Radiant Barrier?

[Texas Proud]

Did you use a 'film' type RB? When my sister was having her siding replace recently they had this roll of film they said is all that was needed... I had an insulated panel with an foil side for mine...

And if you put the film in the attic, do you need to cover the whole area or can you leave 'gaps'? Again, just curious...

[samclem]

Quote:

Originally Posted by Texas Proud And if you put the film in the attic, do you need to cover the whole area or can you leave 'gaps'? Again, just curious...

Gaps are no big deal (it's not the same situation as convective heat transfer, where a small gap can lead to a big heat loss). If you cover 50% of the area, you get 50% of the benefit. In fact, there will have to be some gaps or some other provisions to allow the air in the attic to get out of the ridge/gable vents and to be replaced by the air from the soffit vents. These vents do more than cool the attic, they are essential parts of the moisture management system in homes. If the air in the attic can't get out, neither can the moisture that builds up from the human activities in the house. If the attic is cooler than the house, the moisture in this air condenses on the insulation and wood resulting in mold and rot.

Rule of thumb: If you've got a good vapor barrier in your ceiling and a very tight house with little airflow into the attic, the total net free vent area (equal amounts of soffit vents and upper gable/ridge vents) should equal 1/300th of the ceiling area. Without an effective vapor barrier in/immediately above the ceiling and with more typical "tightness", the net free vent area should total 1/150th of the ceiling area.

[eridanus]

I think it's too late in the season in Texas to install radiant barrier.

[HFWR]

Quote:

Originally Posted by eridanus I think it's too late in the season in Texas to install radiant barrier.

It was only 132.8 in my attic yesterday...

[ls99]

A somewhat off the wall, roof really, observation. If you have a roof largely covered with solar cell panels, presumably on brackets, then the roof is mostly in a shade. Solar panels not being highly efficient electrical generators, maybe 20% efficient, generate a good bit of heat. This heat coupled with with air space between it (them) and the roof set up some nice convective air flow.

With solar panels on standoffs, the roof is mostly shaded, the attic temperatures should be near outdoor ambient temperature.

An interesting test may be to measure temperature in the air space between solar panels and roof, attic temperature and temps between any radiant barrier and roof underside.

[ziggy29]

Quote:

Originally Posted by eridanus I think it's too late in the season in Texas to install radiant barrier.

There's a company that advertises on Austin radio that they will do radiant barrier jobs for half price through the end of May. A couple of thoughts on that:

(1) I suspect what's really happening isn't a half-price job before June, but rather a double-price job during the summer months.

(2) Given the mid-July weather Texas has had over the last week, it really must suck to be charging springtime prices for summertime conditions up in the attic for those guys...

[Nords]

Quote:

Originally Posted by ls99 A somewhat off the wall, roof really, observation. If you have a roof largely covered with solar cell panels, presumably on brackets, then the roof is mostly in a shade. Solar panels not being highly efficient electrical generators, maybe 20% efficient, generate a good bit of heat. This heat coupled with with air space between it (them) and the roof set up some nice convective air flow. With solar panels on standoffs, the roof is mostly shaded, the attic temperatures should be near outdoor ambient temperature. An interesting test may be to measure temperature in the air space between solar panels and roof, attic temperature and temps between any radiant barrier and roof underside.

For anyone tempted to try this, I'd recommend not allowing any gaps between panels that could allow sunlight to hit the roof. I suspect that the sun does just fine at heating our uncovered 10% of the roof directly and allowing conduction to heat the remaining 90%...

We just put radiant foil in our south wall but we don't really have enough hot-weather data to claim victory yet. If that doesn't do the trick then the next shading step would be mounting south-facing panels on a west-facing roof, which is gonna be expensive.

[HFWR]

Well, in a fit of bassakwardness, I decided to proceed with putting radiant barrier in my attic. I can't get a roofer in a timely manner to redo the ridge vents, and I want to do some tidying up in the attic before blowing in more insulation. Since I'll be back to w*rk soon, and since Lowe's has been sending me 20% off coupons, I bought a 4' x 25' roll today, and installed it without much fanfare, aside from the 110 degree attic temperature...

It will take three more of the aforementioned rolls to complete the "living" wing of the house. The "bedroom" wing has more obstructions, so I think I'll buy 2' wide rolls for that.

It'll be interesting to see how this affects the attic temp. Max temp today was 136...

And I'll be doing any future work very early in the morn!

[Leonidas]

Quote:

Originally Posted by HFWR Since I'll be back to w*rk soon, and since Lowe's has been sending me 20% off coupons, I bought a 4' x 25' roll today,

Was the brand name Reflectix? Just checking to see if your Lowe's is selling the same stuff. Buying a couple of rolls is on my list of crap to do tomorrow.

Quote:

Originally Posted by HFWR and installed it without much fanfare, aside from the 110 degree attic temperature...

Only 110? Did you go up there at 4 AM?

[HFWR]

Quote:

Originally Posted by Leonidas Was the brand name Reflectix? Just checking to see if your Lowe's is selling the same stuff. Buying a couple of rolls is on my list of crap to do tomorrow. Only 110? Did you go up there at 4 AM?

Yes, Reflectix. About $50 for the 4' wide roll. Basically, a roll of bubble wrap with foil on both sides.

It was 115 in the attic at 7:30PM when I started, and was all the way down to 100 by 8:30. This morning it was 80-something...