When insulating a wood roof structure, 475’s goal, as always, is to build in the most healthy, structurally sound and ecologically friendly way – thereby achieving truly high performance. This means we generally try to avoid using foam above or below the roof deck (see our 475 series Foam Fails).
With foam out of the equation, we examine how to avoid condensation, rot and mold in our structures. Ideally we vent our walls and roofs on the exterior (see 475 blog post, A high performance roof should be vented – how to do this properly). However, in many renovation projects this is not practical or financially feasible, for instance if a new roof was recently installed. For this reason we have established The Eight Golden Rules for Foam-Free Unvented Asphalt Shingled Roofs. By following these rules you are not only ensured a robust roof assembly, but you can also avoid the need for WUFI calculations.
NOTE: The building code currently asks for air-impermeable insulation below the roof deck in such situations. However, the intent of the code can be readily met with an assembly that includes fibrous insulation and an inboard air barrier and vapor control layer – and such foam-free projects are regularly approved for use. If you are in a cold climate and are considering one of our suggested assemblies, review it with your local building department official. We are happy to help you.
The Eight Golden Rules
1. Use roof shingles with a Solar Reflectivity Index (SRI) value below 40 (in Climate Zone 5 and higher)
The colder the climate and the higher the percentage of cloud cover – the higher the absorption value of a roof should be.
This sounds counter intuitive, as we’ve all been directed towards lighter roofs to avoid summer overheating. However, in colder climates the additional heat from the sun will allow any humidity within the wood structure/insulation to be driven inwards (the only way the assembly can dry). In Germany, >80% absorption is recommended (SRI of 20). In the US, lighter roofs can be acceptable in Zones 2-4 (east of the Rockies) because of higher solar radiation levels – but we recommend a WUFI calculation to determine how light.
2. Don’t shade the roof
This means no house in the middle of the woods under full tree cover. Consistent shading will prevent the sun from driving the moisture out of the assembly.
3. Check wood moisture content before insulation and installation of airtight vapor control layer
Verify and document the moisture content before installation of the interior air barrier and insulation. Solid wood should be 12-18 percent moisture content (M%) and OSB/Plywood between 9-15 M%.
4. Use an intelligent vapor retarder inboard
Class III vapor retarders (painted gypsum is 8 to 15 perms) are semi-permeable and will allow too much moisture into the assembly in winter – potentially leading to condensation on the sheathing.
Fixed interior vapor retarders (Class I and II) will prevent winter humidity ingress, but also reduce the inward drying capabilities in summer to close to zero. Class I retarders or not allowed per R806.5.2 on the warm side of unvented assemblies. This is because an interior vapor barrier will inhibit inward drying capacity, leading to humidity build-up and structural damage over time, as humidity from vapor ingress through small air/water leaks get stuck between two vapor retarders.
The simplest and most effective solution is airtightness and smart vapor control with INTELLO Plus. Located at the warm side of the roof, INTELLO Plus prevents wetting from the interior in relatively dry wintertime conditions – acting as a vapor barrier at 0.17 perms – while allowing drying to the interior over the relatively more humid summer season when it can open up to over 13 perms. By preventing wetting and promoting drying, INTELLO Plus provides maximum protection.
5. Do not install humid insulation
Do not install humid insulation or dampspray cellulose or damp fiberglass/mineral wool batts into the cavity, as this will introduce a large and unnecessary amount of moisture into the structure. If installing batt insulation during winter, you should install the smart vapor retarder immediately after the insulation to prevent moisture levels in the structure from rising.
6. Avoid cavities or air spaces in the insulation
Uninsulated cavities are colder than their surroundings – and reduce the R-value of the assembly. These empty cavities will create cold convective loops, and consequently moisture will gather and condense in the high points of these spaces, potentially leading to moisture damage.
7. Verify airtightness
Airtightness is the cornerstone of any high performance assembly. It is absolutely critical to blower door test the roof assembly along with the overall building. The roof should be airtight inboard (with INTELLO Plus membrane and TESCON tape) and outboard (with taped sheathing or roof underlayment).
8. If you break rules 1 or 2, contact 475
If you are in a forest, or install light shingles or solar panels – don’t immediately resort to foam. We can WUFI your roof and come up with foam free solutions in most cases. Contact us at firstname.lastname@example.org and indicate your roof assembly from outside to inside. At a minimum include location, preferred materials, roofing color and section drawings.