Item for thought and discussion:
A typical “cold roof” over structural insulated panels, from bottom up, generally is panel, furring strips, 1/2″ or 5/8″ CDX, felt paper or maybe Tri-Flex or RooftopGuard-type product, and shingles or other roofing material.
All we really need is for the roof to breathe. Could we put something like Home Slicker or a cedar breather on the roof in place of the strapping and sheathing, and save a few thousand $$ in plywood, nails and strapping? We still would need to provide a ridge vent and soffit or eave vent, but that seems to be a minor issue. It looks like the cost in materials, at current rates, would be about half of plywood and strapping, and labor considerably cheaper (i.e. no humping the plywood up to the roof.) It doesn’t seem like there would be an issue with “compression”, unless one is worried about blowing roofing nails right through the shingles? Hand nailing, anyone?
Or is there another reason why we would need a large airspace between the panel and second layer of sheathing, with an apparent thermal break every 16″ to 24″?
I’m seriously considering trying this, it would be applied over Grace Tri-Flex 30 which is already on the panels.
Discuss. 🙂
Replies
Why do we need the roof to breathe?
The roof will be unvented otherwise.
Why does it need venting? SIP panel are airtight through the thickness. It's a system that is easily sealed at its joints. So inner vapour barrier, no air leakage at joints......no reason to vent.
Shingle life: There are research cases where unvented large attics have lower air and sheathing temperatures than vented cathedral ceilings. At the top of the air channels in the vented cathedral ceiling, the sheathing temperatures varied little from unvented cavities.
See: http://www.cmhc.ca/en/burema/gesein/abhose/abhose_ce13.cfm
In this documment you find statements such as:
(1) What To Do About a Wet Attic
The usual response is to increase attic ventilation. This is the wrong approach. In some cases, adding ventilation will actually pull more moist house air up into the attic and make the problem worse.
(2) Attic ventilation is overrated. In winter, the cold outside air cannot hold much humidity or carry moisture away from the attic. In summer, attic temperatures are more affected by the sun and shingle colour than by the amount of ventilation.
(3) A well-sealed roof will not need ventilation.
(5) A well-sealed and insulated attic will generally not have ice dams.
(6) Ice dams caused by cathedral ceilings are more difficult. The same principles apply to preventing ice dams — stopping house air leaks, good insulation, perhaps ventilation
I am assuming when you say "airtight" you are rolling the vapour barrier and "air" barrier into one .. I know its hard to stop thinking this way, but we now know better. As I understand it nothing is impermiable to water vapour, it will even diffuse through 6 ml poly, which is why we have to give it a way to escape from the walls and roofs ..
In a house with average winter humidities of say 30-50%, with a decent vapour retarder(6 mil poly), the amount of moisture by diffusion will be so low as to be neglible That's why about 98-99% of effort should be put into the air barrier since this is whats been causing the moisture in the attics, not the lack of a vapour barrier. In the past, in dry buildings (30-40% RH) without a vapour barrier, 99% of moisture movement was from air leakage.
In 1976, Building Digest #175 from Canada's National Research Council titled "Vapour Barriers: What are they? Are they effective?" states:
"Air leakage is now considered to be the prime cause of most condensation problems in walls and roof spaces. If, therefore, a building can be made tight against air leakage it may not need a vapour barrier, as defined. On the other hand, if there are openings that permit air to leak from the warm side to the cold side of the insulation, adding a vapour barrier (even of zero permeance) that does not seal off the openings will be useless."
(My comment: A zero permeance barrier would be tin foil.)
Article Summary
A particular wall subjected to particular conditions has been analysed in order to assess its ability to control the accumulation of harmful quantities of water resulting from vapour diffusion and migrating air currents. In the first instance it was deliberately made weak in its ability to resist vapour diffusion in that no vapour barrier was incorporated in it. Even so, it could be shown that vapour diffusion was unlikely to create a problem. The accumulation by this mechanism of any water within the wall could easily be stopped by the simple expedient of using several coats of a more impermeable paint. Such a vapour barrier would not have to be continuous since diffusion through small cracks would be negligible. Air leakage, on the other hand, would be a more serious problem and considerably more difficult to control.
