Replies

1. GBA Editor
   Martin Holladay | Nov 14, 2012 02:36pm | #1

   Neil,
   If you install an airtight layer of roof sheathing (for example, taped plywood), you can install all of your roof insulation on the outside of your house. If you go this route, install several layers of rigid foam with staggered seams, followed by strapping and roofing (or strapping, more sheathing, and roofing).

   Another approach is to install a structural insulated panel (SIP) roof.

   If you choose instead to install insulation between your rafters, most people would probably advise you to choose cellulose and to avoid spray polyurethane foam.

   For more information, see How to Build an Insulated Cathedral Ceiling.

2. Expert Member
   Dana Dorsett | Nov 14, 2012 02:55pm | #2

   Rigid polyisocyanurate foam placed above a sealed roof deck would have very low risk. It's blown with pentane which outgases quickly with very low lingering VOCs, plus above the roof deck you'd be putting it outside the pressure boundary of the house rather than inside.

   You'd still need to ventilate between the roof deck and fiber, but semi-rigid rock wool batts that run ~R4/inch or higher have very low VOCs and none of the friable particulate/aerosol issues of fiberglass. Most rock wool products use the same phenolic formaldehyde binders used in fiberglass as part of the process, but unlike fiberglass batts those chemicals get cooked off in production to extremely low levels by the time it's a finished batt. (Roxul claims something like 13 parts per BILLION formaldehyde in the entrained air of a rock wool batt.)

   Most non-expanding injection foams (eg: Corefill 500, TriPolymer, etc.) are zero VOC (blown with air) and chemically inert , and run ~R5/inch. But it's more expensive per unit R than polyiso, which is already more expensive than rock wool.

   Also for a lot of money, foamed cement (eg AirKrete, etc) is pretty safe, but I'm not sure it's really the right thing for cathedral ceiling, and it would take a lot of inches of depth to meet code.

   Knowing the class of chemicals to which you're sensitive might steer you to one vs. the other based on the MSDS sheet info.

3. Expert Member
   Dana Dorsett | Nov 14, 2012 03:00pm | #3

   Martin- some people may have issues with newsprint-ink and fire retardents found in cellulose, which have substantial amounts of borates, &/or sulfates. (Borate-only versions are probably less of an issue, and are DEFINITELY nicer to metals, should it ever get wet.) Depends on the sensitivities.

   Amongst readily available fiber insulation, rock wool is about as chemical-free as it gets, since anything volatile has long since been burned away in the spinning of the fiber, and even the batt-binders are mostly cooked down to (next to) nothing.