Q: Can I install radiant barrier if I have spray foam up on the rafters of my attic? How does AtticFoil work with spray foam?
We get this question a lot – someone is usually in the process of new construction and they want to incorporate both the AtticFoil radiant barrier AND some spray foam in their attic space. First, I’d like to say that the two products compliment one another very nicely. Foam isn’t going to perform like radiant barrier because (like traditional insulation) foam slows conductive heat gain/loss, but it doesn’t actually stop it. Radiant barrier foil is different than spray foam because AtticFoil’s reflectivity of radiant heat is 97%, meaning you stop the heat from ever even entering the home.
That being said, The foil is most effective at this when it’s closest to the outside layer of the home. For this reason, adding the foil with the required air gap it uses, can be tricky. There are several ways to go about this. First, you could use a traditional Cathedral Ceiling Method for Installing Radiant Barrier. In that case, you’d have some spacers added to the bottom of the roof deck, then the foil, then an intermediate layer like foam board, then spray foam over the foam board. The reason for the intermediate layer is because in many cases, the spray foam does not attach well directly to the pure aluminum radiant barrier.
Another, more economical option, is seen in this scenario pictured below:
The roof decking is being laid on (or replaced) and you can clearly see that the foam in between each rafter cavity leaves a small air gap between the foam and the deck itself. AtticFoil radiant barrier only requires a 1/2 to 3/4″ air space in order to work effectively; in this situation, you could run the foil across the top of the rafters and staple it down before new decking way laid on top. Another option would be to attach the foil directly to the bottom of the decking (or use a pre-manufactured radiant barrier decking) and then lay the decking down, with the foil facing the small air gaps. This is a perfect example of using radiant barrier and spay foam together and this is an ideal installation because the foil will serve as the first line of defense, reflecting 97% of the sun’s radiant heat from entering the layers below the foil and then the little bit of heat passing through the foil will covert to conductive heat and the spay foam will significantly slow down the transfer of the conductive heat into the attic and/or home as your second line of defense.
Q: What about adding the foil from the inside, below the rafters filled with spray foam?
For blocking heat (ie. summertime benefit) the foil is not going to offer much benefit being placed that far inside the envelope. There might be some benefit for heat retention in winter, since the foil would be right near the heat leaving the interior of the home, but generally people are adding foil to roofs/attics with spray foam because they are trying to lighten the heat load in the summer, even if they also want the benefit of keeping heat in during the winter.
Bottom line: for a situation where you want to stop heat gain and/or you are looking for year-round benefit from the radiant barrier foil, it’s best to place the foil closest to the exterior of the home/building.
I’ve talked about attic ventilation issues before, in fact it’s probably one of the most popular posts on the blog to date. In that article I touch on this issue and in this video I decided to go a little more in depth based on the frequency this problem comes up in conversations.
What do you think? Is this a problem you’ve personally experienced in your attic? Did you have situations like the examples in the video? Tell me about it in the comments!
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People often ask me, “How does (insert brand name here) compare to AtticFoil® Radiant Barrier?” As a response, I decided to make this video to compare AtticFoil® with a brand sold in local home improvement stores, ie. Home Depot and Lowes, called Enerflex® Radiant Barrier. There are two main differences between the two that have an impact on your decision of which one to purchase.
I think the differences are pretty clear. Now, tell me which one you’d rather have in your attic?
The one question I probably get asked most often is: “What all do I need to know to install AtticFoil Radiant Barrier Foil Insulation?”
The big concept is you’re trying to get a piece of radiant barrier foil between the roof and the insulation; the goal for heat rejection in the summer being to keep attic insulation cooler by preventing it from absorbing radiant heat from the roof. In this video I will explain it as quickly and concisely as I can.
So, in summary here is what you need to focus on:
- Get the foil between the insulation and the roof line.
- Staple it up across the rafters; it doesn’t have to be smooth or pretty. You can use 48″ wide radiant barrier foil to work horizontally, or 26″ wide radiant barrier foil to work vertically.
- The staples are standard size – 1/4 or 5/16th size work fine.
- Cover as much as possible; the more space you can cover, the better your results. Even partial coverage works!
- Allow ventilation a free path to flow in to the attic from down low at the soffits and escape the attic up high, near the ridge (via a ridge vent, gable vent, attic fan, etc.).
If you would like to see some photos of finished installs, I recommend you take a look at these Do-It-Yourself radiant barrier foil installation photos.
Still have questions? Leave me a comment below!
How does TechShield® compare to AtticFoil®? I get this question just about everyday. First, what is Techshield®? Techshield® is a roof decking material – usually OSB that has a sheet of aluminum foil laminated on one side. Techshield® is made by LP Building Products or Louisianna-Pacific Corporation and is probably the most popular brand of radiant barrier decking. Other brands are Solarboard by Norbord and Thermostat by Georgia-Pacific.
The main difference between Techshield® and AtticFoil® is that Techshield® is used almost exclusively for new construction or whenever a roof deck is being replaced. AtticFoil® is most commonly used inside the attic of existing homes.
