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Question DetailsAsked on 3/8/2012

Can I install spray foam and radiant barrier? Do I need both? Which is better?

I live in Dallas and have just bought a 3,000 sq ft ranch home built in 1970. We're putting on a new roof and are debating what kind of insulation to put in the attic/roof. I didn't know what was better and whether or not it was worth/possible to install both. Thanks!

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7 Answers

Voted Best Answer

Obviously this is not a timely response to the initial question. However, for those who may be reading these answers at a later time, a couple of added thoughts:

1) the radiant barrier being discussed is basically heavy-duty metal foil or metallized surface on a plastic sheet, intended to reflect RADIATED heat (infrared radiation - think heat light, or heat you can feel at a distance radiated from a fireplace), the same way a mirror reflects light. Radiated heat is how a standard oven broils and how steam and hot water baseboard heat predominately work.

2) you generally should do NOT place a radiant barrier over the insulation that lies between and over the joists in a normal attic, especially in a region where the attic temperature can frequently reach condensation temperature (below about 45-50 degrees) - it may reflect back some of the house heat that is coming up from the house, but by destroying most of the temperature gradient from the house to the attic air destroys much of the driving force that moves moisture to the attic air and subsequent venting. Between that greater heat and the fact the barrier is also a moisture barrier, that makes a perfect condition for mold and rot in your insulation and attic wood, and has become quite an issue in energy upgraded homes because of retrofits that cut off airflow outside the insulation, but do not cut off the moisture source leaking thorough from the house. The proper and ONLY place for a vapor barrier in a normal attic insulation system is on the pressurized and normal warm, humid side of the insulation zone - directly above the ceiling drywall in the top floor, fastened to the UNDERSIDE of the ceiling joists or trusses, NOT anywhere above that. Perforated barriers are supposed to reduce this tendency, but the perforation area percentage is so small that typically they still act as a vapor varrier, just not a totally effective one.

3) radiant barriers reflect radiated heat ewith up to 99% efficiency but have basically zero resistance to CONDUCTION (body to body heat transfer at points of contact - think heat transfer from your warm hand to a frozen cold drink can, or hot pavement heat transfer to the bottom of your feet) - so there needs to be an air gap between the radiant barrier and the hot item passing the heat to it, otherwise the heat will just pass through it by conduction. Therefore, applying it directly to the sheathing (above or below) or manufacturing it directly on the surface of the sheathing defeats its purpose, even though this is commonly done.

4) there is a lot of discussion, particularly in the professional design community, about attic radiant heat barrier effectiveness and problems. Because they are being installed on the bottom of the sheathing or underside of roof joists, they act as a heat trap for the energy being conducted through the roof which would normally radiate into the attic air or be transferred by CONVECTION (fluid flow heat transfer) to the attic air, and be vented through roof vents, ridge vents, gable vents, etc. By trapping that heat, they are causing the underside of the shingles and particularly the felt and sheathing to get a lot hotter than they otherside would, essentially changing it from a system where the shingle top surface might reach 120-180 F and the inside surface of the sheathing about 80-140F in the summer, to making the entire roof system equal to the outside surface temperature. This causes more rapid shingle deterioration and cracking, and makes the felt or plastic moisture barrier under the shingles brittle and subject to failure.

Also, any moisture above the radiant barrier (from roof leaks or humid air coming into the area) is prevented from evaporating by the attic airflow which would normally remove it, so it starts acting like a steamer. I have seen both wood and metal lofts and attics become a major mold farm in months because of this effect, and a couple of roofs which started sagging due to rotted sheathing within 2 years of reroofing with tightly adhered radiant barrier. Some radiant barriers are vapor-permeable to reduce the moisture issue, many are not, but few actually are effective in letting moisture freely escape.

Having seen these products in use, and having analyzed and specified building products for use from the Middle East to the Arctic for decades, and having a Masters in Arctic Engineering (a degree predominately in energy conservation and heat flow), my personal opinion is that these radiant barriers will be banned by code within 10-15 years for unheated (so-called "cold" roofs) roofs, because they just do not use the principles of thermodynamics correctly. For more info on this issue Google the following search phrase - moisture trapping by radiant attic barriers and read the government (not the manufacturer) literature on the issue.

