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Old 07-14-2019, 09:44 PM   #1
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Radiant Barrier installation questions?


Hello All,
I started stapling some radiant barrier to the bottom of my rafters and would like some input please. We have a hip roof with two electric ventilators, btw.


1) It's my understanding that I am to begin stapling the first run of radiant barrier near the bottom of the rafters but not all the way down. I am supposed to leave an opening at the very bottom of the rafters so the cooler air from the soffits can enter.

So does this mean the opening just needs to be available for the outside wall/soffits -or- does the opening need to also be open to the attic space? In other words, the first run of radiant barrier must remain above my blown-in insulation and open to the attic space.


2) How much of an opening am I supposed to allow at the bottom and top of the rafters for the air to flow? Do I allow 8in, 10in or 12in?


3) Lastly. Because we have a hip roof most of my rafter lengths will vary. The jack rafters begin on the outside wall but they end at varying lengths on the hip rafter which of course goes all the way to the top ridge. Please confirm the obvious, I am supposed to cut holes at the top of all of these rafters so the hot air can then exit to the very top of the attic?


Thanks,
Ralph
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Old 07-14-2019, 10:27 PM   #2
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Re: Radiant Barrier installation questions?


An inch from the lower end at the walls, should suffice, and an inch from the HIP RAFTER, should be good to allow the air to continue rising to the vent fans.

ED
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Old 07-15-2019, 06:05 AM   #3
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Re: Radiant Barrier installation questions?


To be honest I deal very little with climates as hot as yours, but given that your attic will still be warming up, not everything can be covered with the RB, then I would want the incoming air from the soffit to be able to flow into the attic.

Covering the bottoms of the rafters changes the air flow, such as night time cooling. Most naturally ventilated attics cool through a combination of air flow and radiant heat loss, you RB will be limiting that radiant portion and interfering with the air flow. I still like RBs but we have no examples of your installation to predict the results.

Just FYI,
Bud
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Old 07-15-2019, 06:09 AM   #4
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Re: Radiant Barrier installation questions?


You do need a space at the top of all the jack rafters.
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Old 07-16-2019, 11:10 PM   #5
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Re: Radiant Barrier installation questions?


Thanks for all the input.



I contacted RadiantGuard support who is one of the top manufacturers of radiant barriers and they stated I need to allow a 12in gap above the attic floor insulation. This will allow the soffits to both ventilate the attic space as well as behind the barrier.


God Bless,
Ralph
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Old 07-16-2019, 11:12 PM   #6
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Re: Radiant Barrier installation questions?


Quote:
Originally Posted by Ralph III View Post
Thanks for all the input.



I contacted RadiantGuard support who is one of the top manufacturers of radiant barriers and they stated I need to allow a 12in gap above the attic floor insulation. This will allow the soffits to both ventilate the attic space as well as behind the barrier.


God Bless,
Ralph
What did they say about where the jacks hit the hip?
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Old 07-16-2019, 11:29 PM   #7
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Re: Radiant Barrier installation questions?


Maybe you could treat it like a valley on the roof and run one straight up the corner and leave a space under the hip.
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Old 07-16-2019, 11:34 PM   #8
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Re: Radiant Barrier installation questions?


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What did they say about where the jacks hit the hip?
I didn't ask that specific question because I'm absolutely positive there will need to be ventilation holes cut for them. It's really no different than a gable roof rafter except that jack rafters vary in length. The radiant barrier needs to be vented and that is what they stated.


In regards to how much of a hole should be cut for the top of the rafters? I plan on leaving about 10in. That should be adequate to ventilate my rafter space. As others have stated, the radiant barrier acts as a shade so no matter how much I add it will be beneficial.


Thanks,
Ralph
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Old 09-20-2019, 11:25 AM   #9
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Re: Radiant Barrier installation questions?


The key is not to overthink it. I tell customers this everyday. Here is a picture a customer provided mid-installation. A hole is a hole, shape does not matter. You do NOT need 12" at bottom. You just need a couple inches above the insulation so the air can either go between the radiant barrier and the roof deck OR be drawn into the attic. Remember, the air is NOT under pressure like an air duct. So, even 1/2" will allow enough air to pass through due to the stack effect. You just don't want the air to hit a "dead end" either at a section of cross blocking, ridges or at a hip rafter. Treat it like water heading to a drain in the top of your attic. Just give it a path to get to the top and find it's way out whatever topside ventilation you have.

Hope this helps,
Ed
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Old 09-20-2019, 04:49 PM   #10
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Re: Radiant Barrier installation questions?


Thanks Ed. That is pretty much exactly what I did and your image looks identical to my installation. I did however allow about 8in at the bottom and had planned on allowing that much at the top but will probably allow around 4in now. I plan on adding additional blown in insulation in early spring, so it will end up being about 3in at the bottom.


I have completed almost 40 percent of the attic already which is a major plus for me. I hadn't planned on starting on it until winter but I have been able to spend a few hours every Saturday morning on it. I'm real happy with the progress and my two roles of 1000 sq ft has gone a long way. I think one more role will do it.


God Bless,
Ralph
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