Vaulted Web truss Ceiling with dense pack cellulose ventilated a good idea?
Hi,
I am building an ICF walled home in Saskatchewan. It is ICF right up to the gables. I have 24 inch deep web trusses on 2 foot centres. 2/3 of roof is 8:12 and 1/3 of roof over a dormer is 3:12. I am planning on using metal standing seam roofing , over plywood.
It seems that we require ventilation here for code. So I am planing on putting Tyvek house wrap over the trusses and then using 1×4 scabbed along the edge of trusses to provide a ventilation channel.
The idea is that the house wrap will do a few things it will
1)contain the dense pack cellulose on the top (cellulose will be installed from inside),
2) it will provide the ventilation channel
3) it will prevent wind washing and even help as an exterior air barrier.
4) it will provide a secondary barrier against liquid moisture (and until metal is on roof)
Underneath the trusses I will be using mesh (to contain the cellulose during install) then a poly vapour barrier ( Again I think this is required by code) then 2×4 strapping across trusses, then finally drywall.
There will be ventilation from under the eaves into the channels formed by the house-wrap over the trusses
Does this seem durable enough to you? From what I understand there is a starch that will harden in the cellulose so there shouldn’t be too much pressure on the house wrap after that hardens.
Is there a less finnicky way to achieve the same thing?
BTW
I had hoped to try and use take some of the vented air off at the top of the roof for use as pre-warmed air for the HRV or heating garage etc, but that seems like it will be a little too complicated.
I prefer the idea of dense-pack cellulose to fiberglass or spray-foam.
Thanks in advance
David
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Replies
David,
Densepack cellulose requires a rigid vent baffle above it. The Tyvek will simply be pushed up against the plywood sheathing and close the vent cavity.
What you'll need to do is attach 1x2s to the sides of the upper truss chords and attach a rigid material like thin 1/4" plywood or MDF to create the vent channels. Then the rest of your system will work.
The starch in cellulose is activated only during damp spray applications (and some manufacturers add additional water-activated glue), and has no effect in dry applications. Dry cellulose has to be installed at approximately double the settled density of 1.5 pcf so it always exerts an expansive pressure.
David,
Your plan sounds difficult to install. It will be hard to stretch the Tyvek tight on top of your roof trusses before the roof sheathing is installed.
Although it will cost more, I suggest you install a ventilation channel on top of your roof sheathing, between the sheathing and the metal roofing. Use two layers of 2x4s -- one layer parallel to the rakes, and a layer on top parallel to the eaves (to allow fastening of the metal roofing).
If you are going to install strapping over the bottom of your trusses, you might as well install a layer of foil-faced polyiso there first, instead of polyethylene. That will improve the R-value of your roof system.
With 24" deep roof trusses, there's hardly a need to improve the R-value, though that's certainly an option ($$$).
And most standing seam metal roofs require a solid roof deck to prevent deflection. A 24g roof will be stiffer than the more common 26g, but I still wouldn't install it over purlins.
Robert and Martin,
Amazingly quick answers!
I was planning on using the tyvek with 1x4 strapping along side of upper truss chord -expecting a bulge of a couple inches but hoping that would still leave adequate ventilation space.
I was planning on using the strapping to tighten the tyvek as I went along from 1 truss to the next.
I thought that whatever was between the cellulose and the ventilation space had to be vapour permeable in order to make the ventilation useful as a way of allowing drying to the outside. Am I wrong on that?
A compromise I had thought of was of instead of using 1/4 inch plywood was to use 1 by 4 or recycled pallet pieces across between the trusses every 2 or 3 feet - giving the tyvek a little more support - but that is even more labour intensive!
My understanding was that the standing seam roofs required a solid decking. Perhaps another way of doing it would be to just put 1/4 inch plywood over the trusses, then 2x4s parallel and directly over the truss chords then the 1/2 inch plywood for the standing seam? Is the 1/4 inch plywood then going to be vapour permeable enough to make this considered a ventilated roof?
And more importantly - will it perform well - dry and durable?
Re the polyiso, that sounds like a good idea, what kind of tape do you use to seal it, I wonder how long the red tuck tape will last. The polyiso might be a little more durable than poly too.
