Can I install spray foam and radiant barrier Do I need both Which is better (Home Insulation

Can I install spray foam and radiant barrier Do I need both Which is better (Home Insulation

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Question Details Asked on 3/8/2012

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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.

Can I install spray foam and radiant barrier Do I need both Which is better (Home Insulation

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 1 year ago by LCD


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