Anyone ever insulate rafters?

[_strickland]

I'm sure this topic has been discussed before. I know most would suggest radiant barrier but I'm actually not particularly interested in it especially considering I have roofing nails all over sticking through the decking. I live in Texas and the a/c system is in an uninsulated attic. As such the ductwork is mostly R8 with some R6 here and there. When the killer heat arrives in the summer the a/c has to work pretty hard of course and so I've tried over the years to do small things to keep a little of the heat out. My most recent one was putting a small gable fan in the attic to at least exhaust some of the heat. I've made sure the soffits are cleared for proper ventilation but the pull from the fan is fairly minimal in such a large attic.

Anyway has anyone ever tried using kraft batt insulation in your rafters before? I think you have to insert some sort of moisture barrier first correct? Was this worth it to you and could you tell any sort of difference in temperature afterward on a consistent basis?

 

[Kpaige]

Properly venting and insulating is just as important in the heat as it is in the cold. You do not want insulation over the roof boards you want an air space of 1” from eave to ridge you want air coming in at the eave running up out the ridge. You want the uncontrolled space wether it’s the 1” space or an entire attic to be as close to the same temp as it is outside as possible. If it’s 90 outside that’s what it should be in the attic. Improperly ventilated spaces will be much hotter

 

[acer66]

Maybe this can give you some ideas.

 

[pcmeiners]

Agree, my attic is basically open at one end (long story), providing a very large air flow, and has a ridgevent. It stays at a couple degrees over outside ambient temperature. My dark brown roof shingles are 37 years old and are in like new condition due to the cool attic condition.

 

[billconner]

Roof color will make a major difference so next reroof, go light. Much bigger difference than venting.

Ducts and HVAC in an attic outside the thermal envelope is a really bad idea. Different strategies for how to insulate - cathedralize - the roof, but here's some science on it.

RR-0917: Measurement of Attic Temperatures and Cooling Energy Use in Vented and Sealed Attics in Las Vegas, Nevada | buildingscience.com

buildingscience.com

 

[cbracer]

That video was great! When it's 90 degrees outside, it's 110 on the roof and 105 in the attic. There's no amount of ventilation in my attic that would provide enough cooling for it to be anywhere near 90. There is no way around it. Studies that were done at universities say that insulation under the rafters works best. Increased heat on the shingles did not affect their life any appreciable amount. The science is right, but just like pex plumbing, it will take a life cycle of people to rotate into the business of making houses to actually change perception. I had a company that built steel houses with prefabbed walls, no wood, and they were amazing. Not your typical commercial steel studs, not completely engineered studs and wall construction. Cost maybe 5% more on the total of a home. But nobody wanted to pay a small premium and builders can't invision changing their process.

So yes, put your fiberglass on the rafters. It will help. The radiant cooling products don't actually do much on their own. Real insulation does.
-->> All that said, It's incredibly important to know about moisture barriers because that's what can bite you if you don't do it right. I live in southern california so I luckily don't have to worry about moisture, but 80% of the US does.

 

[mike93lx]

I added 2" poly iso foam board across the truss top chords on the south facing side of my attic, leaving it open at the bottom and top for air to flow through. Attic is still hot as ****, but slightly less now