Many walls are relatively porous and allow air to pass directly through them; others are initially air-tight but develop cracks as a result of shrinkages and deflections. Field investigations indicate that holes big enough to permit a hand to be passed through them have sometimes been left inadvertently in walls, either through faulty design or poor construction. Designers and builders must make every effort at all stages to avoid such errors and to anticipate and allow for wall movements by providing suitably caulked or gasketed joints. The control of air movement is probably the single most important factor in obtaining a problem-free building envelope."
your playing with semantics, and so is the author of this digest (btw .. have you read the digest about the automated test houses they set up to test all this tight house, air vs vapour barrier stuff .. seems one of the relays to turn on a shower failed to close, shower ran for an extended period of time, .. it was enough to temporarly heat the house .. I diverge) of course "air leakage" is a greater problem, since air leakage provides both air and vapour movement. What you should also know is that even at NRC, when the R2000 program, tight house movement began, the staff divided into 2 camps, leaky vs tight, and the only reason tight got all the press was because it was new and offered to save people big bucks .. but that was before people started getting sick, mold, and all other kind of issues arose to compensate for tight houses (air handlers, etc) ..
I happen to know the R2000 system very well. From 1984-92, I was an inspector/site advisor as well as "troubleshooter" for a province. I took separate research contracts to study the homes after they were lived in for a while and.......two of my contractor clients were named Canada's R2000 Builder of the Year for 1989 and 1993.
Early R2000 homes did not have moisture/mould and other problems but houses built "like R2000 but without HRV's" outside the tight design/inspection control of did and they smeared the R2000 work horribly.
The main problems with early R2000 homes were:
(1) the early continuous ventilation rates were way too high. Having no previous whole house ventilation to learn from,they erred on the side of safety. I had and still get calls about houses being too dry. Some early program homes were vented as high as 50 cfm per person 24 hours a day. 15 cfm per person is more than enough in a clean house. The ASHRAE Air Quality Standard 62-89 recommends ventilation for "Smoking Lounges and Bars" of 60 cfm per person only while occupied!!!!! Problem houses I visted had children with nosebleeds, shrinkage in hardwood floors and lots of shocks!! The program has slowly reduced the rates (3 times) over the years and there will probably be one more rate reduction or some variation thereof such as required programmable timers/preset timed periods of exchange or exchange/re-circulation/rest state options at the main remote controller.
Also overventilation works against saving energy as you still have to re-heat a portion of the extra incoming cold air.
(2) Another problem was poor location of supply vents which delivered air at below room temperatures directly on to locations where people may sit. This was a whole new field for housing- trying to bring air as cold as -32 to -38 degC into our homes. SOme of the installers had come from the HVAC heating field and had no feel for the systems.
(3) every HRV invented in Canada was done in some small guy's basement or garage shop. The big guys- York, Carrier, Lennox, Broan, Trane- were not interested in maybe 10-100 units a year. So the early equipment was a bit noisy or poor quality parts were used or there were no filters or exaggerated claims were made, etc. I started installing in 1981 and had to go through some of the growing pains such as defrost damper motors failing within 2-3 months of installation, main fan motors within 1 year, defrost not working and the cores turning into blocks of ice. This even happened when one of the bigger ventilation companies got into HRV's- I had to replace a fan module 3 times in 3 years in the mid 90's!
(4) Complaints of minor condensation on bottoms of window sashes. in colder weather. We did not have Low E + argon windows + warm edge sealed glass spacers available. So even with the house being essentially overdried by the oversized HRV, a bit of condensation would occur at glass edges due to the sealed pane cold edge sandwich of glass/metal-spacer-glass and the window sash geometry. I had one client call with a complaint that the HRV wasn't working....solved that one over the phone. Asked him to check where the dehumidistat control was "clicking" as he adjusted it up and down......31%, actually quite dry. So I described the window design problem to him and all was OK.
R2000 had there own minor problems but they weren't mould and poor air quality.
Wane:
These engineers and Phd building scientists were not playing with semantics.....you can't measure and see semantics like you can condensation, air leakage and frost. There is a series of approximately 250 digests avilable on the web; read a few and see what was going on as far back as 1960.... and before. In 1953, (hell I was only 3) there was a talk by the head of the NRC Div. of Building Research to the national engineers association right here in Halifax about "uncontrolled air leakage and the respect it deserves due toheating/cooling/moisture/comfort concerns"
experienced, I know, I've read the digests, I know many of the authors, I can assure you at least one of them is having a chuckle about this right now ..
Quote:
"the only reason tight got all the press was because it was new and offered to save people big bucks"
When and if there were two camps at DBR-NRC is not the issue! "Tight got the nod, not because it was new but because it was was right. It stopped
(1) the attic condensation/ dripping problems,
(2) mould growth on the roof sheathing with some leading to rot
(3) severe frost/ice build up and then the attic rain with water dripping down through light fixtures, etc (I have seen 2 houses with 15-20 pots, pans, buckets catching water)
(4) It provided more comfortable draft free rooms
(5) and of course , it saved money on top of stopping other aformentioned problems!!!