We’re big fans of Fine Homebuilding. In the most recent issue of Fine Homebuilding, Martin Holladay wrote a piece on Insulating Unvented Roof Assemblies. We agree that avoiding sheathing rot is critical, and that air permeable insulation such as fiberglass or cellulose cannot be used alone. But in it, unfortunately, all four options provided are dominated by foam. They did not consider fibrous insulation options installed with the addition of an interior air barrier, sans foam. Nor did Fine Homebuilding consider an alternative to foam boards above the sheathing – where wood fiberboard like GUTEX or mineral wool boards would be credible and preferred options. And the Options 3 and 4 provided are potential disasters to homeowners – where toxic spray foam is applied to the underside of an existing roof of a too often occupied building. Like we’re stuck in a dystopian 1960’s sci-fi movie, it’s still just foam, foam, foam.
There is a more sustainable and robust way. 475 believes that foam fails, and should be avoided. Less is Best is our motto. Therefore, in addition to simply replacing foam boards with GUTEX or Roxul, we have proposed some alternative ways to insulate an unvented roof that integrate the use of an interior air barrier, INTELLO PLUS, which is also a smart vapor retarder and airtight membrane – providing a previously unachievable degree of protection from moisture damage.
The membrane changes permeability from .17 perms in the winter to over 13 perms in the summer, depending on the relative humidity around it, allowing the insulation to dry out faster and more reliably.
One of the main reasons for the use of foam is its vapor retarding properties. Using INTELLO PLUS with thermal insulation provides the necessary air and vapor barrier needed to prevent structural damage.
As we note in our blog post Yes, Unvented Roof Assemblies Can Be Insulated With Fiberglass – A WUFI Post, the building code, written for the foam industry, does ask for air-impermeable insulation, or foam. 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 is regularly approved for use as such. 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.
Here are our own 4 options for building an unvented roof assembly, all including INTELLO PLUS and all foam free.
1. Fiberglass Batts
Fiberglass is an inexpensive insulative material, coming in at an average of 1/3 – 1/4 the price of spray foam. Fiberglass batts allow for easy installation and can be purchased at higher densities to meet your insulation needs. Fiberglass is composed of spun glass fibers, making it naturally flame retardant, and can be made from recycled materials. The main drawback of batt insulation is the potential for gaps in the insulative layer if it is not correctly cut to fill the entire cavity.
2. Mineral Wool
Like fiberglass, mineral wool can be installed in batts. Since it is made from non-directional stone fibers, the insulation is water repellent, microbial growth resistant, fire resistant, and denser than other insulative materials. It also provides superior sound control. Mineral wool has an R-value comparable to spray cellulose or high density fiberglass batts.
3. Dense-packed Cellulose
Cellulose is a good choice for the especially environmentally conscious, as it is made from recycled materials. The added fire retardant is borate, the same thing you find the your all-natural cleaners and laundry detergents. It is non-toxic, affordable and provides excellent thermal control. Intello Plus is especially well suited for dense pack cellulose, as it is reinforced and can withstand the pressure from the insulating material.
4. Blown-in Fiberglass
Blown fiberglass may be chosen over batt fiberglass as it provides more complete coverage. Where batts have the potential to leave spaces when installed, blown insulation comes in smaller pieces and fills those potential gaps.
All of these installations perform best with an airtight, vapor-open weather resistant barrier. SOLITEX MENTO 1000 is a popular choice, as it is weather resistant, vapor open and allows the insulating layer to dry out. For extra reinforcement suitable for dense-pack insulation, choose SOLITEX MENTO PLUS. You’ll find examples of all of the above in real world projects on our video index. These projects are excellent examples of how to achieve foam free and high performance on a budget you can afford.