Getting new insulation blown in - radiant barrier paint option???

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Forum topic by Mike_D_S posted 06-17-2016 01:48 AM 1137 views 0 times favorited 9 replies Add to Favorites Watch
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482 posts in 2361 days

06-17-2016 01:48 AM

So the insulation in my attic was never that deep to start with and with the settling over the years, it’s getting pretty bare in spots. I got a quote today for about $1.15/sq ft for R49 including eave baffles, hanging the existing AC ducts with strapping and installing an attic tent over the access ladder.

For me the insulation is a no brainer as last year, my electric bill in the summer is $200-$225 more than the winter, so I’ll make my money back in a couple of years.

But he also offered to do a radiant barrier paint for $400. While I know the paint is not as effective as the foil, I also figure that even if it only rejects 20%-30% of the radiant heat gain it’ll probably help out, but the payout will probably be longer given that the insulation will be doing the bulk of the work.

In any case, he wasn’t pushing it and was happy to do the job without it, so I don’t get the feeling it’s hidden profit or anything. So the question for my fellow LJ’ers is whether anybody has had this done and if they think it made a big difference?

I’m leaning towards just having it done, but I’m interested in opinions from people who’ve done it.


-- No honey, that's not new, I've had that forever......

9 replies so far

View DrDirt's profile


4464 posts in 3889 days

#1 posted 06-21-2016 01:01 PM

Haven’t done it – These guys focus on the foil, but also give test results of the different paint solutions and their effectiveness, with links to the study.

-- “The two most important days in your life are the day you are born and the day you find out why.” Mark Twain

View Bill White's profile

Bill White

5072 posts in 4107 days

#2 posted 06-21-2016 01:38 PM

I’ve seen results of shingle damage from the use of radiant foil or paint.
Makes me think it is not such a good idea as a retro fit.


View patcollins's profile


1687 posts in 3012 days

#3 posted 06-28-2016 12:53 AM

I have my suspicions about radiant barrier inside of an attic having much of an effect. If it was actually exposed to the sun of course it would, however with the shingles and roof decking blocking the direct radiation I am skeptical.

On the other hand it is pretty cheap and you are not out much

View Mike_D_S's profile


482 posts in 2361 days

#4 posted 06-28-2016 04:59 PM

I decided not to go with the radiant barrier for a couple of reasons. First I do think that re-radiated heat probably does shorten shingle life to some extent, but I believe that roof decking and shingle temp are probably a lot more strongly impacted by attic ventilation. So I decided to skip the radiant barrier and have the guy install some additional soffit vents to increase my under-ventilated attic.
The second reason was the age of my roof. I will need to re-roof in no more than 3 years and decided I would bite the bullet and re-deck with techshield at that time.

Radiant barriers work on the emissivity of the material. There is a metric ton of math in the background, but basically the radiant energy from the sun is absorbed by the roof and transmitted primarily by conduction (direct contact) through the shingles and roof decking until it gets to the radiant barrier (assuming a radiant barrier paint or foil in direct contact with the roof decking like techshield). The emissivity of the radiant barrier is low meaning that the radiant barrier re-radiates only a portion of the heat it absorbed. The rest of the heat is lost from the radiant barrier via conduction back into the roof deck (which I actually referred to technically incorrectly above as re-radiated), convection through air contact into the attic space raising the air temperature.
So on the top of a shingle, you want high reflectivity which means the solar radiant energy (heat flow) is rejected from ever entering the barrier in the first place and on the underside of the roof you want low emissivity which means the heat energy is not radiated into the attic space.

The explanation above is probably not 100% correctly technically worded, but it gets the point across.

In my opinion, the order of these in terms of importance would be:
1. Good insulation. Slowing down the heat transfer into the house is the place to start.
2. Proper ventilation. This only applies to vented attics obviously, but getting the air temp down is also helpful.
3. Radiant barriers. You’ve got good insulation and a well ventilated attic, so you’ve done something about conductive and convective heat gain, so addressing radiant heat gain can help as well.

