Ask The Guru: Radiant Barrier

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1. Rating: +0

   

   I live in Michigan and want to use Radiant Barrier in my attic. I want to staple to the bottom of the roof rafters. I am going to leave 3 inch space on the bottom and three inches on top. I like to use non-perforated barrier, I think it does a better job. Since both sides will be ventilated and the same temperature, do I have to worry about condensation, or can I use it?

   The perforated and non-perforated radiant barriers work exactly the same. The holes are tiny pinholes about 1/2″ apart. The difference in effectiveness of the two products – (reflectivity and emissivity) is so small it’s not even measurable. Vapor barriers should be used with caution regardless of location. The general theory is that you NEVER use a vapor barrier unless you are SPECIFICALLY trying to stop/trap moisture in it’s vapor form. In an attic, you WANT air and moisture to flow freely to maximize the drying process. Therefore, it is NEVER suggested to use a vapor barrier in a vented attic application. If the effectiveness is the same, why risk introducing something that can slow/stop moisture flow out of the attic?
   Also, you CAN get condensation on ANY surface if the temperature is below dew-point and you have a source of warm-moist air. Ventilation helps, but you REALLY want to eliminate the source of warm-moist air.

2. Rating: +0

   

   I am planning to install your Radiant Barrier over the winter. Almost everything that I need to know has been readily available on your site. I just have 1 question. Is there any diference in running the Radiant Barrier up and down the rafters instead of across. Due to the number of interferences in my attic, going up and down would be an easier install versus going across. Thanks in advance for the information.

   It does not matter which way the AtticFoil Brand Radiant Barrier is run. Generally, it's easier IF you can go horizontal, but some attics it works best to run up and down. If you need a smaller width roll - just use a power saw (miter type or table) to cut the whole roll. You can easily make 3 x 16" or 2 x 24" rolls.

3. Rating: +0

   

   I covered the underside of the roof starting 3 feet from the soffit and ending about 6-8 inches from the roof vent apex. Is it enough to get a good benefit from the radiant barrier? In your figures you show the barrier starting at the vent. Will the air from the soffit not be able to get behind the radiant barrier and carry the heat away if the barrier is not low enough? Thanks for any help.

   You are fine. Air will naturally find it's way to the top of the attic whether it's behind, or in front of, the radiant barrier AtticFoil. Starting 3 feet up is not ideal, but radiant barrier has a cumulative effect. The more of the roof you cover the more benefit you get - just like shade. If it's not too difficult, I'd try to cover as much as possible.

4. Rating: +1

   

   I live in the Washington, DC metro area and want to increase the insulation in my attic. I received an estimate from a company to add additional cellulose insulation in my attic and then add a radiant perforated barrier on top. Does any space have to be left between the cellulose insulation and the radiant barrier? Thanks for your help.

   You can lay AtticFoil directly on top of the cellulose insulation.  The required airspace will be on top of the foil.  I would not "mash' the foil down into the cellulose insulation.  Gently lay on top and try to maintain minimal contact if possible.

5. Rating: +0

   

   An HVAC guy told me that radient barriers will change the dynamics of a balanced system. My system is poorly designed so that The MBR upstairs gets very little airflow and is either too hot or cold. I have an 11 year old, 2800 sq. ft. 2 story home with one zone. He states that by over insulating or otherwise dramatically altering the temp. control demands of your system you may end up with mold or continuously cycling of the a/c unit since it is designed to work at particular specs. In essence you may end up with a unit that is too big for its needs. I cannot afford to have the ductwork redone properly. I'm interested in your response. Thank you.

   Thanks for the comment. The fact is that MOST air conditioning systems are oversized. In fact, we tend to size our AC systems to handle "full load" (which is usually the hottest day of the summer); however, we are usually only at full load" about 2% of the time. So, in reality, most systems are oversized 98% of the time.