5) Unfortunately, the right way to handle this issue is to put the radiant surface on the OUTSIDE of the house - by using reflective materials on the roofing material. This is already done with flat roofs, house trailers, and industrial structures by spraying with alumiunum paint, and a few brands offer reflective aggregate shingles that are slightly more reflective and radiant than normal shingles. People obviously do not like this reflective surface from an aesthetic standpoint, though with solar cells coming into more general use this may soon be more widely adopted. The idea should be to keep the solar energy from penetrating into the building envelope at all, not try to re-reflect it away after it has penetrated throguh the roof system.

The sprayed-in foam has a couple of issues you need to be aware of:

6) it needs to be the low-pressure expanding type mixed for use around window frames, as fully expanding foam can bow joists or trusses and pop drywall ceilings free as it expands, and non-expanding foam actually shrinks as it cures, leaving gaps for air and heat flow alongside the ceiling joists.

7) being closed-cell it is essentially impervious to moisture, so the vapor barrier on the house side has to be EXCELLENT (incuding sealingof all penetrations), or it will trap household moisture escaping into the attic and promote mold and rot in the ceiling drywall and joists.

8) it tends to bleed chemical fumes into the house for a long period of time (can be noticeable for years), which may be objectionable to some people from an odor or environmental standpoint, and especially should be considered if any residents have severe allergy issues or respiratory problems.

9) I emphatically recommend AGAINST use of sprayed-in foam between ceiling joists or truss members in any area that can have cold attic air that could cause moisture condensation in the insulation, though this is probably not a significant problem where you live, assuming your Dallas is the city in Texas. For essentially year-around air-conditioned homes in hot climates, the problem can actually be condensation of attic air moisture on and in the colder ceiling surface insulation and on cold attic runs of air conditioned air, so attic ventilation becomes a critical issue to remove the moisture before it condenses.

In summary, having seen an awful lot of attic moisture and thermal problems, my personal recommendation would be to ensure excellent sealing of the house from the attic, use normal UNFACED fiberglass insulation, and instead of a radiant barrier ensure adequate full-attic ventilation. If you decide to got with a radiant barrier, then I would recommend a perforated one, sloping up towards the sides a foot or two and stopping a foot or so clear at the sides so moist air under it can escape to the roof joist spaces and be vented from the attic. I have seen this done several times with a fine nylon net strung above the insulation in the attic, supporting the barrier, resulting in something very similar to the double-roof system used in bedouin tents, where airflow between the two layers keep the hot air away from the living space.

Answered 7 years ago by LCD


Ranch homes in the South and Southwest oftenhave duct work in the attic. This can effect how insulation, air sealingand radiate barrier product are installed. Are your ducts in the attic?

Answered 8 years ago by Cost Rite


Both can help but you are limited in what you can do. You'll have to remove all of the old insulation in the attic to prepare for the spray foam between the ceiling joists. You can not spray foam insulation between the rafters. There have been several discussion forums on the topic across the web. I believe it was Owen's Corning that put out a bulletin on the topic a while back but I'd have to search for it. Your roofing manufacturer's warranty will be effectively nulled because you no longer will have air flow under the decking. This traps heat in the shingles and cooks them from both sides, significantly reducing their life span. You can place radiant barrier under the roof because it does not provide a significant source of insulating R-value which traps the heat.

Make sure you will never remodel your home if you spray foam the ceiling. You will have a very difficult time getting to the electrical boxes or air register boxes after the fact and will add significant cost to the remodel expense. For bang for your buck in retrofit applications here in Texas you'll do best to have blown insulation added to boost your attic R-value to about a R-38. Also, put the foil radiant barrier up. Between the two you won't get much better on attic heat blocking. Make sure you have adaquate attic cross ventilation in terms of gable, soffit, and ridge vents. Any other more costly moves will be limited in productivity based on the age and construction of the home without further remodel including replacement of windows, doors, HVAC system with ducting, etc.

