# Add Blown Insulation or Radiant Barrier?



## Ralph III (Oct 7, 2011)

Hello All,

We are purchasing a home with the following details.

*Blown in insulation (R-30 +-).
*Light colored brown or tan shingles.
*Two electric vent fans. 
*There are no shade tree's around the home.
*Hip type roof.


How would I best be served.....

1. To add more blown-in insulation?

2. To add radiant barrier? Ebay has some listed fairly inexpensive as seen HERE. I've always had ridge vents and have read the electric fans are terrible. So maybe the radiant barrier and tan shingles will limit their usage?

3. Or a combination of the above? 

I will be doing the job myself but do have a limited budget for this year. I can always add more later.

God Bless,
Ralph


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## Bud9051 (Nov 11, 2015)

Hi Ralph,
Optimize your natural ventilation, soffit vents to either roof or ridge vents. Then dig through that r-30 and at least locate the major air leaks. Would have been easier before they buried everything. I'll add a link on air sealing to help your search. Rake the current insulation out of the way, seal what is needed and rake it back into place. When done you can give everything an extra few inches to make it look and work better.
https://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/TBC_Guide_062507.pdf

Make sure your soffit vents are not filled with insulation and that you have baffles to keep the insulation from blocking the air path. Every rafter bay should have a soffit vent but total NFA (net free area is what you are after in warm country).

Bud

PS, if there are any ac ducts or equipment up there it all needs to be well insulated and air sealed. Guidance available.


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## NotYerUncleBob2 (Dec 29, 2017)

I'm not a fan of attic fans. The problem is that they might exhaust some hot air, but where is the make up air coming from? If it's not coming from the soffit intake vents, then it's pulling your air conditioned air out of the house and pulling in hot humid outside air in through whatever gaps and cracks are present. For most homes, that's through dusty, mouse pee saturated wall cavities or up from a musty basement. 
Radiant barrier? Not sure what that'll really do vs what they claim it will do.


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## Ralph III (Oct 7, 2011)

Hello and thanks for the replies!

I've done home remodeling and construction most of my life, so I have a fair understanding of these things. 

1. Yes, when we move in I will look to fill in all top plate holes. This is a good practice not only for draft purposes but for fire purposes as well. I will also inspect all duck-work and add mastic if needed.

2. I will insure the roof and soffits have proper ventilation as well. There are no ridge vents (as noted) and I'm not good on steep pitches. So I'm trying to limit how much the attic ventilation fans will come on. 

Otherwise, how much do you think someone would charge to install ridge vents? I've always used natural ventilation and have read quite a bit of negative aspects of electric fan ventilation. Most notably that it can suck conditioned air out of your home but is that truly an issue? If you have adequate soffit ventilation with all top plate holes filled and buried under insulation; then it would seem any air pulled from within the home would be quite minimal? I don't know. Maybe it pulls air from the cavity between the brick and exterior walls as well? 

3. I've read mixed opinions about adding insulation to existing attic duck. At our previous residence, I actually added unfaced fiberglass insulation to our duck-board and then covered up the flex duck as much as possible with blown in insulation. It would seem to be more of a benefit than an issue but maybe some of you can offer a professional opinion.

4. I have no experience with radiant barriers. The hype seems to suggest it is highly beneficial though and as I want to limit attic heat; then maybe it is worth it?

Your input and clarifications are appreciated!

Thanks,
Ralph


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## Bud9051 (Nov 11, 2015)

I forgot to comment on the radiant barrier question. In the deep south they sell and use sheathing on the roof that has a foil face on the bottom and it is considered a good investment. In your case a RB could be applied to the bottom of the roof or across the bottom of the rafters. If you put it across the bottom of the rafters you would need to be sure it did not interfere with the ventilation.

Bud


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## NotYerUncleBob2 (Dec 29, 2017)

If you bury a reflector, doesn't it just become a conductor?


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## Bud9051 (Nov 11, 2015)

The RB should be installed up below the rafters and not buried. Its benefit is that solar energy has only one primary path from the bottom of the roof down to the insulation below and that path is by radiation. A radiant barrier works in both directions to block radiant energy transmission across the attic air gap. in the winter it would reflect heat back down away from the roof.

Bud


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## user_12345a (Nov 23, 2014)

you don't need a radiant barrier unless you have air ducts in the attic.

R-30 is low by today's standards and you should go R-50, unless you live in a area that doesn't get very hot or cold.

insulation is important in summer too, not just for winter.


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## Ralph III (Oct 7, 2011)

Bud9051 said:


> The RB should be installed up below the rafters and not buried. Its benefit is that solar energy has only one primary path from the bottom of the roof down to the insulation below and that path is by radiation. A radiant barrier works in both directions to block radiant energy transmission across the attic air gap. in the winter it would reflect heat back down away from the roof.
> 
> Bud


 Bingo! I was wondering about this because in the winter it would actually be beneficial to have some heat in your attic. Thanks for posting that because it encourages my consideration of RB.





