Radiant Barrier Advice and Installation - AVS Forum
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post #1 of 3 Old 03-10-2009, 08:12 AM - Thread Starter
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Hello All,

I'm currently in the process of optimising my living room for home theatre duties. I bought a 30 year old house in Florida which I can only describe as a "handyman special" (so much needed/still needs to be done). I've been making steady progress, pulling network wiring, adding whole-house audio capabilities, pulling for remote video distribution, etc.

Part of my project has been to try and improve the overall insulation of the existing structure, both for audio and for thermal control. For the audio side, I've installed double-drywall in the master bedroom (see pic below).



To try and improve the thermal performance of the house, I've spent (what feels like) days in the attic putting in acrylic caulking on the header joints and filling in all the holes where electrical wiring, DWV and ducting penetrate the headers and ceiling. I've also caulked all of the electrical outlet boxes and switch boxes in the house and added foam gaskets. I've even pulled off all the baseboards and sealed the drywall footer gap with backer rod and caulking.

Now, my challenge.


The wall shown above is the reverse of the double-drywall wall in the master bedroom. This wall is nearly 6" thick now, with the living room side having 1" rough cut cedar over 1/2" drywall, over studs then another 1/2" drywall and my new layer of 5/8" drywall (two tubes of PL200 adhesive per panel).



The rear of the house has a massive vaulted ceiling that at its peak rises to 17'. The upper half (~8ft vertical across the entire house) of this facing wall abuts an unfinished attic space with only a single layer of aging faced insulation behind the 1/2" drywall. I get a massive amount of heat transference through this facing wall in the summer and want to do all I can to mititgate this for the coming year. I have read up on radiant barriers, but can't find specific instructions on how a radiant barrier should be applied to this sort of wall.

10 minutes later in Sketchup...



Should the radiant barrier be applied directly "behind" the drywall, under the insulation, or should the radiant barrier be "on top of" the existing roll insulation?

"A" or "B" ?

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post #2 of 3 Old 03-10-2009, 09:32 AM
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kingleer,

Definately not option "B" , radiant barriers basicall need at least some air space to work.
Its hard to make a reccomendation with out fully understand the rest of your attic space. when applying a radiant barrier, you have a couple of options.

1. apply the barrier directly on top of insulation with reflective side facing the Roof side of your house.. this way the radiant energy is reflected back into your attic and the insulation basically stays cooler and heat flow transfered through the insulation is less.
if this approach is taken make sure the barrier chosen allows water vapor to pass through it to prevent condensation between your barrier and insulation.

although the eaiset This approach is typically not reccomended because of the moisture issue mentioned and that over time the dust settles on the surface of the barrier and reduces it reflectivity.

2. Apply the barrier to the bottom of your roof rafters with the reflective side facing the roof.. you will have an air space equal to the depth of your rafters.. this will reduce the radiant energy entering your attic space.



Hope this helps

My Basement HT Construction ~ Faster than the speed of Dark

"I've cut that piece 3 times and its still too short..."
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post #3 of 3 Old 03-10-2009, 10:03 AM - Thread Starter
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Quote:
Originally Posted by KNKKNK View Post

kingleer,

Definately not option "B" , radiant barriers basicall need at least some air space to work.
Its hard to make a reccomendation with out fully understand the rest of your attic space. when applying a radiant barrier, you have a couple of options.

I didn't think it would be "B" but all of the radiant barrier installation resources online don't really address insulating an interior wall that faces an unfinished attic.

The "facing wall" covered earlier in this thread is framed with 2x4 vertical studs 16" on center. The roof is arched with a ridge vent running 3/4 of the total length of the house. The soffits are vented underneath the roof overhang. The roof substrate is 1"x10" planking (pine I think, but it's much denser than any pine I've ever seen before) topped with a firm styro layer and then asphalt shingles over that. The rafters are all framed in grade A pine (no knots to speak of ) and everything is hurricane wrapped. I had the house inspected for wind mitigation last year and the inspector was suitably impressed with the integrity of the construction.

I have ~8" of loose fill insulation (the white fluffy type) laid in over the ceiling drywall. After 30 years areas of this have been shifted and some of it is quite compacted.

As can be seen in the photo below, the "rear" of the house has no attic space at all and no access between the interior rough-cut cedar planking and the exterior roofing surface. Sadly, this feature of the house I will have to live with until it comes time to re-roof, at which time I plan on increasing the insulation properties of this section of the roof substantially.


Back of House Roof


Front of House Roof

Quote:
Originally Posted by KNKKNK View Post

1. apply the barrier directly on top of insulation with reflective side facing the Roof side of your house.. this way the radiant energy is reflected back into your attic and the insulation basically stays cooler and heat flow transfered through the insulation is less.
if this approach is taken make sure the barrier chosen allows water vapor to pass through it to prevent condensation between your barrier and insulation.

although the eaiset This approach is typically not reccomended because of the moisture issue mentioned and that over time the dust settles on the surface of the barrier and reduces it reflectivity.

The attic is far too dusty to use this method as I would quickly encounter reduced reflectivity.

Quote:
Originally Posted by KNKKNK View Post

2. Apply the barrier to the bottom of your roof rafters with the reflective side facing the roof.. you will have an air space equal to the depth of your rafters.. this will reduce the radiant energy entering your attic space.

Now, this is something I've been considering but I can't get a straight answer as to how much this will increase the temperatue of the exterior roofing. There is a concern of "overbaking" the roofing shingles since I live in Florida and the sun here is amazingly intense. The radiant barrier suppliers would have you believe that any increase is negligible, whilst roofing shingle manufacturers sometimes claim that adding a radiant barrier will increase the temperature of the shingles into a failure zone.

My eventual plan after completing all wiring and caulking in the attic is to "re-fluff" the compacted insulation and add another 8" or so of new loose-fill material on top of the existing material.

Progress Energy raised electricity rates 39% as of January 1, 2009, which is nothing short of ludicrous. I'm trying to do all I can to not run up against a $300 power bill this summer.

In terms of home theatre, I've run over 1000' of CAT6 through the attic and walls and have pulled multi-room HDMI, component and audio distribution throughout the house. I plan on creating a thread to show all this progress in the near future.

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