Cathedral ceiling insulation

The Icynene insulation is too expensive to use everywhere, so it's only on the roof and everywhere else in the house has fiberglass insulation. In this picture of the attic you see where the untrimmed Icynene is used for maximum insulation value, fiberglass for economy, and trimmed Icynene where the cathedral ceiling is.

No, it's not much of a Cathedral, that's the term of art for a room's ceiling that is also an exterior wall, and it's always an insulation challenge. In this case, that part of the roof is framed with 2x6 rafters, so there's really not enough insulation between us and the 160-degree roof shingles baking in the summer sun (the roof design is unvented).

The plan is to implement what I call a double radiant barrier below the Icynene. The Icynene will be covered with perforated aluminum foil (actually an aluminized housewrap-like product called RadiantGuard). Then 1" strapping will be applied, then another layer of non-perforated foil underneath the sheetrock. The two foil surfaces have high reflectivity and low emissivity - they reflect radiant heat, and they don't radiate the heat they absorb. This is the same principle as a Thermos bottle, except that the surfaces are separated by air not vacuum.

It's hard to translate the potential benefit of this insulation approach to conventional metrics. Some analyses suggest that a single radiant barrier makes an R20 wall perform like an R30 wall, which is about what we're hoping for. We'll be able to measure the surface temperatures on the foam versus the drywall, and get a sense of its performance.

Addendum: performance measurements in warm weather

Roof insulation performance, warm weather

The cathedral ceiling in the third-floor/attic, an unvented roof insulated with icynene foam and a double radiant barrier, seems to perform well in warm weather. We didn't install central air conditioning (too expensive both upfront and operating cost), and we both have office space in the third floor, so we depend on that roof insulation system for comfort both winter and summer. So far, in hot weather the attic temperature is always lower than the outside temperature, which I think is pretty good.

Boston's had our first heat wave the past few days, and I took some measurements today. We keep most windows closed during the day, opening them at night to pull cooler air through with a small "whole-house" attic fan.

Location	11 am	3 pm
Outside air	89 F	93 F	air temp
Exterior siding (south wall)	95	114	surface temp
Exterior shingles (roof)	132	152	surface temp
Attic ceiling	86	92	surface temp
Attic air	83	89	air temp
Second floor air	81	84	air temp
First floor air	80	82	air temp

The cathedral ceiling is only 6" thick (made of 2x6 rafters), so all that separates the interior drywall from the 150-degree shingles is 6" of icynene foam and the 1" radiant-barrier airspace.

Icynene foam insulation

Anderson Insulation insulated the attic with Icynene foam today. Very cool stuff: here's a quick movie of the fellow who spent about an hour spraying the attic, followed by before and after pictures of the attic peak.

Roof trim detail

The roof trim is that of a traditional Maine colonial, with

• Roof pitch 10/12


• 12" eave overhang


• 6" flying rake

In this picture, the slight notch at the end of the frieze and fascia boards doesn't match. The frieze is going to be narrowed slightly to make them consistent.

Note that there's no vent in the soffit, in fact there's no vent anywhere in the roof. The roof will be sealed and insulated with icynene foam - what's called a "hot roof" with no ventilation required.

Energy efficiency

Aside from providing our 9 year-old with her own bedroom, and connecting the house to our back yard, a key objective of this renovation is to increase the energy efficiency of our house. We looked at fancy stuff like geothermal heat pumps and photovoltaic solar, but all those technologies are evolving rapidly, and now doesn't seem like the best time to invest. So, we're planning for what I'd call conventional "best practice" energy efficiency:

• No central air conditioning
• No fireplace or internal chimney
• Exterior 6" walls and attic insulated with Icynene
• Efficient double-pane low-E windows
• Hydronic baseboard heating, one zone per module
• High-efficiency, modulated-flame, condensing gas boiler
  
• Tankless gas domestic hot water
• Compact fluorescent and LED lighting where appropriate
  

After this renovation, our next energy priority will be to upgrade my commuting car to all-electric. By the time that's done, thin-film photovoltaic or other distributed-generation options will be available, and we'll look to make our next step.

Energy efficiency measures

At the outset of the project, a key objective of ours was to increase the energy efficiency of the new house. We looked at fancy stuff like geothermal heat pumps and photovoltaic solar, but all those technologies are evolving rapidly, and now doesn't seem like the best time to invest. So, we planned for what I'd call conventional "best practice" energy efficiency. Here's a summary of those measures, and how they've worked out so far:

• No central air conditioning

There are usually about 10-20 days in Boston with temperatures near or above 90. So far, on those days we can keep the indoor temperature down just by closing windows during the heat of the day, then opening the house and using the whole house fan in the evening.


• No fireplace or internal chimney

This was a big aesthetic compromise, but I think it really helps keep the house tight.


• Exterior 6" walls, insulated to R20


• Efficient double-pane low-E windows

We have a lot of large windows, and clearly lose a lot of heat through them in the winter. We did try to minimize glass on the North side, however.


• Attic roof insulated with icynene foam and double radiant barrier

The semi-finished attic has a small crawlspace on top, which is vented in the summer with a Broan attic fan (1200 CFM), acting as a simple and quiet whole-house fan.


• Hydronic (hot water) baseboard heating, one zone per floor



• High-efficiency, modulated-flame, condensing gas boiler

A Weil-Mclain Ultra 80 boiler, properly sized, theoretically runs at about 92% efficiency. "Modulated" means that it reduces its flame when it's not too cold outside, analagous to shutting off cylinders in
a car engine that doesn't need full power. It doesn't run at full capacity until it's -10 degrees F outside.


• Indirect water heater

A Weil-Mclain Ultra 60 gallon tank is heated by a dedicated "zone" of the main boiler. Water temp is turned down to about 120 deg F, since the dishwasher and clothes washer (both Bosch)
have their own internal heating for sanitization.


• Compact fluorescent and LED lighting where appropriate



• Non-metallic ducting for all exhaust fan penetrations

Sometimes the ducts for bathroom fans or dryer vents are made of aluminum and create an efficient "thermal bridge" from the inside to out. We used plastic components. For the kitchen exhaust fan, by Nutone, we found a rubberized canvas sleeve by Excelon to connect to the outer vent.


• Bathroom fans are still a problem...

I think we lose a fair amount of heat through the bathroom fans in the winter. There are energy-recovery systems for these that we should look at sometime.