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Daniel Ernst
Posted on Friday, November 05, 2004 - 08:08 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Hello All,

I'm still in the process of designing the details of my roof. I've been researching to determine several questions:

Do I install a vapor barrier? Which side of the insulation cavity?

Do I ventilate the roof? If so, what is the best method? What is the effect in winter / summer?

What color roof should I use?

My goal is to design a roof that performs well in my climate, uses natural materials, and has a long life. Along the way I've learned quite a bit on vapor retarders, moisture control, insulation, etc.

One excellent site is http://www.buildingscience.com/default.htm

They focus almost entirely on the use of modern materials (i.e. Grace IWS, vinyl siding, etc), but otherwise their work is enlightening. I'm in the process of ordering their design guide (specifically for "mixed climates").

The Florida Solar Energy Center is another valuable resource for those wanting to learn about energy efficient construction techniques.

One of their "Zero Energy Homes" can be found at this address: http://www.fsec.ucf.edu/bldg/active/zeh/lakeland/index.htm

I am building in Arkansas - so I have to deal with a moderate number of heating degree days, as well as cooling degree days. We plan to heat with a masonry woodstove, use thermal mass and solar orientation to keep the house cool. We would like to avoid AC if possible.

And although I've searched all over the web for information on slate solar reflectivity values, I can't find anything.

One concept that I've learned concerns roof color, which largely corresponds to a material's solar reflectivity. In hot climates, the roof's solar reflectance determines how much solar heat is transmitted through the roof structure. Ventilating the roof / attic helps reduce some of the heat gain, but only affects convective heat. The majority of summer time heat gain is in the form of radiant heat, which is not affected by ventilation.

Does anyone out there know of any studies on slate and solar reflectance? I haven't settled on slate color yet. Given the fact that many slates fade in color, this could be key to making this decision. I don't have much hands-on experience with slate. When I compare the slate samples I've received with pictures from various sources - I am amazed at the color change in some varieties (sea green, rustics, mottled purple / green). Even the unfading green looks much lighter after years of weathering.

If anyone could point me in the right direction, I would much appreciate the help!

Daniel Ernst
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slateworks
Posted on Saturday, November 06, 2004 - 06:37 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Hi,The lighter color slates would be Sea Green or Unfading Green,Do they radiate less heat than Black or Purple slate I really do not know,All I can tell you is when working on a slate roof(no matter what color) when it is in the 80's with the sun shining you cannot touch the slate with bare skin for very long it becomes almost to hot to handle. Also the thickness of slate would be a factor to consider,more mass would hold and radiate more heat.Slate must have some reflective ability,but it seems to me to absorb the sun and hold that heat for a good while,at the same time radiating heat into the roof space,especially on roofs with no felt paper,even with felt since it is black would also radiate heat. There is a synthetic felt out on the market called Titanium,we have used it this summer on a few jobs(Tear off and replacment)It is a light gray color ,lightwieght,does not wrinkle ect,this product may help stop some of the radiant heat transfer from the slate,while installing this product thru the summer it stays cool,very low solar absorbtion,this product with proper roof ventilation and a well insulated attic may help reduce the amount of radiant heat into the attic space given off by the Slate.Just some of my thoughts from working on the roof.
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Peter
Posted on Sunday, November 07, 2004 - 03:37 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Hi Daniel,

I agree with slateworks, the slate does hold a lot of heat and cools down over a period of time in the evening.

There is a product similar to aluminium foil that can be used under the slate to reflect radiant heat, it also reflects the cool or warm air generated in the building.

I know the product is included in the "Energy Star" web site, if you are having problems finding it I will look through the archieves in the office.

Kind Regards,

Peter Crawley, M.I.o.R.

www.crawleyroofing.com

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