   Normally, this is not a huge issue. However, anything you do to drop the load more (including installing radiant barrier) can cause most systems to be even MORE oversized (the difference between what it can do and what you need it to do becomes a big difference), which could result in a lack of dehumidification but this really depends on you climate area. If your air conditioner is "short cycling" then you have a couple of options. The easiest method is to install a dehumidifier to pull out extra moisture. Or, you can install a two stage (high & low capacity) air conditioner. Personally, I like to use air conditioners that use "inverter" systems. These systems are made by companies like: Mitsubishi, Daikin, LG, Sanyo, Fujitsu and many more. Inverter type air conditioners are similar to a cruise control on a car. The adjust the flow of refrigerant based on the "load." An inverter system will run longer but pull less amps resulting in more dehumidification, lower energy bills and less of a "muggy" feeling in the home.

   For you particular situation, I would install the radiant barrier. Then, if the humidity is too high, install a plug in dehumidifier.

6. Rating: +0

   

   We plan to install foil radiant barrier in our attic. As I understand it, it is better to install the foil beneath the rafters rather than between them, right? And running it horizontal would make it easier to install. Would outside AND inside radiant barrier treatment help, or not? We could paint the roof (dark asphalt in Texas sun) with radiant barrier paint (reflective to 95%) after we install the foil radiant barrier in the attic. Would this help?

   Yes, you want to install the foil across the bottoms of the rafters so you can eliminate the thermal bypass that would occur if the ends were not covered.

   And yes, you could paint the roof if you don't mind the color of the radiant barrier paint. There are actually reflective coatings specifically designed for roof applications. As far as doubling up the barrier though (on the roof and in the attic) it would only really help if there was a large portion of the attic you could not get to. Otherwise the foil is blocking 97% of the radiant heat on it's own, plus the cost is substantially less than the reflective coatings, so you're better off going that route if you have to choose one.

7. Rating: +0

   

   I live in a top floor condo in Dallas that was built in 1974. I would really love to install a radiant barrier. I don't have an attic or even a crawl space. I think the only two options I have are: 1. take down the drywall staple the attic foil then replace drywall ceiling or 2. furring strips over the existing ceiling to create the air gap I need and attach new sheetrock. So basically drop my ceiling by an inch. Will this second option work since the radiant barrier will be below the insulation above the original sheet rock ceiling?

   Yes, we have had cases where customers did an install similar to this (attaching wooden strips over the existing drywall, stapling foil to the wood and then adding a new layer of drywall over it). However, the foil really needs to be closest to the outside in order for you to see a real benefit from it as a summertime heat blocker. If there is NO insulation in that ceiling (which is unlikely), then you could get a benefit from the foil, even though the drywall is there. However, if there IS insulation up there between the rafters, then I hate to say it, but without removing the assembly and adding the radiant barrier closest to the roof, you aren't gonna get the great results you are probably hoping for.

   In that case (having insulation above your ceiling between the ceiling and the roof deck), your best bet is to bulk up what existing insulation you have. I recommend you get 3/4" to 1.5" foam board and apply it directly over the existing ceiling and the drywall over the foam board. The only way I'd recommend you using a radiant barrier is if you were going to do the cathedral ceiling install method (3rd video on this page: http://www.atticfoil.com/faq.htm) and you were able to get the foil up against the roof so you could results that would be worth your time, money and energy.

8. Rating: +0

   

