solar roof folks that re-decked their roof: did you opt for radiant barrier?

[funkadelic]

Our roof is skip sheathing so we'll need a new (solid) deck added after tear off. I've had radiant barrier in our previous house (done post roof install) & it made a noticeable difference in the attic temps (that house also had the HVAC in the attic, which our current one doesn't).

Checked w/my project manager and the decking w/radiant barrier is only a additional $600. Those black tiles are going to love to absorb heat, so radiant barrier sounds like a good idea, but anyone have a bad experience or heard otherwise against radiant barriers w/the solar roof?

 

[trautmane2]

I'm not familiar with radiant barriers and don't recall reading about other solar roofs with them. I am interested in how the solar roof affects my attic temperature and wish that I had the forethought to measure temperatures before my install to compare later.

My understanding is that the temperature of the PV portions of your roof are important, so I could imagine that if a radiant barrier increased the temperature of your PV tiles that could be bad.

 

[wjgjr]

I'm not familiar with radiant barriers and don't recall reading about other solar roofs with them. I am interested in how the solar roof affects my attic temperature and wish that I had the forethought to measure temperatures before my install to compare later.

My understanding is that the temperature of the PV portions of your roof are important, so I could imagine that if a radiant barrier increased the temperature of your PV tiles that could be bad.

I am in the same boat about wishing I had measured the temperature before install - I now have a sensor to check the temperature, but I don't have something to compare it to. Tesla used to claim the roof would be more insulating (though I don't see it on their site currently) and I think that might be true in that I noted (with mostly summer experience) that the house does not seem to cool down as quickly. I do not think the house (absent use of A/C) or attic got hotter during the day. So far, seems our attic gets no more than about 15 degrees warmer than our house in summer and 10 degrees cooler in the winter, but I do wish I had something to compare to to know if the roof made a difference (and it might also be an issue of insulation between 2nd floor and roof.)

Good point on the radiant barrier - no idea if it would have an impact on the tiles

 

[jboy210]

I did not pay anything extra for a radiant barrier since we already had OSB sheeting The attic is quite cool even in mid-summer. Some of this may be due to the underlayment they were using, Firestone Clad-Gard SF FR, at the time. But, I can't see much downside to the barrier, other than the small expense. If I were choosing I would get the radiant barrier.

 

[BGbreeder]

I think that in your (our) climate, you won't regret a radiant heat barrier. The only downside beyond the small cost that I know of is that it will attenuate RF signals. So, your cell, Starlink in the attic, )), and over the air TV, signals may be affected.

Make sure that the radiant heat barrier has an air gap on one side for greatest effectiveness. Not all roofers understand how to install it. If you have something touching both sides, it doesn't work.

All the best,

BG

 

[charlesj]

Do these radiant barriers have an air space in front of it? If not, I don't see how it would reflect radiant energy above it.

 

[jboy210]

Do these radiant barriers have an air space in front of it? If not, I don't see how it would reflect radiant energy above it.

Looking online it looks like the barrier is directly applied to the OSB boards. And you put the metal foil barrier side on the framing. The goal is to minimize heat transfer to the space below.

 

[funkadelic]

My understanding is that the temperature of the PV portions of your roof are important, so I could imagine that if a radiant barrier increased the temperature of your PV tiles that could be bad.

i recall reading some tests Oak Ridge National Laboratory did years back on radiant barriers and the effect on the roof deck temp and I think it was something like 2-10 degree F increase (basically a nominal increase). That said, that temp increase probably has some effect on the PV production, but in turn I'd use less A/C due to less heat radiating into the home, so :shrug: maybe a wash.

Make sure that the radiant heat barrier has an air gap on one side for greatest effectiveness. Not all roofers understand how to install it. If you have something touching both sides, it doesn't work.

Do these radiant barriers have an air space in front of it? If not, I don't see how it would reflect radiant energy above it.

the plywood will have it on one side, but now that i think of it, it'll be applied over the existing skip sheathing, so it'll only have an air gap in basically 1/2 of the normal area, so it wouldn't be as effective if the skip sheathing wasn't there. i guess any reduced attic temp is better than no reduced attic temp

 

[arnolddeleon]

I think that in your (our) climate, you won't regret a radiant heat barrier. The only downside beyond the small cost that I know of is that it will attenuate RF signals. So, your cell, Starlink in the attic, )), and over the air TV, signals may be affected.

