Low E, radiant barriers, low slope roof, and attic venting questions

#1

Hello I have a few questions about my roofing project which I hope someone can help answer. I’m going to break them into sections to make them easier to read and discuss.

Low E:
First, I am trying to decide whether to get Low E for my roof. One of the roofing companies highly recommends Low E to keep our house cool during the summer. The only problem is they will be installing it over synthetic underlayment and beneath asphalt shingles so there won’t be any airspace required to create a radiant barrier. They’re aware of this but said the thermal barrier (even without radiant barrier due to lack of airspace) would still keep the attic space and house cooler. I won’t be able to benefit from something like LP tech shield OSB because I’m replacing a wood shake roof that has skip sheathing so radiant barrier sheathing would not be effective with skip sheathing. So if I do add Low E, I would only be adding it for the thermal insulation and not the radiant barrier The question I have is, would the thin polyethylene foam sandwiched between two aluminum foils in Low E adequate in providing an effective thermal barrier as adding R38 traditional insulation? Or is the R38 insulation more effective in keep the house cooler and also more cost effective since cheaper? The Low E will add on over $2600 to roofing cost.

When people say the attic space will be 10-30 degrees cooler, do they mean 10-30 degrees cooler compared to outside temperature or 10-30 degrees cooler compared to roof surface.

What if I have a segment of the house that doesn’t have an attic crawl space for traditional insulation? Should I add Low E over that segment of the home?

And finally, since Low E can be used as a sole underlayment for roof, if it is applied over synthetic underlayment, would it give me an additional barrier against leak?

Low slope roof:
I have a segment of the home that has low slope (4 1/2 pitch). Would you use double synthetic underlayment or water and ice barrier for that entire segment?

Valleys:
Do you guys use double synthetic underlayment under valleys?

Attic venting:
My home does not have soffit vents because it is an older home so I will need to install bottom intake vents and ridge vents for exhaust. I have gable vents in place currently. Should I seal off the gable vents since it may alter and disturb the air flow from bottom vents to ridge vents? I’m reading mixed opinions online.

Thanks so much for all your help.

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#2

I’m sure I’ll get kick back on this but forget about low E material. This is actually an old technology that was used in the 80’s in lieu of insulation. Didn’t work then and, in my opinion, doesn’t work now despite all the hype. Proper installation is critical and often is ignored. Ventilation is, by far, the best solution to attic heat. Gable end vents short circuit ridge venting, no mixed opinion about that. Make sure the eave and ridge vents are installed properly with sufficient air flow unblocked by attic insulation. Lighter colored shingles help a lot.
Use ice and water in the valley, absolutely.

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#3

Don pretty much nailed it. Put insulation where it belongs in the ceiling. In my opinion radiant barrier ect might do something but it in no way replaced insulation. The most cost efficient way to insulate a standard structure is fiberglass in the ceiling. Insulation in the ceiling also lets the roof breath as long as there is proper ventilation. R-38 in the attic would be equivlant to 6" of ISO foam on the roof.

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