Download PDF version (121.2k) Log In or Register to view the full article as a PDF document.
Q.Our insulation subcontractor advises us to install dense-pack cellulose in the rafter bays over a cathedral ceiling, without ventilation baffles, and the local building inspector approves. If we don’t ventilate under the roof sheathing, what happens to our asphalt shingle warranty?

A.Contributing editor Ted Cushman responds: With few exceptions, installing shingles on an unventilated roof deck will void the warranty. The fact that your local building department okayed the installation probably won’t help: As one warranty (from the Canadian firm EMCO, which makes Esgard shingles) states, "Where local building codes have specific [ventilation] requirements that differ from the National Building Codes, the more stringent requirement must be followed."

In fact, as you might guess from that quote, just having the roof ventilated may not be good enough. The shingle maker has to agree that the ventilation is up to its standards. If the warranty excludes installations with "improper ventilation," that language may be enough to deny a claim.

Unfortunately, shingle warranties provide little protection, whether or not your roof deck is ventilated. Almost no roof is put on perfectly; and if you’ve deviated from the manufacturer’s instructions in any way, your claim can be denied. In the end, it comes down to trust: If manufacturers want to back the product up, they will, and if they don’t, they won’t. You’ve got to decide if you want to trust them. And even if a warranty is honored, the money you’ll get won’t begin to cover costs like tearoff, disposal, or labor.

The real question is, how will the shingles hold up on an unventilated roof deck? They might give out a little sooner than shingles on a ventilated roof, but probably not enough to notice. The most important factor in longevity isn’t the level of ventilation; it’s the quality of the shingle. The better brands of fiberglass shingles are the ones labeled as passing ASTM Standard D-3462. If you start with a good shingle, what you lose in shingle life (if anything) you probably more than earn back in energy savings from the added insulation thickness.

At least one manufacturer, CertainTeed, will honor its warranty on an unventilated roof deck, although for a reduced term of ten years (prorated from year one, and with no wind speed rating). CertainTeed has funded a lot of research on the causes of shingle failure, including some long-term studies at university sites in three different climates. In these studies, shingles were applied on ventilated and unventilated cathedral roofs side by side. The research indicates that high temperatures do cut the lifespan of shingles, but only marginally; and it shows that roof ventilation doesn’t have much of an effect on shingle temperature anyway (shingle color and roof orientation are more important).

However, ventilation affects not only shingle temperature but also the level of moisture in the roof assembly, as well as the melting and refreezing of snow on the roof. Since these factors can affect how a shingle ages, it may be reasonable for manufacturers to limit or exclude warranty coverage on unventilated roofs.