Interior Roof Insulation Retrofit for (Cathedral Ceiling) Rigid Foam.

The toughest details are those that have to match up with someone else's work, or those done — even done well — when energy was really cheap. These details are a collection of some common — and tough — dovetails of existing work with retrofits or additions. Bear in mind that green remodeling means creating a new operating regime that is better, not worse, than the one that may well have been working just fine before. Integration of energy efficiency, moisture management, and indoor air quality is much more important and challenging in remodeling than in new construction.

Where Roofs Meet Walls is a Critical Connection

Corners and connections are where insulation and air barriers can have trouble. Compressed or insufficient insulation can cause cold spots, which lead to condensation, mold, and rot. Air leaks at this connection can cut the effectiveness of the insulation substantially. In cold climates, this is where ice dams begin.

To keep the air barrier continuous, span the wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. over the framing connection and use adhesive or sealants at framing connections as shown.

Roofs and walls need to dry
Moisture from both outside and inside a house can thwart your best efforts at keeping the building dry. Moisture in roof and wall assemblies is inevitable, so it's a good idea to design them so that they can dry. Roofs and walls that can dry to either the outside or inside are good, but those that can dry both directions are even better.

  1. Designing to dry out means doing two things well:
  2. 1. Choosing materials carefully—each layer affects the vapor profileA vapor profile is an assessment of the relative vapor permeabilities of each individual component in a building assembly and a determination of the assembly's overall drying potential and drying direction based on vapor permeabilities of all of the components. The vapor profile addresses not only how the building's enclosure assembly protects itself from getting wet, but also how it dries when it gets wet. For a detailed treatment of this subject, see Building Science Corporation's article Understanding Vapor Barriers. of the assembly.
  3. 2. Planning the construction to be forgiving—flashing keeps water out, and ventilation removes water vapor.

Vented attics can keep a roof dry
Outdoor air that enters soffit vents and is exhausted through a ridge vent can help roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. to dry if it ever gets damp.

Ventilation may help keep roofs cool
Although roof ventilation is often perceived as an effective way to cool off attics during the summer, studies show that it's hard to lower attic temperatures very much by ventilation alone. However, most building scientists agree that attic ventilation can lower the chance of ice damming in snowy climates. For more information on preventing ice dams, see Prevent Ice Dams With Air Sealing and Insulation.

Learn more in the Green Building Encyclopedia

Enclosure overview
Exterior walls
Roofs: Attics, Structure, Claddings

DRAWING DETAIL

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2.
Oct 30, 2012 12:38 PM ET

Response to Ken Ackerman
by Martin Holladay

Ken,
The best place to ask your question is on GBA’s Q&A page:
http://www.greenbuildingadvisor.com/qa

If you post your question there, you’ll be able to get answers from a wide number of people (including me).

Briefly, either of your suggested interior approaches will work (removing the drywall and installing spray foam -- an approach that can be done with ventilation chutes in place if you prefer -- or adding interior polyiso on the underside of the existing drywall). Your exterior suggestion will not work.


1.
Oct 30, 2012 12:26 PM ET

Edited Oct 30, 2012 12:29 PM ET.

Inner Retro Fit of Insulation in Cathedral Ceiling
by Ken Ackerman

I have a 36 y/o home with 2 cathedral ceilings. A new roof was just put on (strip old roof, replace decking as needed, new 30# felt, new OC Duration energy reflective composition shingles (Shasta White). The re-roof process was on site inspected by an independent engineer to meet the Texas Windstorm Insurance Agency requirements. There was not enough time or money available to add Polyiso sheets over decking during the re-roof process. I live in Corpus Christi, TX, very hot & humid, no snow. I had continuous ridge vents installed and all old turtle backs and other roof vents removed. I am now starting to add Cor-a-Vent continuous soffit vents to balance the new ridge vent area.

Questions:
1) Is it possible to remove the ridge vents temporarily and have an insulation contractor put liquid foam in each of the rafter cavities? I assume that due to the age of the home, that FG batts were probably installed originally. It's also possible to also open the soffits if that provides needed access. Just thinking about trying this approach gives me great reservations because it may actually block the venting cavities between the rafters.

2) If #1 above is not appropriate (more than likely not), then I assume the inside approach is the only one left. The current ceiling is sheetrock, is it better to take it down or just put polyiso sheets right over it? The diagram in this article above shows the polyiso put directly over the rafters. I'm assuming that taking down the existing sheetrock will more than likely allow the FG batts to fall out and prove to be a nightmare, unless the batts were stapled to the rafters.

3) What are the downsides to applying the polyiso directly over the current ceiling and then sheet rock over them, if any?

4) If the current sheetrock ceiling was ripped out and the batt insulation with it, would that allow an insulation contractor to liquid foam the back of the roof decking and still keep a vent channel to the ridge vents? If so, would it be necessary to put rigid foam inserts after the liquid foam is applied to insure the vent channel stays open?
OR install rigid foam sections one at a time prior to liquid foaming above it?

As you can see, I'm certainly not an expert in this area, but I'd like to do it once and only once, the right way, the first time.

Ken