Thanks again.
David
David,
1. You can use any type of plywood or OSB for roof sheathing. If it is ventilated on the top side, it will dry out if it ever gets wet. I think you should use a minimum 1/2-inch plywood, not 1/4 inch -- if only for the safety of the people installing the roof sheathing.
2. Housewrap tape is a durable tape for foil-faced polyiso. Other builders use foil-faced tape. To learn more about taping foam, see Air-Sealing Tapes and Gaskets.
David,
It would be easier to use the double roof deck approach (which I've recommended elsewhere). In either case, you'll need vertical insulation dams at the outer wall line to prevent spillage into the soffits.
Tyvek is not a good barrier for cellulose, since it's far too vapor permeable and will allow diffusion in both directions, and because it can become a condensation plane which prevents liquid water from escaping.
CDX is an excellent material for this, however, since it's far more rigid (you might want to use 3/8" for the secondary roof deck/vent baffle), it is modestly permeable when dry (>1 perm), increases in permeance as it gets damp (like MemBrain), and can absorb condensation and redistribute it along the wood grains. You could cover it with #15 felt if you want a secondary drainage plane, since it has a perm of 5.
Even a ½" layer of foil-faced polyiso would serve as both air/vapor barrier and radiant barrier if strapped before drywall, and seams should be taped with aluminum foil tape (which is incredibly sticky and durable when applied to foil).
I have to disagree with Martin on all counts.
APA 3/8" CDX sheathing is rated for 24" oc roof spans.
OSB will not perform like CDX. It has more wax and resin, it has a lower dry perm, it does not increase its perm very much as it gets saturated, and it doesn't redistribute moisture laterally the way the continuous plies of plywood do.
And housewrap tapes are not nearly as sticky, reliable or durable as foil tapes.
Another question for you guys.
From a building inspectors point of view, will a double deck approach with dense-pack cellulose underneath be considered ventilated?
Is the ventilation primarily to deal with overheating the roof and condensation from the roof from above, or; is it to get rid of moisture from in the insulation?
The system I would be using would be:
trusses- 3/8 plywood - 2x strapping directly on and parallel to trusses - 7/16 OSB roof decking - waterproofing layer - standing seam roof.
Robert, where can I find a description of your double roof system?
I will be having a vapour barrier on the ceiling below the insulation, so there shouldn't be any moisture in the insulation, but just thinking what the building inspector might be looking for. Will the cdx be permeable enought to keep inspectors happy? (in theory?).
CDX does not seem to be available here - most people here have never heard of it.- only seem to sell exterior grade plywood).
As a compromise for cost reasons I will likely use 3/8 exterior plywood on the bottom and OSB for the roof decking layer just below the standing seam.
Although I don't like OSB it seems like that would be the best place to use it if I need to.
Thanks again.
David
David,
Q. "From a building inspector's point of view, will a double deck approach with dense-pack cellulose underneath be considered ventilated?"
A. That's a question for your local building inspector. I'm not sure what the Saskatchewan codes require.
Q. "Is the ventilation primarily to deal with overheating the roof and condensation from the roof from above, or; is it to get rid of moisture from in the insulation?"
A. Your question would make Dr. William Rose chuckle, I suspect, since the history of code requirements for venting roofs is full of misconceptions, red herrings, and junk science. Suffice it to say that the primary reason that we ventilate roofs is to satisfy the code inspector.
Q. "CDX does not seem to be available here - most people here have never heard of it.- only seem to sell exterior grade plywood."
A. CDX is exterior plywood.
You just described it. (It's not a system I've ever used myself because I use flat ceilings and vent the attic).
Yes, it should satisfy all code requirements, but you'll have to clear it with your local inspector.
Pure rubbish. I have never, nor would I ever, build a cold-climate house without a vented roof, and almost every building science expert agrees that vented roofs are preferable to unvented roofs.
Bill Rose, ASHRAE, Illinois Building Research Council:
Airtight ceilings are a more reliable way to ensure a dry attic than venting, but in practice most houses fall into a middle ground where venting balances moisture input.
