A couple of weeks ago, after talking with my insulator, I discovered we had a serious mis-communication forcing us to radically switch the design of the roof system. The original plan was to have an un-vented roof system with 32″ of dense pack cellulose (4 pounds per cubic foot) filling the parallel chord trusses. My understanding is that there was nothing intrinsically wrong with this approach. The cellulose would stop any air movement that might carry moisture to the colder exterior part of the roof system. However, I have since learned that it is virtually impossible to dense pack such a huge thickness. Furthermore, the added weight on the trusses was not factored into their design.
The new plan is to vent the roof by:
- Creating an airspace underneath the sheathing by installing Typar on the underside of the top chord of the truss.
- Installing one 4″ circular soffit vent per bay
- Cutting holes in the wall sheathing where it meets the roof sheathing, install screen to prevent pest entry
- Cutting holes in the lookouts where they pass through the interior bay so those bays can vent all the way up
- Removing the ridge cap, cutting a slot for the air to escape through the ridge, replace the ridge cap with Coravent (a thick bug-screen material)
- Switch from having the sheathing be the air barrier to having the interior ceiling plane be the air barrier. This will mean using a combination of sealing the drywall to the framing, and using Tu-Tuf cross laced polyethylene behind the wood ceilings.
Although we will lose some 30 R-value points, the R scale is not a linear one and the decrease has less impact than one might think. As one goes up on the scale each successive R-value insulates less than the previous one. After R-80 there is very little payback. So, from an insulation point of view, we were going to pay for about $800 worth of cellulose that wouldn’t benefit us except to make dense pack system work.
The new roof system will have 24″ of cellulose blown in at 3 pounds per cubic foot for what will be around R-85. Since this lighter pack of cellulose can’t be relied upon to block air flow through it we will need to install a an air and vapor barrier underneath the cellulose.
Adam and I have been dreading doing the ridge cap work for obvious reasons. Over the past two days we got it done–in large part prompted by the arrival of our solar hot water panels. Adam, as he put it, doesn’t remark about the difficulty of work; not so this time. Thank you Adam!