Make sure that the radiant heat barrier has an air gap on one side for greatest effectiveness. Not all roofers understand how to install it. If you have something touching both sides, it doesn't work.

All the best,

BG

I had radiant barrier installed 18 years ago when I was re-roofing. We also had skip sheathing. Our roofer installed 2x2s over the rafters so the the radiant barrier won't be on the skip sheathing directly. It also conveniently created an air channel to feed our ridge vents.

https://photos.app.goo.gl/YBScAfLQWwyuhBQR7

I've joked before about living in Faraday cage between the radiant barrier and the metal lattice from the stucco in the walls.

 

[slcasner]

I have skip sheathing, too. In part because I have a concern about total roof weight staying under a 7.5lb/sq.ft. limit, I would want the skip sheathing to be removed before the new deck was applied. Would that be a very uncommon request? (I'm just in the planning phase at this point.)

 

[jboy210]

[QUOTE="funkadelic, post: 5208213, member: 88795"It'll be applied over the existing skip sheathing, so it'll only have an air gap in basically 1/2 of the normal area, so it wouldn't be as effective if the skip sheathing wasn't there. i guess any reduced attic temp is better than no reduced attic temp[/QUOTE]

I have never seen OSB sheets installed over skip sheeting. The jobs in my neighborhood have all started with removing everything down to the framing and them putting OSB decking on top of the framing.

 

[woferry]

On my v2 install they just attached the OSB to the existing skip sheathing.

 

[charlesj]

Looking online it looks like the barrier is directly applied to the OSB boards. And you put the metal foil barrier side on the framing. The goal is to minimize heat transfer to the space below.
...

Thanks. Heat transfer physics is somewhat complex. Having that foil on the underside while it may not radiate as much as the OSB board would, it radiates and the attic air will also be heated by convection from the foil surface into the attic. Since the attic floor is insulated most likely, heat will build up to equilibrium temp, heat escapes at the rate of build up.
The OSB itself will get warmer as it cannot radiate into the attic as fast as it would without that foil. What will that roof temp do?
I am skeptical of that foil especially cost effectiveness.

(10) Proof That Foil Radiant Barriers Don't Work Directly Under Roofing Shingles - MUST Have an Air Space - YouTube

 

[charlesj]

Forgot to add that the part that applies starts at 2:35, at 3:00 is what happens with this decking and foil underneath it and ends at 3:30. Rest is not applicable here.

 

[BGbreeder]

FWIW: We put OSB, then 2" of foam, radiant layer, air gap that ventilates the roof, OSB, underlayment, battens, tile.

In hindsight, I would have run the tile battens differently to better shed the water that runs down under the tiles, but that's not germane to this. (Battens on spacers to lift them to allow better drainage.)

The data that I saw makes it clear that radiant layers only work with air gaps, ideally on both sides to minimize heat gain. (Think across 2x4 on a wall or ceiling, with air on both sides.) The installer certainly did not understand how it worked. He thought it was like insulation, roll it out and it does it's thing.

All the best,

BG

 

[charlesj]

F... The installer certainly did not understand how it worked. He thought it was like insulation, roll it out and it does it's thing.

All the best,

BG

Unfortunately there are lots of myths out there about radiant barriers. Heat transfer and reflection of radiant energy is complex.
if you have material on both sides of the foil, it is a conductor of energy, nothing more, a waste of $$$ and is an expensive underlayment for shingles.

 

[charlesj]

i recall reading some tests Oak Ridge National Laboratory did years back on radiant barriers and the effect on the roof deck temp and I think it was something like 2-10 degree F increase (basically a nominal increase). That said, that temp increase probably has some effect on the PV production, but in turn I'd use less A/C due to less heat radiating into the home, so :shrug: maybe a wash.
...

If that radian barrier is attached to the underside of rafters, yes, it will reflect the radiant energy of the roof decking back to the decking and adjacent rafters. It has to go someplace, hence that temp rise Oak Ridge measured. The roof under the panels should not get any radiant energy from the sun so the decking would be perhaps ambient air temp?

ps.

Am discovering that the reflective foil attached to roof deck facing into the attic relies on emittance, the ability to radiate or not. A theoretical black body is assigned as 1, aluminum foil is 0.03-0.05. So, it doesn't like to radiate but energy has to go or build up internally in that ply decking. The foil would also conduct to the air in the attic perhaps at a low rate?