Don Fugler, research director, Canada Mortgage and Housing Corporation
Houses with ceilings tight enough to meet Canada’s strict R-2000 standard “probably could get by without roof venting.” But most Canadian houses, even new ones, don’t have such perfect ceilings.
Ned Nisson, author, The Superinsulated Home Book, editor, Energy Design Update:
"I hesitate recommending [unvented roofs] to clients unless I’m absolutely assured of impeccable quality control. In my opinion, roof ventilation is cheap insurance against expensive callback problems. Why gamble?"
Wayne Tobiasson, research engineer U.S. Army Corps of Engineers, Cold Regions Research Center:
“Because of the monumental problem of ice damming, there is no question in my mind that the ventilated roof is an order of magnitude better in cold regions.”
Anton TenWolde & William B. Rose, members, ASHRAE:
We recommend venting of attics in cold and mixed climates. However, if there are strong reasons why effective attic vents are undesirable, unvented attics can perform well in cold and mixed climates if measures are taken to control indoor humidity, to minimize heat sources in the attic, and to minimize air leakage into the attic from below, or vice versa. The necessity and effectiveness of vents in cathedral ceilings in cold and mixed climates is still a contested issue. Unvented cathedral ceilings can perform satisfactorily in cold and mixed climates if the cavity is properly insulated, measures are taken to control indoor humidity and minimize air leakage into the roof cavity, and a vapor retarder is installed in the ceiling.
Paul Fisette, director of Building Materials Technology and Management at the University of Massachusetts, Amherst:
"There are many ways to treat the symptoms [of ice damming], but proper air sealing, insulation, and attic venting are the best ways to eliminate the problem."
Joe Lstiburek, Building Science Corporation:
"Vented attic/roof designs have the advantage of a long, proven historical track-record. However, they work best with airtight ceiling/attic interfaces and where ductwork and air handlers are not located within attic spaces. The increase in the use of complex roof shapes and cathedral ceilings has resulted in problems with vented roofs."
"In extreme snow regions it is necessary to add a vented air space between the roof cladding (shingles) and the rigid insulation to avoid ice damming. The vented air space is needed to flush heat away trapped by the insulating value of relatively thick snow."
Journal of ASTM International, Volume 6, Issue 4 (April 2009)
Peter E. Nelson P.E., Senior Principal,Simpson Gumpertz and Heger, Inc., Jason S. Der Ananian P.E., Senior Staff I-Building Technology,Simpson Gumpertz and Heger, Inc.:
All of the unvented roof assemblies are intolerant of incidental water leakage and the moisture-sensitive layers. The least tolerant roof assemblies in either climate are the unvented closed-cell polyurethane roof assembly due to trapped moisture and slow drying of the roof sheathing (up to 12 months in Miami, FL and 27 months in Boston, MA).
Robert,
I'm well aware of the many reasons that builders include ventilation channels. I'm sure you're aware of that, because I discuss the need for ventilation in my recent blog on conditioned attics, which you posted a comment on.
The history of these requirements is complicated, as my comment noted. I maintain that the PRIMARY reason that most builders include these ventilation channels is to satisfy code requirements. It's not the only reason.
Martin,
There are not just "many reasons that builders include ventilation channels" - there are many legitimate and important reasons, and those reasons are supported by current building science as well as the natural intuitive wisdom of generations of builders.
You can't sidestep your utter and complete dismissal of the validity of roof ventilation (an attitude which you've repeated regularly on this forum), for this is what you posted and what I called "pure rubbish":
Your statement is hardly a balanced and objective interpretation, is completely negative and dismissive, implying that everything about the history of roof venting is superstition, and that the primary reason it's still used is for code compliance rather than to actually improve the performance of our buildings.
You have consistently shown a strong bias against roof venting, claiming the support of building science when - in fact - almost all prominent building scientists lean towards venting. The only reason that the cold-climate building industry has moved away from vented roofs is to accommodate spray foams and easier cathedral ceilings, neither of which are particularly green. And, ironically, you insist on roof venting with dense-pack cellulose (where it's least needed) and ignore it with spray foam (where it's most needed).