   Dear Guru, I have a question concerning Radiant Barriers. I have looked for any similar inquiries elsewhere and so far have not found any so I decided to ask you. I am looking into installing a radiant barrier in my existing house (built 2006 but I acquired only last year). I planned on doing the Foil type attached to my rafters possibly next spring when it is not so hot. I was also planning on decking over top of all my insulation for two reasons: (1) to improve ease of installing the foil radiant barrier later, and (2) to increase my R-value (I realize if I collapse the insulation the R-value goes down which I won’t do, but an extra layer of wood should add R-value and also allow the insulation to better perform to its potential by making the insulation layer “Dead air” so it does not transfer heat as easily.) I was going to lay 2x4’s down first in order to have an air gap between the blown in insulation and the decking. My projected cost for this decking was already a few hundred for OSB but I had another thought. I have heard that some people have tried radiant foil on top of their insulation and also attached to the rafters in the same house (doubling up) with pretty good results. Since I am decking I did not want to deal with loose foil under the OSB so I thought I could use OSB Techshield face down (only costs a little more than plain OSB). This also allows me to affect my energy bill this summer since I could budget in the decking this month instead of waiting till next spring (More Savings = Happy Homeowner). From my limited knowledge/experience in this area I just want to make sure I was thinking about it correctly. Techshield as a roof sheathing is face down with the attic side being the air gap, so I should be able to lay it face down (walking on OSB side so the foil backing isn’t damaged) with an air gap maintained by using 2x4’s as spacers. My total cost increase for switching to Techshield is minimal but the potential energy savings would make it worth it. I guess it all boils down into two questions. 1 Will decking (plain OSB/plywood) my entire attic improve my R-value as I suspect? 2 If decking does improve my R-value (or at least does not reduce it) will Techshield perform as a good radiant barrier in that placement? Thank you very much for your time. I appreciate your expertise.

   1. Yes, adding some plywood on your attic floor over your existing insulation by about 2 per inch of plywood; however it is really not the best choice for insulating this space. The only reason I can see for you to go this route is if you are wanting to create flooring up in your attic to store things. If you want to bulk up the insulation, your money will go further if you invest in adding more traditional insulation or adding foam board.
   2. If you want to use Techshield, facing down toward an air gap, it will work fine to help keep radiant heat from escaping through your home's ceiling and into the cold attic.
   Does that help?

9. Rating: +0

   

   We will be replacing an existing roof due to hail damage. Our roof already has a 1/4" or 1/2" plywood decking WITHOUT A RADIANT BARRIER layer on it. Would it work to lay down another plywood decking WITH a radiant barrier layer on top of the existing plywood decking -- i.e. 2 plywood decks with a radiant barrier in between the 2 plywood decks? Would that work like other radiant barrier applications? Would there be any fire danger in doing so?

   This won't work unless you have an airspace on at least one side of the foil. By definition radiant heat is heat transfer by NON CONTACT across an air space. Without an airspace, you cannot even have radiant heat; therefore, you cannot have a radiant barrier.

   Complete information here: http://www.atticfoil.com/radiant-barrier-shingles.htm

   One option you could try would be to raise the 2nd deck with furring strips. This is what I did on my home and did full sealed attic with foam insulation on the bottom of the lower deck.

10. Rating: +0

    

    Hello Ed, Thanks for the great insight on your site. Although I have seen your video "How to Install Radiant Barrier Foil into a Cathedral or Vaulted Ceiling", my (non-attic) vaulted-ceiling situation is different than the one in the video. My rafters are made of 2x12s, so they can accommodate R-40 batts (non-faced; already purchased) and still leave a 3.5-inch air space on top for ventilation channels (soffit vents and roof vent already installed). Within this 3.5-inch air space, what would be the optimum placing of single-sided radiant barrier? 1) Attached to the underside of the sheathing (at the top of the air space) facing down into the air space? 2) Attached between the rafters at the point that would put it (face down) in direct contact with the insulation, but with the ventilated air space above it (like in your video)? (Also, would any dust accumulating on the paper side (which would face up) have any negative effect?) 3) Attached face down midway in the airspace, essentially creating two layers of airspace? Also, I read a several years ago that a two-inch air space was needed. Perhaps experience over the years shows that 3/4-inch is actually enough (like in your video mentioned above). Could you expound on that a bit? For reference, I’m using a light-weight, laminated metal roofing product (not black, but dark brown) that contains a layer of aluminum. I used furring strips to create vertical venting channels all the way up the roof. (I saw the exact same product, same color, in one of your other videos.) So, with the interior (below-the-sheathing) ventilation channels mentioned above, a cross section of my roof-ceiling structure would consist of: METAL ROOFING—1/2-INCH AIR SPACE—TAR PAPER—SHEATHING—3.5-INCH AIR SPACE (with RADIANT BARRIER at top, bottom or in the middle)—R-40 INSULATION—VAPOR BARRIER—WESTERN RED CEDAR. Thank you for your time and attention. Sincerely, Paul