Todd Shell
Todd's Home Services
San Antonio, TX

Answered 8 years ago by Todd's Home Services


yes the duct work is in the attic

Answered 8 years ago by Joedawg9


As old energy conservation contractor I am a huge fan of celulose (ground up paper treated with boron for insects) as a CAP over existing insulation, It has a high R value and really helps slow the exfiltration of conditioned air from the interior of the home. IE makes the home more air tight, Most big box stores will lend you a blowing machine with the purchase of raw paper. As to radiant barrier I also think is good idea ONLY if it is installed on the roof rafters with adjustments to allow for good attic ventilation, Dont staple product all the way to the top with continious ridge vent or over the area where a soffit vent feeds air into the attic or where a flat roof vent ( a 505) is installed. My home 9736 Ascot Drive Omaha Ne experienced a 50% drop in electricity bill in summer of 2011 after installing a radiant barrier. I do not sell either product, I am a Gutter Cover contractor. PS Dont think roofing manufators warranty's are worth a pile of beans fyi


Answered 8 years ago by jccasper


The best solution is to remove the attic insultation that is between the attic floor joists (but you can re-use this - it won't be tossed - just moved aside).

Then apply at least 3" (3.5" is optimal) of closed cell foam between the attic floor joists (NOT to the roof). Beyond 3.5" the benefits of foam greatly diminish. This will create a completely sealed air/moisture barrier above the conditioned area of the house. A light .5 to 1" coating should also be applied to the ducts in the attic - again to insulate and SEAL them. Make sure the soffits are not blocked off - they should be protected from the spray foam by blockers already.

Then - pile the old insulation back above the closed cell foam - again all this is the attic floor joists. This will add additional R value and you already have the material.

The benefits of a 3 - 3.5" solid foam barrier are VASTLY better than a massive layer of blown-in insulation. It will be night and day. Your house will be very comfortable and your utility bills will DROP - a LOT.

The spray foam should cost about $1 per sq foot per inch installed. And a bit more for the extra labor to move the old insulation around - but that won't be all that much.

Answered 7 years ago by Jefferson


A couple of annotations regarding the most recent post. While I would not recommend the spray-in insulation, if you do go that route a few things to remember:

1) I would advise, if you do go with spray-in insulation, do NOT let them coat the top of the joists - in case they are exposed to moisture on the house side due to poor or holed vapor barrier, you want the joists to have a free surface to evaporate moisture from to reduce the risk of dry rot.

2) If you take the suggestion about spraying ducts, bear in mind 3 things - makes it a much harder job to work on them if they need work in the future, on horizontal runs make sure they are supported every few feet or the weight of the wet foam can crumple them before it sets up (or use thin coats), and be especially careful about the weight of the wet foam (before it gains strength) pulling flexible duct apart or loose of connections. Also test a short piece first - some spray foam chemicals (when still wet) dissolve the plastic coating on some flexible duct materials - I have seen it on a spiral-reinforced bathroom duct hose and on A/C flex duct, where it ate right through the plastic and half-filled the duct with foam. Also,, check all first to be sure there are no holes, splits, or gaps where the foam could fill the duct up.

3) Keep at least 18 inches clear of mechanical equipment like fans or HVAC units to leave work space and to provide cooling air, and do not "box" any in so heat cannot rise off the unit or fresh air flow to it at base level.

4) Do NOT spray insulate can type recessed ceiling lights - they have to either have free air flow to them, or have a specified open air space around them, depending on rating marked on or in the can. If yours are the type that cannot be insulated, this would be a good time to change those out, because they provide a continuous year-around free flow of moist air to the attic from the house, and hot summer air down into the house during high pressure times. Fully encapsulate in foam and you are asking for a fire. Even if rated for full insulation wrapping, wrap in a minimum 3 inch layer of fiberglass before spraying, because the cans get hot enough to melt and set fire to sprayed foam.

5) Put a thin layer of fiberglass over each electrical box exposed from the ceiling below - makes it much easier to work on in the future, and keeps the expanding foam from filling the box and risking overheating of the wires in the box, not to mention making it impossible to work on in the future.

6) It is illegal to encapsulate electric wires in the foam - they are rated for some air movement, such as occurs in fiberglass, and not only will their amperage capacity be significantly degraded by encapsulation, but the foam melts many types of electric wire insulation before it sets up. This is commonly ignored and has been the cause of electrical fires, particularly in houses where most runs go to the attic and then down into the walls rather than from basement up. In this type of house wiring, most of the wires are run through the middle of the attic floor joists, so it is pretty much impossible to spray foam insulate without embedding them, so foam insualtion is out of the picture for that case. Going ahead an encapsulting them, in addition to making future work on them almost impossible, is as dangerous as running normal romex through conduit - it needs the ability to radiate heat around the wires or they can overheat.