For most new construction projects, I recommend using TechShield® (or another brand). The products work well, the cost of upgrading from regular OSB decking to radiant barrier decking is pretty minimal and there is NO additional labor cost since the Techshield® is installed just like a regular OSB roof deck. The foil side MUST face the attic air space (foil facing DOWN). If you install the foil facing up and then put roofing felt and shingles on top, you will get NO benefit.
Do they work the same? Yes and No. Techshield® works off a quality called EMISSIVITY. Basically, this is the ability to NOT convert energy to radiant heat. The roof deck will get hot and would normally want to emit radiant heat both upward and downward. By putting the foil on the bottom of the roof deck, it greatly reduces the ability of the deck to radiant heat downward. This is similar to taking two HOT baked potatoes and wrapping ONE with foil. The potato with foil will stay hot longer since the foil will reduce or slow down the amount of energy (heat) emitted.
If you were to wrap only half the hot potato with foil, then you have something similar to TechShield®. In this case, the potato would emit more heat upward through the area without foil compared to the side with foil. By reducing the heat emitted into the attic, this causes the attic contents (wood, insulation, framing) to be cooler. Combined with adequate attic ventilation the attic air temperature will also be significantly cooler.
AtticFoil® works off the quality of REFLECTIVITY since there is an airspace between the source of heat (the roof deck) and the foil. The radiant heat is STILL emitted by the roof deck, but hits the foil and is reflected back keeping everything below the foil cooler since the items never get a chance to absorb the radiant heat coming off the roof.
Don’t worry about overheating the roof deck – it will only increase between 2º-10º degrees.
By stapling to the bottom of the rafters, you will usually achieve a greater reduction in overall BTU’s coming into the attic compared to radaint barrier decking.
Can I make my own radiant barrier decking like TechShield using AtticFoil®? You can and it is VERY easy! Simply roll AtticFoil® Radiant Barrier foil out on standard 4’x8’ sheets of OSB or plywood. Then use a hammer staple and ¼” staples to tack it down. When installed over the rafters it will jut like a piece of radiant barrier decking. Or, if you want to install a radiant barrier for new construction, you can run the foil across the rafters and let it “droop” down about 4-5” between the rafters. By doing this method, the AtticFoil® will work off the reflectivity quality compared to the emissivity quality of foil. Be sure to leave a gap at the top and bottom of each rafter run for air to find its way up and out of the attic.
It happens every year. I get a few calls from customers who say “I installed AtticFoil® – it’s a great product and I could really tell a difference in the summer, but now I am having trouble getting my house to cool off at night. Is the radiant barrier working right?”
Remember heat ALWAYS flows from hot to cold. So, if it is hotter outside than inside your home, heat is flowing IN – typical summer weather. Or, if it is cold outside and warm inside heat is flowing OUT – typical winter weather. The problem is that there is typically a period in the spring and fall when the heat flow FLIPS in the middle of the day. Example: It may be warm – 80º in the afternoon and heat is flowing IN, then it cools off at night down to 50º heat is now flowing OUT of the house.
The challenge is that radiant barrier does not know that you want your home to cool off on nights like this. It is just doing its job. Radiant Barrier will ALWAYS reduce heat flow from hot to cold – even if you don’t want it too. In the afternoon, AtticFoil® radiant barrier is reflecting heat OUT of the home to keep the home cooler; at night it will help KEEP the heat inside to maintain a warmer home.
Yes, it is true. There are some days each year when an installed radiant barrier will cause the opposite effect of what you may want, but the solution is usually pretty easy to solve. In the evening, once the air temperature cools off, you need to flush out the heat your home is holding inside. The best way to do this is to crack open some windows and run bathroom exhaust fans and/or a kitchen vent hood fan to help pull out the warm air and heat inside the home. I even heard of one customer who would pull down their attic staircase in order to allow air to enter through the windows and escape through the attic.
Fortunately, this uncomfortable period usually only last for a few weeks before the weather changes to consistently hot or cold. Therefore, the overall benefit of installing a radiant barrier far outweighs this short-term inconvenience.
Interestingly, we have many customers who do not have air conditioning. They live in mild climates or higher altitudes that cool off significantly at night. However, even in these areas there are usually a few weeks of the year where they wished they did have air conditioning. We often hear that installing a radiant barrier has made a huge difference during this period and keeps their home comfortable by not heating up as much – even on the hottest days.
Last year I had the adventure of building a new home for my family. I used Wayne Atkins of SterlingBrooks Custom Homes. Wayne and I have been friends for a long time and he “gets it” when it comes to building a high performance, extremely well built home.
We decided to build a full foam encapsulated home with a non-vented attic. We used 6” of open cell foam on the bottom of the roof deck, sealed the attic and spray foamed all the exterior walls. Basically we built a giant Styrofoam box that was practically airtight. However, what we really wanted was a giant Styrofoam box wrapped with foil. Foam is great for slowing conductive heat, but in a hot Texas summer we really needed to put the home in the shade (shade from the radiant heat, not the light) – we NEEDED a RADIANT BARRIER!
