planes, and why bother including an un-lived in space in the envelope?
Baffles keep the cellulose from blocking the soffit vents.
IMO, the major need for ventilating attics is to prevent moisture buildup; if you can do that below, it reduces the need to ventilate.
But I still think ventilated is better than unventilated, because (i) perfect vapor sealing isn't likely and (ii) I think it better to keep the attic as cool as possible in the summer.
Even with an insulated attic floor, a hot attic will radiate heat back into the envelope: the hotter, the more heat radiated.
Allow me to disagree.Insulating the roof rafter area is not that difficult if you're using a foam product like Icynene or Corbond. Insulating exterior surface à la RayMoore's PERSIST system is even easier. Just consider how many different ways that the space into the attic is penetrated, now compare that to a regular roof.Furthermore, consider the widespread use of attics to run ductwork for AC systems. Besides making the AC system work that much harder, it's also a collosal energy waster in the summertime and a huge heat loss in the wintertime (even if unused). Any other things you have up there will also benefit, such as water lines for sprinkler systems, drains for AC systems, drains for condensing appliances, etc.If you can, have a look at the research the folk at building science corporation did on the temperatures in vented vs. unvented attics in the summertime. Their research indicates that unvented, insulated attics stay cooler in Las Vegas than the vented "cold roof" variety. It's one of the reasons I elected to go with an insulated, unvented roof.
>>Furthermore, consider the widespread use of attics to run ductwork for AC systems.I should have thought of that; it;s a r regional thing: rare in my area.
>>Their research indicates that unvented, insulated attics stay cooler in Las Vegas than the vented "cold roof" variety. It's one of the reasons I elected to go with an insulated, unvented roof.
Again, possibly regional. I haven't done rigorous tests, but as a home inspector in NW Ohio I can tell you well vented attics are cooler than poorly vented.
I did some crude tests several years ago, measuring the temp in attics in the summer when I first opened the hatch; and then later after a couple of hours of being open. If I recall, after a couple of hours, the attics would be roughly 5 - 10 degrees cooler. (I do attics towards the end of an inspection, but I open the hatch at the beginning for that cool-down.)
And try traversing a decently ventilated attic on a summer day, and then coming back to feel the cooler air pouring up into the attic through the hatch. I'm not sure what that proves, but it is very noticeable.
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Sojourners: Christians for Justice and Peace
Edited 7/23/2005 7:31 pm ET by Bob Walker
Bob-
The attics that you are speaking of have the insulation at the ceiling plane and would of course get hotter when poorly ventilated.
The unvented attics that we are speaking of have the insulation applied to the bottom of the roof deck and are conditioned space. The temperature of my attic does not exceed 80 degrees F and I'm near Austin TX.
I have 3400 sq ft of living area and 800 sq ft of decked off conditioned attic and my total electric bill for July is 120$. My average bill is 100$. My attic conditions right now are 76 degrees F and 44% RH. Current outside conditions are 83 degrees F 69% RH and sunny. We reach 95-100 degrees F on most summer days. Our dewpoints are usually around 70 degrees F in the summer.
I do not believe it to be possible to achieve these results with a vented attic.
