How to Insulate Roof Eaves - "Tie Downs"
Truly effective thermal barriers (read "insulation") have two properties, they are continuous, no gaps or holes or thin spots, and they incorporate an air barrier, also continuous, to stop air movement through the insulation. This ought to make intuitive sense, but I am compelled to mention it by how often field practice is unaware of or ignores this. I especially see this in the way that the air-thermal barrier in walls is joined to air-thermal barrier in roofs.
An insulated roof or ceiling assembly must be connected or "tied-down" to the exterior walls at the eaves and the gable end. Often, not enough attention is paid to detailing these roof-to-wall junctions for air tightness. Air permeable insulations, such as batting or loose-fill cellulose, cover openings here but do not seal-off air movement; moving air removes heat otherwise retained, and performance of the insulation is severely compromised.
Without special detailing at eaves, there is nothing to stop warm air from leaving, and cold air from entering, at the eaves. Unfortunately, these are not easy areas to air seal because collar ties and roof rafters break up the area to be sealed in regular intervals. The need for the air and thermal barrier to be just as good here as elsewhere, and the fussy detailing needed to really close these openings by tight carpentry and sealants suggests using materials and techniques that can seal quickly and effectively, even the materials are at reasonable premium.
When insulation is to be installed in a roof, as opposed to in a ceiling below, there clearly must be an insulating connection between walls and roof. The downside of not doing this at all is clear, but the downside of not doing it well is less widely appreciated. The square footage involved is small compared with the total area of walls and roof. An appraisal of "too small to matter" or "the best we can do" is common. Yet, the "damage" that can be done by small areas that are highly air-leaky or have low R-value compared to other building "skin" is illustrated by a more frequently considered component, windows.
The aggregate impact of old, loose, drafty windows is clear to anyone who has ever had to live with, or even "visit" with them. Moreover, my recent calculations of conducted heat loss through R-2, double glazing on typical homes in which glazed area is 10% +/- of total wall area show conducted heat loss through windows to on the order of 30% to 40% or heat loss conducted through the balance of the walls. Put another way, why would anyone with the chance to do better put rows of leaky windows at all of the eaves of an otherwise well-insulated roof? Enough said.
The need for both air barrier and insulation, quickly applied, suggests spray applied foam for these details. No argument, this is a premium material beyond many project budgets for general use. For fussy details, where it can save time, and give near continuous, premium performance where not interrupted by framing, sealing the most irregular opening, it's use is more than justified. The total volume of foam may be such that a "kit" delivering 40+ cubic feet will cover without the high fixed cost of a foam contractor.
Further economy is possible by spanning most of the opening with minimum 3" thick, loosely cut insulation board, 1" + clear on all sides. Batch cut these for standard framing centers and foam them into place. This is also a workable technique for edge baffles to prevent wind washing where the insulation is on top of ceiling. Note for those installing vent chutes: Most spray foams adhere poorly to polyethylene plastics.
Scrupulously observe all safety precautions regarding spray applied foam! If you don't honestly feel up to this, rely on a professionally trained contractor.
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