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Old 05-23-2012, 04:37 PM   #1
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(Partial) Radiant Barrier Above Finished Attic?


I just read that for a radiant barrier to be effective, 80% of the surface area must be covered.

My house is in climate zone 5A. I have an original finished attic room that I am just beginning to insulate properly. The house was built in 1950. For the past six decades this house has leaked like a sieve. (However, all the ventilation has kept the roof in exquisite condition.)

One of the knee walls had NO insulation, there was no air seal on either knee wall, insulation was poor and poorly installed, there is no insulation on the south exterior wall, etc. The litany is endless. Already in mid-May, the attic has reached 120-deg F. The finished attic has already reached 90-deg F. As is, it will soon be uninhabitable.

While rewiring above the ceiling of the finished attic room, I intend to improve the insulation. This space is six feet wide and about three feet high. It runs N-S. There is a gable vent at the southern end, but the northern end of this 'tunnel' dead-ends. It was necessary to cut away 16-inch sections of four 1x6 planks to gain access to the tunnel. The roof structure is composed of these 1x6 planks laid on top of the rafters. The asphalt shingle roof is relatively new.

The sloped ceiling of the attic room comes up to the rafters on each side. Here, fiberglass insulation fills each channel between the rafters. The batts are 4-inch thick with a thicker black paper facing down towards the ceiling, and a thin, crumbling, tan paper facing up towards the roof. This tan paper provides a consistent channel of between 1”-2” for air flow from the soffits through the rafter channels to a ridge vent.

Because of the summer heat in the attic room, my idea was to install a foil radiant barrier in the attic channel above this room. It would be difficult to install anything in the 14” x 6” channels above the sloped ceiling sections. I will be insulating the knee walls.

Question 1: Does it make sense to install a radiant barrier in this tunnel above the finished attic room? My thought is that it will be more difficult to keep this space comfortable during the summer. Heat from the house below will maintain winter comfort. I am not too concerned about winter solar gain on this section of the roof. Besides, I will have ground source heating and cooling.

Question 2: What does “80% coverage” mean in this context?

Question 3: Should the barrier be installed between the rafters, or across the rafters? It seems to me that there is an advantage to protecting fragile material by placing it between the rafters. Anyone crazy enough to visit this confined space in the future will surely damage exposed material.

Question 4: Is there any advantage between the various radiant barrier products? Keep in mind that I cannot move large sheets into this space. I must use a rolled material. In addition to the issue of exposure to physical damage, headroom should be retained.

Question 5: Can a radiant barrier also be used as a venting baffle? Or, can a radiant barrier be incorporated into baffles? If so, can I install it down the rafter channels above the sloped attic room ceiling?

Question 6: Any other thoughts/recommendations?

TIA

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Old 05-23-2012, 09:28 PM   #2
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(Partial) Radiant Barrier Above Finished Attic?


Posting a picture will net you more responses. Radiant barriers need an airspace to work correctly.

After that, they are not solutions for inadequate insulation levels.

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Old 05-24-2012, 05:01 PM   #3
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(Partial) Radiant Barrier Above Finished Attic?


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Posting a picture will net you more responses.
I didn't think this was a dating site!

Quote:
Originally Posted by Windows on Wash View Post
Radiant barriers need an airspace to work correctly.
Both options mentioned in my initial posting incorporate an airspace both above and below the radiant barrier, albeit of different depths.

Quote:
Originally Posted by Windows on Wash View Post
After that, they are not solutions for inadequate insulation levels.
I am not proposing a substitution, only an enhancement that may be suitable to the particular context that is thoroughly described in the original posting.

Last edited by Lascaux; 05-24-2012 at 05:04 PM.
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Old 05-25-2012, 03:15 PM   #4
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(Partial) Radiant Barrier Above Finished Attic?


Here is the peak area of the attic that is above the finished attic room:
(Partial) Radiant Barrier Above Finished Attic?-south-wall-attic-peak-x280.jpg
Notice that the south wall (at the far end) is uninsulated.

Here is where it was necessary to cut into the planking from the other roof section in order to access the attic over the finished room:
(Partial) Radiant Barrier Above Finished Attic?-attic-peak-x280.jpg

Here is one of the channels between the rafters leading down towards the knee wall:
(Partial) Radiant Barrier Above Finished Attic?-attick-peak-rafter-channel-x280.jpg
This is fiberglass batt with a dark, heavier paper facing down towards the plaster board of this 1950s house. On the top side of the batt is a very thin, crumbling tan paper; now covered with dust and cobwebs.
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Old 05-27-2012, 11:51 PM   #5
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(Partial) Radiant Barrier Above Finished Attic?


That South wall (if not bordering a conditioned space) does not require insulation.

You do not need a radiant barrier in this application. I would recommend that you remove that existing insulation, air seal everything that can be air sealed, slide a chute do that roof to room intersection to allow for make up air at the soffits or other intake area, blow in additional insulation to get you to R-49 levels.

Radiant barriers will do nothing to help you with anything else in this application.

How is the intake air for the attic handled?