Thank you for your observations... let me ask you though: Have you ever inspected a fully-insulated and unvented attic before? That is, an attic where foam was applied to the rafter space or 4+ inches of XPS (or whatever) had been applied to the outside?For me, the question is simply one of preventing the heat gain or heat loss to occur in the first place. Keep the heat out, keep the cold out, and the whole argument for requiring ventilation goes out the window at the same time. When there are no wild temperature swings within the envelope of the house, there cannot be any dew, and no dew means no rot.I cannot think of a more punishing environment than the likes of Las Vegas in the summertime to measure the effects on temperature and humidity in the attic space. If the buildingscience graphs show a relatively flat temperature + humidity curve for an insulated, sealed roof system in an environment as brutal as that, I'm convinced that a home witht he same construction in MA will be happy as well. Meanwhile, the temperature/humidity graphs for the "regular" (and properly-done) cold roof construction in Las Vegas looked like a roller-coaster. I can attest to the same in my current rental apartment where the walk-up attic has huge temperature and humidity swings every day. Given that it's brutally hot during the day, and cold during the night, I assume that the venting is working...As for the rush of cold air you feel as you open up the attic access, I imagine it has a lot to do with the stack effect, where the temperatures inside the home are lower and the opening of the attic hatch initiates the same kind of air movement that a chimney creates during normal operation. If the home below was as hot as the (presumably vented) eaves, I imagine the flow of air would be reduced. Lastly, also consider how the attic hatch is a much larger opening than any eave vent I have ever seen. The larger hole (even when accounting for surface area), the lower the restriction on the movement of air. Large slots will always move more air than tight mesh, for example.
No, I haven't seen that sort of construction, it isn't done around here, although I do see some Capes where they have stuffed insulation in the slanted ceiling.My opinions on these sorts of issues are based on observation of a lot of homes in NW Ohio. A very different environment than LV.Las Vegas might be good for learning about the effects of heat, but I doubt if it's much for learning about humidity.The concern I would have with an unvented roof structure is the obvious one: what are the effects of the extra heat buildup on the roofing materials.In my area, the south facing roofs age more quickly than north facing, with maybe a 2-3 year difference in life expectancy over 20 years. I don't know how much of that is attributable to heat and how much to infra red and other components of direct sunlight.But I also know from experience that, in my area, a second layer of shingles is going to typically lose 1 to 3 years on both slopes of the roof, and that can only be attributable to the extra heat buildup , as far as I can see, and not from differences in light content.>>When there are no wild temperature swings within the envelope of the house, there cannot be any dew, and no dew means no rot.Crawl spaces are common in my area, and in much of it there is heavy clay soil and the land is flat.Like: ____________________________________________ flat.The biggest determinant for condensation and mildew and mold on the bottom of roof surfaces is the amount of moisture in thew crawl space.That factor outweighs all others, ventilation, moisture barriers in the house and attic temperatures by a large margin.4" of insulation is not going to stop heat transfer, just slow it down.Put a lot of moisture in an attic when during the day and it will condense on the bottom of the roof surface as the roof cools down.
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Sojourners: Christians for Justice and Peace
Mr. Walker,
I put together a small list of building science articles in a related thread that might be of interest to you. They can be found here. They address some of your concerns such as the migration of moisture within a home that has an unvented roof.
Cheers!
Thanks, looks like a terrific list!
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Sojourners: Christians for Justice and Peace
what are the effects of the extra heat buildup on the roofing materials.
Which is an interesting question.
I'm trying to remember what the long-hair insulating materials research was coming up with back in the Shuttle insulation days. This is apt, as they were studying the effects of direct heat on a surface with insulation bonded to it. I may have to snail mail some folk for titles of a paper or two.
The shingle mfgrs seem to all go "violates warrantee" when asked about insulating the roof deck. I'm trying to see what gain there is in ventilating a roof deck to get a 10º delta T when that's going from 140º to 130º. My "reflex" reaction is that bonding the roof deck to insulation has to be 'better' than 10º of cooling. That's a personal observation, entirely and under-documentedly empirical.
But, then again, I've noticed a draft or two through scuttles & attic stairs, but that may have more to do with 40-50º delta T to the conditioned space.
That's probably not a common occurance in NW Ohio <g>.Occupational hazard of my occupation not being around (sorry Bubba)
I have lived in a Condo the last 5 yrs in Idaho and dreaded summers as the night tend to get hotter thru midnight until I checked the attic. Previous owners were over zealous in adding insulation that covered sofit vents.
I put in an attice fan on a themostate near the ridge and WoW!!! It made all the difference in the world. $70 Lowes or Home Depot Good Luck