# (Partial) Radiant Barrier Above Finished Attic?



## Windows on Wash (Aug 30, 2011)

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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## Lascaux (Apr 17, 2012)

Windows on Wash said:


> Posting a picture will net you more responses.


I didn't think this was a dating site! :laughing:



Windows on Wash said:


> 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.



Windows on Wash said:


> 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.


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## Lascaux (Apr 17, 2012)

*Images for (Partial) Radiant Barrier Above Finished Attic?*

Here is the peak area of the attic that is above the finished attic room:







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:








Here is one of the channels between the rafters leading down towards the knee wall:







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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## Windows on Wash (Aug 30, 2011)

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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## Lascaux (Apr 17, 2012)

Windows on Wash said:


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





Windows on Wash said:


> 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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## Windows on Wash (Aug 30, 2011)

Lascaux said:


> 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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## Taxman (Aug 31, 2012)

Hi,
I'm working on a similar project. Please advise how you completed this and forward any good suggestions. Thanks!


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## Lascaux (Apr 17, 2012)

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.


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## Lascaux (Apr 17, 2012)

Information on the "Mooney wall"...

http://www.builditsolar.com/Projects/Conservation/MooneyWall/MooneyWall.htm


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## Gary in WA (Mar 11, 2009)

Just keep air ventilation above the cellulose; http://www.buildingscience.com/docu...archterm=attic+fiberglass+batts+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/documents/published-articles/pa-crash-course-in-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/documents/information-sheets/installation-of-cavity-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/documents/information-sheets/air-barriers-airtight-drywall-approach/

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

Gary


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## Lascaux (Apr 17, 2012)

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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