7) As stated in the other recent comment, you have to be sure to keep your eave openings open with eave baffles or air chutes, not only from foam but also if you overstack with the old fiberglass, you need a good clearance at least the depth of the rafters, and preferably a couple of inches more, so air can move around as needed to vent heat and moisture from the attic.

8) Be sure penetrations like bathroom and kitchen fans, lights, etc are well sealed to the vapor barrier and covered with a thin layer of fiberglass, because they commonly have openings around them through the ceiling under the trim ring around them or snap-on cover - I have seen a brand new kitchen sprayed all over with attic spray foam because it sprayed through the air gaps around the recessed ceiling light fixtures that had not had trim rings put on yet - was about $30,000 in damage in about 2 minutes of spraying, because the applicator was trying real hard to get those gaps filled well !

9) Ditto to eaves - they need to be temporarily blocked from the inside with paper during spraying, or the overspray will either block the bug screening on the eave openings, plug up the eave cover vents, or spray out the eave openings all over the underside of the eaves, side of the house and deck or patio outside.

10) It is illegal to cover over any type of electrical junction or splice box without providing open access to it, so that needs to be boxed out for access or raised above the joist level. You can box it out with wood in a taper opening away from the box and a couple of layers of unstapled visqueen and a "handle" of wood or rope sticking up and then spray the inside of the boxout, creating a pull-out foam block when you pull the rope or handle that gives access to the electrical box. If this is done, requires a yellow notice on the front that there is a splice or junction box below, the circuit number(s), and the voltage.

11) It is illegal to spray foam on or have foam within, if I remember correctly, 6 inches (but don't hold me to that) of a chimney flue or furnace/water heater chimney / exhaust vent pipe, or exhaust of any other combustion device. If you have an attic mounted furnace or hot water heater the safe distance is greater, depending on model of furnace - it should have safe distance information on a label. I would not have exposed spray foam anywhere near a furnace - I would have drywall or at least 4 inches of fibreglass as the closest material.

12) Be sure to have any attic ventilation fans OFF during the spraying operation, as the fine spray mist will stick to the blades and motor and unbalance them and cause overheating. In fact, any electrical equipment in the attic should be turned OFF (and tagged out) and wrapped in plastic sheeting during the insulation removal/moving process and during the spraying process. Just don't forget to remove the covering before operating them afterward. If the contractor gets lazy and does not seal off equipment until the spraying process starts, all equipment should be well vacuumed FIRST, and then blown out with compressed air after all the work is done, else you risk motor failure from dust and fiberglass contamination in the motors. This can really shorten a motor's life, so proper protection (garbage bags tied around the base with duct tape work well) before any dust is stirred up is the best prevention. Do not ignore the lockout / switch covering, because a minute or tewo running under plastic coering will total most electric motors.

13) if you have an HVAC unit in the attic, you should check (and probably change out) the filter about a week or so after the job is done and residual dust has had a chance to clear out. If your filters are expensive, it might be an idea to put a used but usable one in before the job, then change out for a new one after the job is done. It definitely should NOT be run when insulation or foam is being handled and dust stirred around.

14) Be sure contractor's personnel understand that the trusses and sheathing above the joist level is NOT to be sprayed - I have seen operators think they were doing a favor by shooting a layer of foam on the underside of the sheathing without extra charge, as free insulation. This is destructive to the roof materials and should not be done, as Todd Shell pointed out.

15) As you can see, any fool can jump in and spray foam an attic, but if not properly done, you can cuase a lot more problems, and more serious ones, than you are solving.

16) If there is any living space in the attic, or thoughts of that in the future, the foam has to be fire rated. In all probability, you have 1/2" sheetrock for ceilings, which also is not thick enough for the required 1 hour fire rating against non-rated foam, so that needs consideration.

Answered 7 years ago by LCD


Please research the "Unvented Attic System." This is tried and true building science that has worked effectively for many, many years in hot climates. All these home grown solutions are confusing, often wrong and, quite frankly, the reason spray foam sometimes gets a bad name. There really is a right and wrong way to install spray foam.

Properly installed spray foam (unvented attic system) will give you a clean, quiet, comfortable and energy efficient home. It is an awesome system.

And, yes we are insulation contractor.

Answered 7 years ago by Beaucastle

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