Since the foil radiant barrier is the first line of defense against radiant heat and the foam is the 2nd line of defense against conductive heat, combining the two would result in the ultimate reduction of heat flow.
The challenge with a spray-foam enclosed attic/roof is that you cannot typically have a radiant barrier because of the lack of a proper air gap. The shingles are connected to the roof deck, and the deck is connected to foam. Some heat will still flow by conduction through the roof/foam assembly. With this type of assembly there is no air space for the heat energy to jump and be converted to radiant heat. You CANNOT just put foil under shingles on a typical roof and have a radiant barrier. You MUST have an air space on ONE side of the foil. Radiant heat by definition is heat transfer by non-contact across an air space. Without an air space you cannot have radiant heat and without radiant heat you cannot have (or install) a radiant barrier. Once you can convert heat to its radiant form you OWN IT! Conductive heat can only be slowed down, but radiant heat can be REFLECTED.
So how can you incorporate an air space and a layer of foil into a roofing system?
The options are limited. Metal, tile, concrete or other raised- type roofs will work and you can install a radiant barrier on top of the roof deck below the raised roof. The problem is that these type roof systems typically run $4-$5 per square ft. where a traditional shingle roof will run about $2-$3 per square ft.
I decided to get creative and build a double-deck roofing system. This system provides the best of both worlds: we get the radiant barrier and we get it at a reasonable cost.
This is how we did it: first we installed the first (bottom) deck. We stopped it about 10” from the fascia board. This gap allows air to come from the continuous soffit vents and from behind the brick into the soffit cavity and then flow up between the two roof decks. Then, we installed AtticFoil® Radiant Barrier Foil directly over the roof deck. You can use either the double-sided radiant barrier 48” product or a single-sided 60” radiant barrier product (the foil side MUST be facing up, toward the sky).
NOTE: We used a felt marker to mark where the rafters were on the foil. Without the marks on the foil it would have been hard to find each rafter for the next step.
Next we installed furring strips. We used 2×2’s, but you could use 1×2’s if you want. You will want at least a ¾” gap or better between the two decks. We were careful to leave gaps at the tops and bottoms of all hips to allow air to flow freely from the soffits to the peaks.
Then we then installed a foil-backed roofing deck material called TechShield on top of the furring strips. This was our “normal” deck that we tied into the fascia boards and then installed traditional roofing felt and shingles.
Ventilation note: Since the airspace between the two roof decks was only 1 ½” we did not need a whole bunch of ventilation. For every 1,000 square feet of roof deck, there is only about 125 cubic feet of space between the two roof decks. The roof was a hip style roof with a limited amount of ridge, so a ridge vent was not an option. I like 14” wind turbines. They work well, are cost effective and easy to install. When you combine the draw caused by the Bernoulli effect of the wind turbines with the natural stack effect of the air rising (or being pushed up by the cool air) the air between the two roof decks is easily being changed several times per minute. This airflow has a cooling and drying effect on both decks.
Nothing short of amazing!! We were under construction during the brutal heat wave of the summer of 2011. Example: On one hot-sunny day (about 100ºF) the top surface temperature of the shingles was close to 180ºF (using an IR thermometer to measure). The BOTTOM surface temperature of the lower deck was 110ºF. ONLY a 10º INCREASE!! Virtually NO heat was making it through the system. In fact, the construction workers would usually eat lunch INSIDE the home since it was relatively so comfortable.
By forcing the heat to JUMP the airspace between the two decks we OWNED the radiant heat. The top deck with the foil on the bottom worked off the EMISSIVITY quality of foil radiant barrier (the ability NOT to convert energy to radiant heat) and the AtticFoil® installed below the airspace on top of the bottom decked worked off the REFLECTIVITY quality to reflect the radiant heat.
I would speculate that the nominal 10º temperature increase in the bottom deck was mostly caused by thermal bypass through the furring strips. Even with the double-deck system, you still will have some conduction though the assembly where there is no air gap.
If you are building a home with a large roof surface area, or want to use dark shingles, please consider this system to incorporate a radiant barrier with a foam encapsulated attic. The upfront cost is a little more, but you will recoup the cost in additional energy savings and in being able to further downsize the size of the heating/cooling system during construction.
Check out the second half of the radio show interview from On the House where Ed shared his expertise on energy savings.
In the first part of the second half of the radio program, Ed covers using radiant barrier in a metal shed/barn (that is not being conditioned), painting a radiant barrier and using a radiant barrier with your existing insulation.
Ed finishes up his interview by explaining surface temperatures, roof temperatures and the importance of attic ventilation to get the best results and the most comfortable living space. He also explains why predicting savings is tricky since homes have different variables, and why partial coverage works with radiant barrier.
Did you miss the live broadcast of On the House where Ed was featured as the energy expert and radiant barrier guru of the hour? Catch up on the first segment below!
In the first segment Ed talks about how he got in to the radiant barrier business, where to get radiant barrier AtticFoil®, what it is and why it is different from regular kitchen foil. Ed also talks about how AtticFoil® is constructed and the simple science behind how and why radiant barrier works.