SIP Ventilation Part III: The Roof
Welcome to Part 3 of my series regarding appropriate venting applications when building with Structural Insulated Panels (SIPs). You may recall that in the previous blog we dealt with rain screen requirements for walls built with Insulspan SIPs; and previous to that, in Part 1 of the series, we discussed HVAC requirements for buildings built with SIPs. Review the Rain Screen Blog or the HVAC Requirements Blog.
Now to the roof
The type of ventilation discussed in this blog is not to be confused with roof ventilation referred to in most building codes. The International Residential Code (IRC) 2006 and the National Building Code of Canada 2005 each have a similar provision stating that ‘except where it can be shown to be unnecessary’, an enclosed attic which is defined ‘as the space formed by application of finish material to the underside of roof rafters’, must have cross ventilation. The Insulspan SIP system is a closed cavity building component that does not include ‘rafters’ as defined above, and since the EPS insulation is in direct contact with the underside of the OSB top skin of the SIP, there is no opportunity for a condensation plane to develop within the core of the panel. PIB_207_-_Building_Code_Roof_Ventilation_Requirements
To summarize, the cross-ventilation requirement referenced in the code does not apply to a SIP roof.
So, the subject of this blog is ventilation of roof cladding systems above the roof deck or top skin of the SIP. There are some pretty strong arguments for the practice of ‘over-cladding’ a SIP roof.
Effect on Shingle Life
The durability of asphalt shingles correlates directly with temperature and ultra-violet radiation. The greater the temperature and UV exposure, the shorter the life of the shingle. Generally speaking, shingles installed over unvented ‘hot roof’ assemblies like SIPs operate at a slightly higher temperature and therefore some reduction in shingle life may be expected. Having said that, the choice of shingle colour, roof orientation and geographic location will likely have a more profound effect on shingle life than the effect of a slightly higher shingle temperature.
Telegraphing at Panel Joints
The moisture content in the top OSB layer of a SIP roof assembly changes regularly with the seasons and can result in the panel joints ‘telegraphing’ through asphalt shingles (other roofing materials such as wood shingles, shakes and metal roofing are not affected). This can also happen with stick-frame or rafter type roofs (See APA publication K310N – “How to Minimize Buckling of Asphalt Composition Shingles).” One of the key concerns to avoid shingle buckling or ridging than can also lead to this issue is to ensure that the underlayment and roofing are applied over a dry roof deck.
Although the ‘telegraphing’ at panel joints in no way has any structural effect on the roof assembly, there is the small matter of aesthetics. This problem can be avoided by ‘over-cladding’ – adding another layer of sheathing on furring strips to the top of the panels. This could be considered the ‘cold roof’ over ‘hot roof’ approach.
Ice damming occurs when the temperature of the roof surface is above the freezing temperature when the outside air is below freezing. In stick frame roof applications, ice damming occurs due to air leakage from the interior of the house. A SIP roof is inherently more air tight than a stick frame roof so ice damming due to air leakage is less likely.
However, snow has some thermal resistance value and in areas where heavy snow fall is normal, the depth of snow can add additional thermal resistance above the roof deck. The added thermal resistance above the roof shifts the thermal gradient in the roof assembly, which may lead to ice damming if heavy snowfall accumulates on the roof.
Thus in extreme areas where heavy snow accumulation on the roof is possible, adding a vented airspace between the SIP and roofing material allows trapped heat due to the insulating properties of the snow to be flushed away. This reduces the potential for ice-damming when the temperature of the roof cladding is above freezing and when the temperature outside is below freezing with snow on the roof.
So in a nutshell, and to sum it all up –
Asphalt shingles can be applied directly to a SIP roof assembly with a secondary line of defense, such as a permeable underlayment. However, adding a vented space above the roof deck can provide some advantages in certain applications as noted above.
Wood shakes and shingles however, should always be installed over furring strip assemblies or drainage mats that can provide a vented drainage plane.
Metal roofs also work best when installed over drainage mats or furring strips with an additional layer of OSB, but it is not always necessary, and depends largely on the climate. (Reference – Builder’s Guide to Structural Insulated Panels (SIPs) for All Climates by Joseph Lstiburek).
And of course, the builder should always be aware of the specific requirements of the local building code and apply roof cladding accordingly.
This ends our discussing on venting requirements for SIP buildings. I hope this series has been informative and provides architects and contractors some guidance that results in more effective methods for siding and roofing applications.
Stay tuned for my next blog which will delve into the matter of electrical and plumbing applications in a solid SIP shell.