    Ideally your radiant barrier would be closest to the outside (roof deck in this set up) with the air gap between the roof deck and the foil - just like in the video. If you are using a single-sided product then the foil side MUST face the air gap. I say a half-inch air gap is needed, but honestly that is the minimum, if you have more air - then use it.

    Just like you mentioned, these would be the layers:
    METAL ROOFING
    1/2-INCH AIR SPACE
    TAR PAPER
    SHEATHING
    3.5-INCH AIR SPACE
    RADIANT BARRIER
    R-40 INSULATION
    VAPOR BARRIER
    WESTERN RED CEDAR

    Send me some pictures of the install if you wouldn't mind - we have people ask about this all the time.

11. Rating: +0

    

    I can't find any information about Radiant Barrier Plywood on your site or blog. I'm curious about Plytanium Thermostat Radiant Barrier Roof Sheathing from Georgia Pacific specifically. How can this work with no air gap?

    Radiant barrier decking works because in that application, the bottom side of the decking with the foil is facing down toward the open attic space. When standing in an attic you can look up at the roof line and see the foil in between each rafter; because of this air space, the foil works to block radiant heat from emitting (emissivity) into the attic space, and ultimately the home.

    However, the one down side to radiant barrier decking is that it doesn't take in to account thermal bypass on the rafters (meaning the heat is still moving through the rafter ends that are not covered with radiant barrier, and then radiates off the ends into the attic). Using radiant barrier across the rafter bottoms does not allow for thermal bridging, so in some cases people opt for the staple up method to get the best possible coverage.

12. Rating: +0

    

    Ed, Thanks for the good info. I would like to add about 6 in of blown in insulation to my attic. This will make the insulation about 6 in above the ceiling joists. When I add the attic foil will it need to be stapled? If so, I would need to push the foil down to the ceiling joist to have something to staple to and this would counteract the effect of the added insulation.? Can attic foil just be laid in without fastening? Thanks, Alvin Green

    Alvin,
    By putting it on your attic floor directly on top of your insulation, AtticFoil works great in combination with regular insulation to make the regular insulation more effective. First, it will reflect the radiant heat loss BACK into the living space. Second, it will reduce the internal convection within the existing insulation. Laying AtticFoil out on top of the insulation will have the biggest impact on winter heat loss; think of radiant barrier as your first defense against heat loss, and traditional insulation as your second defense.

    When installing AtticFoil over your insulation in colder climates, please follow these recommendations for the best results:
    Roll out sections and cut around supports, ducts, and other penetrations. If ducts are on the attic floor, cover them too.
    Do NOT push the foil onto the insulation - just lay it over the top, like a blanket. If it peaks in certain areas, that's fine & ideal.
    Overlap the foil at least 2-3 inches; if you want you can secure the pieces together with foil tape, but it's not necessary. Stapling is also not necessary since doing so in your case would require compressing the insulation.
    Mark decking, or walk paths with colored tape or spray paint so you can tell where the flooring ends so as to prevent any accidents stepping through the drywall.
    You can NOT cover the foil with anything (i.e. flooring) or else you will eliminate the require air space for the foil to work.
    CRITICAL NOTE: You must be careful NOT to cover any source of moisture from inside your home such as a bathroom exhaust fan or a leaky can light. It is very important that bathroom and kitchen fans are vented to the outside of the attic, or at least past the foil insulation.
    Also, take a look at this page for more info on installing it for cold weather heat retention: http://www.atticfoil.com/over-insulation-instruction.htm