Be sure to fix the kneewall insulation schedule and there are multiple threads about that on here as well.
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Old 05-29-2012, 09:03 AM   #6
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(Partial) Radiant Barrier Above Finished Attic?


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Radiant barriers will do nothing to help you with anything else in this application.
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How is the intake air for the attic handled?
First, I should have stated that there is no conditioned air during the summer in this 1950's home. The finished attic room has two floor vents from the furnace. Second, the attic is vented at the ridge. However, the soffit vents appear to be inadequate and, apparently, they were covered by siding without venting many years ago. In addition, on the interior, I will be adding venting to the soffits.

Because I am seeking to decrease summer solar gain as much as is possible in only this finished attic room that lacks summer air-conditioning, it seems that a selective radiant barrier would apply here.
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Old 05-29-2012, 09:16 AM   #7
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(Partial) Radiant Barrier Above Finished Attic?


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Because I am seeking to decrease summer solar gain as much as is possible in only this finished attic room that lacks summer air-conditioning, it seems that a selective radiant barrier would apply here.
It will not hurt in this case as long as you do not interrupt the venting for the attic.

Beyond an R-19, they are statistically insignificant. That being said, they will not hurt and given the disjointed insulation and envelope layers, it might help.
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Old 08-31-2012, 09:05 AM   #8
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(Partial) Radiant Barrier Above Finished Attic?


Hi,
I'm working on a similar project. Please advise how you completed this and forward any good suggestions. Thanks!
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Old 08-31-2012, 01:51 PM   #9
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(Partial) Radiant Barrier Above Finished Attic?


My current plan is to use Roxul rock wool above the slanted sections of the room, deepen the knee walls with a "Mooney wall", and blow in cellulose above the horizontal ceiling and into the knee walls.

I have ridge and soffit vents. When the temps fall this coming autumn, venting baffles will be installed in the slanted ceiling sections.

There is far too much solar gain during the summer mornings on the east face of the roof. The west side is shaded by several large trees. I will incorporate a radiant barrier on the east side and over the peak. Hopefully, this will make the room livable in the summer. With radiant heating in the floor, this room should be snug in the Zone 5a winter, even with diminished solar gain.

I've air sealed the room, except for the southern exterior wall, which will have to wait until next summer.

Comments welcome.

(Partial) Radiant Barrier Above Finished Attic?-attic-room-roof-plan.png
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Old 09-01-2012, 09:15 PM   #10
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(Partial) Radiant Barrier Above Finished Attic?


Information on the "Mooney wall"...

http://www.builditsolar.com/Projects...MooneyWall.htm
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Old 09-01-2012, 10:56 PM   #11
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(Partial) Radiant Barrier Above Finished Attic?


Just keep air ventilation above the cellulose; http://www.buildingscience.com/docum...+in+flat+attic

The strapping wall existed way before they claimed it, notice they used a pro blower to get the 3-4# dense-pack in the walls. Something the box store blowers are unable to do. So, no radiant above the cellulose, in direct contact, no plastic/foam vent baffles, are you sure with this application there? I'd go with Roxul in the top attic after rigid (foil-faced for radiant) foamboard, canned foamed edges after the airspace--- Site-built baffles, pp5; http://www.buildingscience.com/docum...n-roof-venting

Cavity Roxul with foil-faced (for ignition attic barrier) foam board on the attic side, caulk the knee wall bottom plate/sub-floor, rigid f.b. extending down under the knee walls in to the joist cavity for the thermal/air barrier, cellulose on the side attic floors, Fig.4; http://www.buildingscience.com/docum...ty-insulation/

Right-hand top corner of page 3, but run the f.b. up to cover the edge of Roxul (tight to other f.b. on rake/slope) from wind-washing there; http://www.habitat.org/env/pdf/ceiling_and_attic.pdf

ADA the drywall to stop in/exfiltration at knee wall and sloped ceiling; http://www.buildingscience.com/docum...wall-approach/

At the least, housewrap at attic side to stop wind-washing of cavity insulation, even cellulose (air permeable unless dense-packed).

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Old 09-26-2012, 08:56 PM   #12
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(Partial) Radiant Barrier Above Finished Attic?


Thanks for the suggestions, GBR. I've decided t build my baffles with 1" Poly Shield. I'm cutting 8' lengths that are ~14.5" wide The 'spacers' are 1"x1"x8' remnant Poly Shield strips. Liquid Nails construction adhesive seems to hold the pieces well after curing for a day.

This assembly allows for only a 1" air space below the roof deck. But,with only 5.5" above the ceiling, the 2" deep baffles are rigid enough to slide into the bays without crumbling, while providing the 3.5" space below for sliding in the R-15 Roxul ComfortBatt.

I'm blocking the floor joist bays with 2" XPS after filling the 7" deep bays with 5" unfaced batts. This room will be a music room, so this cheap insulation is for sound dampening.

I haven't yet decided how I will insulate the horizontal ceiling section. I've installed Glimpse LED downlights (spectacular!), so there is no can above. It won't be fun to blow cellulose in the confined space, so I may go with more Roxul batts.

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