This was an interesting episode in which a custom home get’s a 40-year shingle, but because the homeowner used a considerable amount of ‘new’ building techniques through a separate agency, the use of spray-on foam insulation into the rafter cavities of the roof was implemented.
The concern from the roofers was that the absence of soffet and ridge vents in addition of the foam insulation would create a condition in which the shingles would get hot enough to melt them, and thus prematurely deteriorate the product. From this concern came two questions I had:
I presumed the air-tightness of the spray-on foam insulation was going to permit any roof-heat loading that otherwise permeate down into the home would get blocked, inhibited, or reduced, and as such help with Summer cooling requirements. And since this was a custom house, the homeowner certainly showed that money was being spent easily, but on his call.
Secondly, if this was the only concern of the roofers (premature deterioration of the shingle product), then what if someone used another roof product, something like a tacotta or cement-based shingle, or a tin roof?
I guess what bothers me if the idea of the product manufacturers taking a stance that tar shingles should prevent rain from passing through, but should be allowed to pass as much heat through it (either direction depending on season). Is this reasonable? I know it would be accustomed since old habits die hard. 🙂
My biggest question for this homeowner, their architect, and the agency they used for modern building practices was why on earth didn’t they consider SIPS to begin with?
The agency the homeowner used for implementing new building techniques was to get a higher performance out of the home. Ok, so they stick-build instead of SIP build. I’m confused, here.
Replies
Shingle degradation in comp shingles over unvented attics has become a non-issue with most manufacturers. The difference in shingle temperature due to use of radiant barrier sheething or insulation under the roof deck is only a few degrees. By comparison, the difference in surface temperatures of white versus black shingles is forty degrees F. Faced with these facts, most of the manufacturers withdrew their warranty concerns about RBS and then later withdrew their objections to insulated roof decks. Most of the heat build up in comp shingles is reradiated back to space.
SIPS construction is more expensive to implement in most cases unless the structure is specifically designed for their use.
I'm doing my first SIP and this question if of interest to me.
The sales rep from one supplier assured me that most shingle companies were honoring their warranty. I'll have to see more assurances before I decide not to do a secondary cold roof.
blue
It really is a minor concern, Blue. I read one of the studies a few years ago that demonstrated that composition shingles over an insulated, non vented deck will deteeriorate faster. But the difference shown was a percentage that might result in only 3-6 months shortened lifespan in a twenty year roof. That is far less than we see in the diff between north and south sides of a 12/12 roof in these northern climates. The savings in heat energy over the lifespan of the roof will way more than pay fro a year's worth of shortened lifespan. There are a couple manufacturers still voiding their warrantees over hot decks so it is worth checking your brand. I believe that Elk, and Certainteed are fine with it.
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Piffin, I'd be interested in that study. If you can remember the name of it, or something similar, I'd like to see it too.
blue
Florida Solar Institue has done a large number of studies on the effects of roofing material, insulation, radiant barriers, venting, etc, etc on the amount of heat energy passed to the house. They might have something of shingle life, but don't know.
I don't remember the name. I think I got the link via one of the JLC email newsletters, and I'm pretty sure that it was done in the Georgia/ S Carolina area.
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click on this Certainteed link...
http://www.certainteed.com/CertainTeed/Pro/Design+Professional/Roofing/Warranty/
go to Roofing Warranty.... and read the section on page 6...
my understanding is this means they will honor the warranty if the roof is designed to be a hot roof....
if it just happens ( "non-ventilated " roof ) they will alter or disallow the warrantyMike Smith Rhode Island : Design / Build / Repair / Restore
Most track homes in the lower-third of new-home makret have as a norm (from what I've seen) exposed rafters and using ridge-vents with soffet vents. I cannot see how this makes a roof 'not hot' in the middle of Summer. By Noon today, it will be >100ºF in my attic as I am in an example of what I described in a home. :)
Without an attic fan (or two or three), the average summer high-temp in the attic can easily reach 120º, and anyone on the outside of the roof is truly brave (those shingles begin to liquify. I think today I am going to stop in Fry's Electronics and buy 5-6 of those digital thingies to measure the temps in the attic and on my second floor.
My attic is pretty steep in slope for the 'south' as its going up 18' of so. I know its over 16' because the foam-board used as sheather is at least two full panels in their up-right position. I think if I place a thermometer (or electronic temp-reading thingy) at my head elevation (6'), and at the attic floor, again at the ceiling of the second floor, head elevation, etc.
I would think that in the 'south' anyone interested in their confort and not getting it with the minimal-HVAC installs typically found in track homes would love the thermal insulation benefits of 8-16" of foarm insulation that tightly fills the rafter cavities, or a composite roof decking that SIPS can provide.
Has anyone actually seen a Stick+SIP construction taken place outside of a custom home arena? I would think that a SIP-roof would go up in a day, compared to the three days I watch day-laborers take to stick a roof up--especially in the south when the warmer it gets the longer it takes a framer.
I read a study at Buildingscience.com that showed a net 17°F difference between the roof deck temperature of vented cold roofs, vs. unvented, insulated roofs in the summer sun of Las Vegas. Here are some resources I have found useful at buildingscience.com:
The Roof System Primer
Venting on Venting
Unvented roofs, hot-humid climates, and asphalt shingles
The Las Vegas case study
A summary article on unvented roofs
More on unvented roofs
Etc. sorry for the need to edit this, I guess the google entries were out of date. Cheers!
Edited 7/25/2005 7:10 pm ET by Constantin
I could not access either of this links.
Curious, was this between conventional roof construction and the only difference was venting & non-venting, or was this with 12" of either spray-on foam insulation on stick-raftering or SIPS to form the roof deck?
Also, I guess shingles are the most inexpensive form of roofing. I think my home got some 30-day shingles. :)
Here you go, I've edited the links (see above). Cheers!