RB installation clarification


Old 12-20-01, 09:25 AM
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Question RB installation clarification

Question about RB installation. First, the ORNL web page has a document that describes RB systems as consisting of 'sealed' cavities bounded by reflective layers. I've seen other sites that seem to recommend ventilation of the system. Second question: I have an attic storage space, built with storage trusses (open area in the middle). I want to semi-finish the space, but am concerned about summer heat. I'm thinking an RB system would be a big help. I'm in NW Washington, so there isn't a lot of summer sun to worry about, but I need to be able to use it in summer - it is currently uninsulated and probably 120 deg on a hot summer day.
To summarize:
-Ventilate RB cavities or not?
-Recommended RB system for trusses with 2x6 on 24 in. centers - could fur them out if need be for add'l depth.
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Old 12-25-01, 04:31 AM
Join Date: Nov 2001
Location: USA
Posts: 1,873
A thermal mass is anything that can hold and retain either heating or cooling BTU's. The roof and all the wood members qualify as a thermal mass. The insulation in your attic function is to retain heat, which is good for heating but not so good for cooling. That is why radiant heat transfer plays such an important role in cooling, but not so important role in heating. This also accounts for the temperature in your attic being above the temperature outside in the summer. It is the retention of heat inside the insulation that causes the high attic temperature and at the same time it is the same retention that prohibits the heat from conducting into your home. This is also the reason why homes with finished attics usually install a thermostatically controlled attic ventilating fan, because the temperature outside is lower than the temperature in the attic.

The radiant barrier you are referring to will act as a radiant barrier, vapor barrier and provide some insulation value due to the trapped air bubbles in it. The problem with radiant barriers is all heat load, loss and gain calculations are modelled using conduction (diffusion). In other words, I can't calculate it's usefulness to you, though I may understand the principals concerning radiant barriers. It is because of this, I don't use it in my work.

As far as finishing your attic the way I would do it is first install an attic ventilating fan. Then insulate with a vapor barrier, keeping the insulation as far away from the roof as possible so the fan ventilates the entire roof. If the insulation touches the roof, the fan won't ventilate that section of the roof. Then finish off the room. I would also recommend if and when you have to roof your home, use a light colored shingle to reflect the heat from the sun. I am aware that in your state wood roofs are popular.
Old 12-26-01, 04:42 PM
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I'm second guessing this,but, I think they might have ment the attic space. Venting the cavities of a multilayer RB makes no sense and could be detrimental to its performance. It's these types of ambigouis comments that muddies the waters.

If you have no venting or air exchange in the storage space the temperature would get higher than normal than if it was vented or air exchanged. How ever the temperature would drop quickly at sun down because the RB holds not heat engery to speak of.

You don't say what the construction is. If you have a RAFTER surface into this area, that will allow more than the normal amount of heat energy as compared to say a truss system where there is a thermal break.

The RB is the material of choice in this application. You can install two single layer sheets and get maximum benefits, however, I need to know the constuction to determine the best way for you to do this project.

This may be beyond the scope of this web site.

If you wish to contact me for more detailed info and exchanges, go to, [email protected].

Bulk insulation such as FG would probably raise the temp higher than 120 degs since it is only about 10% efficient against radiant heat energy and would trap the heat in the storage cavity.

In this application I would think that the RB temp should not exceed 90-95%. With proper attic venting( ridge vents) it would probably be lower.

Thank you for considering my opinion.

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