So I decided to focus on #1 and #2 and worry about #3 later.


-- No honey, that's not new, I've had that forever......

View patcollins's profile


1687 posts in 3012 days

#5 posted 06-30-2016 01:09 AM

Mike, I understand the concept, 3 of the 8 classes I took to get my masters degree were in heat transfer, it is the practicality that I question. Generally radiation as a heat transfer mechanism is small enough to be ignored unless one of the surfaces is glowing hot like the sun. Since the barrier itself is inside under the shingles and roof decking the amount of heat for it to reflect is small compared to what convection and conduction would transfer.

I have used some of the reflective bubble wrap in the past to insulate my old cheap fiberglass garage door and it worked well, but that was because the sun shone right through the fiberglass, even with three coats of paint on the outside and three on the inside, didn’t do much to keep the heat inside the garage in the winter. I finally replaced that garage door with a steel door with styrofoam insulation and the garage stays even cooler in the summer and much warmer in the winter. Judging from my garage door radiant insulation actually reflecting the sun not keeping the garage as cool as 22 gage steel and an inch and a half of styrofoam is what gives me serious doubts. It is a west facing garage door, so after 5 PM in the summer the sun brutally beats on my garage door. Before I did anything to it my garage was regularly in the 90s.

Better attic airflow would most likely dwarf anything a radiant barrier inside the roof structure could do. It sure would work great as a tent over the house though.

View kiefer's profile


5619 posts in 2814 days

#6 posted 06-30-2016 01:27 AM

Insulation is good to a point but ventilation upgrade is a must when adding insulation gable vent fans are used a lot around here but wind powered fan roof vents do a great job also and no wiring required .


-- Kiefer

View Mike_D_S's profile


482 posts in 2361 days

#7 posted 06-30-2016 02:58 AM

Agree that radiant transfer is the least impactful.

If you want to get serious about radiant heat rejection on the exterior, then a white roof is probably the way to go, but my HOA would probably have a fairly major league conniption.

I got the insulation put in on Monday with soffit vent baffles. It started storming, so they came back today to install the additional soffit vents and the attic tent. We have one of the fancy smart meters, so I can just go online and pull up my daily usage and look at the 15 minute usage for single days.

So after I get a week or so of time with the new insulation, I’ll go in an take a look to see if it made much of a noticeable difference. I’m not expecting miracles, but going from 6 or 7 inches on average to 15 or 16 inches should make some difference. What I expect to see instead of lower max power consumption is the AC usage ramping up slightly later and then ramping down a little sooner. Once I get a weeks history, I’ll go in and pull a report and post it here.

The big issue is that we have been having a lot of intermittent rain and some heavy cloud cover, so a weeks worth of data may not be very statistically valid.


-- No honey, that's not new, I've had that forever......

View patcollins's profile


1687 posts in 3012 days

#8 posted 06-30-2016 11:53 PM

I’ve wondered for years why haven’t there been any advances in roof decking material, plywood/foam core decking material would greatly increase the energy efficiency of a house. They make a foam core vinyl siding that adds insulation to your house, but most importantly doesn’t look nearly as cheap as normal vinyl siding.

View Mike_D_S's profile


482 posts in 2361 days

#9 posted 07-07-2016 11:58 AM

So it’s only been about a week and half, but we have been having some pretty hot days down here in Houston with the last 3 or 4 days being mid 90s (feels like 105ish with the humidity, etc). I have one of the smart meter and it records my usage every 15 minutes.

Pulling my July 2015 usage, on the hot days, my average usage us about 85 kWh/day. For the last week, my usage has been more like 65 kWh/day. When you look at the intra-day chart, you can see that last year when the AC kicked on in the afternoon, it ran for at least 15 to 30 minutes with pauses between cycles of about 15 minutes. Now the AC runs for less than 15 minutes then kicks off and the next cycle doesn’t start for about 30 minutes.

So I can positively say the new insulation significantly slowed down the heat gain into the conditioned space, reducing my usage per day by about 25% on the hot days.

-- No honey, that's not new, I've had that forever......

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