Welcome to Part 2 of our discussion regarding appropriate rain screen and venting applications for SIP walls. You may recall that in the previous blog we dealt with HVAC requirements for buildings built with Insulspan SIPs. [SIP Ventilation Part 1]
Venting (rain screen) requirements for SIP wall designs
The short answer to the question whether exterior wall cladding design requires venting is ‘No’. SIP walls don’t always have to be vented, but because all exterior claddings pass some rain water, especially wind driven rain, SIP wall design should always include some form of a secondary drainage plane. A ‘drainage plane’ is defined as any water repellent material, such as building paper or house-wrap, that is located behind the cladding and designed to drain water that gets past the cladding downwards and outwards.
Whether ventilation in the form of an air space is required between the sheathing and the cladding depends partially on the type of siding being applied, the climate where you are building, and also on what the local building code has to say about the matter.
I can’t comment on every local building code, except to say that the builder should be aware of the specific requirements and apply the cladding accordingly.
As for the type of cladding being applied, only vinyl and aluminum siding should be applied over a sheathing membrane applied to the exterior OSB facer of a SIP wall. This is because they are inherently ‘back ventilated’ due to their profile.
Wood siding and cement fiber board siding should be furred out using spacer strips that are applied over the drainage plane and can be as thin as ¼”, but again this will depend upon the local code requirements and cladding suppliers’ requirements.
For cedar shingles, stucco and stone veneers – drainage mats made of polypropylene mesh resembling large rolls of plastic Brillo pad® work very well. The cedar shingles can be applied directly over the drainage mats, but with the stucco and stone finishes, a layer of ‘tar paper’ should be installed over the mat to keep it from becoming clogged. (Reference Builder’s Guide to Structural Insulated Panels by Joseph Lstiburek).
In summary; although not all cladding requires venting in the form of an air space, all exterior claddings do pass some rain water. So especially in areas where rain is common, it is probably prudent to couple the secondary drainage plane with a vented air space behind the cladding, regardless of what material is used.
Join the conversation again next time for the final segment on SIP ventilation – Part 3 Venting Requirements for SIP Roof Design.