Aside from the use of spray foams and exterior rigid foams, this is the one area where we most strongly disagree, but I think GBA readers have a right to expect a more objective treatment of roof venting than what you've offered here.
Robert,
Don't over-interpret my words.
The vast majority of builders have no idea why they include ventilation in their roof systems, other than that they know that the code requires ventilation.
As William Rose has exhaustively proven, the original code requirements for ventilation were established without any foundation in building science.
That doesn't mean that ventilation doesn't make sense. I never implied that it didn't.
But I was responding to the question on "the primary reason" for roof ventilation. And trust me, that is a question that most building scientists can't answer.
Robert,
More on William Rose's research:
http://docserver.nrca.net/pdfs/technical/7877.pdf
Actually, he "proved" nothing of the kind. He offered what he believed to be an exhaustive compilation of the early research and the possible links from that research to the first code standards for attic ventilation. And he concluded with an opinion, that the strict 1:300 ventilation standard is not necessary, and even suggested that it may be inappropriate for building codes to regulate moisture control !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Early History of Attic Ventilation
William B. Rose
Introduction
The aim of this paper is to describe and assess the early research and professional literature that underpins current attic ventilation practices. The period covered in this paper is 1930-52. In addition, the aim of the paper will be to determine when the argument that attic ventilation enhances shingle service life first appeared. It is not the aim of this paper to review actual attic ventilation practices nor is it the aim to provide professional guidance regarding attic ventilation.
Conclusion
The principal early sources of attic ventilation requirements have been presented. The background presented here is, in the opinion of the author, a complete and exhaustive list of the significant sources of the current regulations and practice.
Professionals in the building industry – design, codes and construction – may view the support for the current regulations, described in this paper, as being strong or weak. In the opinion of the author the support is weak, and a strict interpretation of 1/300 compliance is not appropriate. Indeed, the building industry may wish to question whether ensuring moisture control is an appropriate duty and responsibility of the building codes, and, if it is, whether prescriptive venting regulations are the best way to provide it.
Robert,
In order for scientists to determine the best recommendations for construction practices, it's helpful to understand the origins of common rules of thumb and code regulations. Rose's research has been invaluable in establishing the facts undermining many of these rules of thumbs and code regulations.
Once this history is well understood, it's also necessary to conduct ongoing research and building monitoring to understand how building perform. Rose has also conducted significant research in this field, as have many other building scientists.
I'm sure that Dr. Rose would be among the first to admit that there is a lot we still don't know. After I reported Armin Rudd's surprising finding that inward solar vapor drive can push morning dew through asphalt shingles, asphalt felt, and plywood roof sheathing into insulated attic bays, I did a follow-up interview a couple of years later to ask about current data on his findings.
Rudd explained that new data gathered since we had last spoken had surprised him, and the explanation for the so-called "ping-pong moisture" phenomenon was more murky than ever before. He told me that he was frankly unsure whether the strange moisture spikes he observed were really a result of inward solar vapor drive or some other phenomenon.
He noted, "What we need is more money for research to learn more about the phenomenon. But no one has come forward to offer financial support for the research."
There's more to learn every day.
That assumes that scientists are the most appropriate people to recommend construction practices. Even Rose's introduction to his historical overview made it clear that "It is not the aim of this paper to review actual attic ventilation practices nor is it the aim to provide professional guidance regarding attic ventilation."
And Dr. Joe concludes his "Top Ten Rules of Wood Durability" with "Don't let biologists and wood scientists design real buildings."
It must have been a complete accident that builders have been erecting structures for tens of thousands of years (some of which have lasted that long) without the help of scientists.
Robert,
I never claimed that Dr. Rose was making construction recommendations. Among all the building scientists I know, he has always been the most reluctant to do so.
He is humble about what he does not know, which is an excellent attitude for a scientist.
Others, including Joesph Lstiburek, have been eager to step up to the plate with best-practice recommendations. Dr. Lstiburek thinks like an engineer, because that's his background and training; Dr. Rose thinks like a researcher.
Both of them have conducted original research, and both are scientists. I value their different approaches.