> user_12345a "you don't need a radiant barrier unless you have air ducts in the attic.....you should go R-50".


 That is indeed part of my consideration. All of our ducks are within the attic space. I've never seen a residential home in the deep south that didn't have the duck-work in the attic, otherwise very rare. I desire to have R-45 barrier within the attic which is the number most referenced around here. R-50 would be even better though!

Thanks,
Ralph


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## NotYerUncleBob2 (Dec 29, 2017)

Ralph III said:


> All of our ducks are within the attic space. I've never seen a residential home in the deep south that didn't have the duck-work in the attic, otherwise very rare.


The ducks should be in the pond and your ducts that are in the attic should be well insulated. Otherwise you're heating and cooling one of the coldest/hottest spaces in the house. Never mind that nobody else is doing it right, there are a lot of things that are consistently done wrong simply because it's easier/cheaper to do it wrong.


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## Ralph III (Oct 7, 2011)

NotYerUncleBob2 said:


> The ducks should be in the pond and your ducts that are in the attic should be well insulated. Otherwise you're heating and cooling one of the coldest/hottest spaces in the house. Never mind that nobody else is doing it right, there are a lot of things that are consistently done wrong simply because it's easier/cheaper to do it wrong.


Hello UncleBob,

Can you then confirm that it is ok to wrap the existing duck-board with unfaced or faced fiberglass insulation? Can you also confirm that it is ok to cover the flex duck with blown-in insulation?

I'd really like to increase the duck-work efficiency and the above methods could be done fairly quickly. I however do not want to create some sort of mold issue, due to condensation, which is the concern as I've read. 

Thanks,
Ralph


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## user_12345a (Nov 23, 2014)

for best performance but highest initial cost, insulate the roof deck and close soffit vents.

otherwise, yah, do what you can with radiant barrier.

The danger of buying flex duct in insulation is you make the outer barrier of the duct too cold, resulting in condensation and water damage - rot, mold.

This can also be an issue with fiberglass on duct work unless the wrap is 100% air tight which it never is; would have better luck actually gluing foam to duct board to beef up the insulation.


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## NotYerUncleBob2 (Dec 29, 2017)

This is a duck:








This is a duct:


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## RochesterBen (Feb 5, 2018)

NotYerUncleBob2 said:


> This is a duck:
> 
> 
> 
> ...


:vs_laugh: I've been waiting for this!


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## Porsche986S (Dec 10, 2017)

My 2 cents keep the change :smile: In my last garage I had a mini split AC/heat system . My rafters were 2x4's and I had architectural shingles on the roof . In GA. just like AL. it gets very hot in the summers . That attic space just is roasting hot . I had conventional soffit vents and a ridge vent . On a whim I purchased off of Craigslist several rolls of radiant barrier . 

I simply stapled the radiant barrier to the bottom of the roof rafters creating a 3 1/2 " " air space " between the radiant barrier and the bottom of the roof deck . I only had R13 fiberglass kraft paper for insulation because I was going to add blown in but never did before we sold that house . I ran that mini split winter and summer and tracked electric cost usage , it averaged $20.00 a month more after I added the mini split to the garage . The garage was 24 x 26 with a peaked ceiling of 14 ' . I know for a fact that the addition of the radiant barrier helped . 

I can't say that it radiates 93 % back through the roof as the manufacturer claims but common sense say's that it will help . I am now building another two bay man cave garage that will also have a mini split for AC/heat along with a metal roof . The building started as a pole barn and also has 2x4 rafters with 2x4 perlons then the metal roof . I will again install radiant barrier to the underside of the rafters below the perlons . In this case I will have a 5 " air space between roof panels and barrier . Full length ridge vent and full length soffit vents . Sorry for the long reply just wanted you to know my experience with the barrier .


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## Ralph III (Oct 7, 2011)

Thanks Porsche!

I can get some radiant barrier fairly affordable off Ebay so will look to install some.

--------------------------------------------------------------------------------

In regards to insulating "duct" work in the attic. As I've read and stated here, you risk creating moisture/mold by wrapping insulation around the duct board or flex duct. However, what about just draping fiberglass (un-faced or faced) insulation over the duct work? Wouldn't that be similar to insulating the roof line but instead you are placing the barrier directly over the duct work? 

God Bless,
Ralph


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## Porsche986S (Dec 10, 2017)

Just to be clear I am a fan of the radiant barrier but it is just one piece of the puzzle . You rarely will get hurt by adding more insulation . Had I kept the previous house/garage I would have added more insulation to that garage ceiling as 3 1/2 " R13 is just not enough . In my new garage I will go at least R30 or more PLUS the radiant barrier .


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