Radiant barriers don't work when both sides are snug against another material according to the calcs. I used them from 2003 thru my last qualifying home in 2018. The effect was noticeable. Now that I'm insulating the bottom of the roof deck & using roofing materials with some reflectivity, the radiant barrier has no value in the calcs.

I believe the underside of the roof deck is a little better. Can't remember why.

Are you adding to existing or building new? Roof sheathing with barrier attached is fairly cost effective. But not best practice by any means. As long as you don't have any ducts up there, just add more insulation.

Don't design one element at a time. Describe the climate, the roof construction, the insulation thickness and location and any attic HVAC and ducts.

It is a temperate climate goes as low as 40 in winter, and upto about 100 for a few days in summer (maybe 10 days). We like to keep the house warm (78). Roof is regular roof with attic. Insulation (batts) R19 in attic between ceiling joists. No vaulted ceiling. Run the heater for 6 months of the year and the cooling for less than a month.

I agree with David; add more insulation if you can; the radiant barrier will not be very effecive.

putting it on top of the insulation will be less effective long term because it will get more dirty and dirt reduces the effectiveness of the barrier.

if you heat more than cool, more insulation will be far better than adding radiant barrier. if you can get it to r49 you would reduce heat loss through your ceiling by approx 60%

The underside of the roof deck is extremely effective as long as the radiant barrier is not sandwiched between materials. From 2003- 2018 I used it in combination with R-38 blown at the ceiling joists and all those homes were significantly cooler than those without the barrier.

New CA CEC guidelines have us increasing levels that require another layer of insulation at the roof deck level. A layer of R-11 under the roof in addition to the R38 at the ceiling supposedly is more effective, but it creates a sandwich leaving the radiant out of the picture.

The issue is where to put additional insulation. Tell us how the attic is currently vented.

"Radiant barriers don't work when both sides are snug against another material according to the calcs."

Not sure I quite understand. Radiant barriers reflect infra-red EM radiation - shouldn't have anything to do with what is next to/sanwiched to them. Now, convection or conduction heat exchange is a different matter.

@sktn77a

https://www.youtube.com/watch?v=E1IwS0_lpBc

Its not quite correct to say that infra-red radiation can only pass through air or a vacuum but they absorb and reflect very little or none of it.

However, wood, brick, stone, paper and glass all absorb IR radiation (converting it to heat) and aluminum foil is a strong IR reflector. It is also a poor emitter so it can be used on either side of an air space in a construction detail.

Therefore, a radiant barrier system consists of an air space with one or both of its faces containing a radiant barrier.

Any opinions on https://www.youtube.com/watch?v=qm-N2JEsQMc where the radiant barrier on top of the insulation keeps cold out of the home by acting a top cold air barrier ?

Radiant Barrier via Attic Foil method.

@ Joe Macker - I think this method could work, however the problem for my particular area is that on many occasions there are a lot of things in the attic. People store things up there, cable runs for cable tv. Plumbing, namely water heaters, furnaces, duct work and so on.

If you spread this out the con is that (hopefully) you never have to disturb it. The other is that you also want to have proper ventilation as well. I think proper ventilation (natural ventilation) along with this attic foil may help. (you'd be surprised how often I see attic insulation missing in various spots in attics. Repairs are made after some event and the insulation is not put back.)

I think on it's own with only attic foil only and inadequate ventilation and it probably won't work all that great.

If you have a lot of stuff in your attic it would probably be better to install this on the underside of the roof rafters, which would make installation a bit tougher.

If you have good attic ventilation already, I think this would be at best a marginal improvement. There is no substitute for good ventilation. (attic air exchange --- cooler air enters at soffits --- hot air exits thru ridge vents / whirly birds)

In a colder climate you would need to have the attic well insulated first. If there is a break in insulation / an or around the popular can lights the problem would be condensation on the foil. Warm moist air from the home collides with cold air in the attic and you'd get condensation on the under side of the foil. Which would lead to mold, among possibly other problems.

radiant barrier reflects heat. so that wouldn't work. matter of fact it would hurt you in the winter because it prevents the sun's heat in the afternoon. roofs can still get pretty warm in the winter.

do the extra insulation if you can.

also, res3d asked - do you have space for extra insulation?

This is an existing house so the design solution must be specific to the existing conditions which have not yet been fully revealed.

The argument made in the video is that insulation does not prevent air from seeping through. Since cold air tends to settle downwards, it will go right through the insulation. So even a thicker insulation won't help.

The moisture could be a problem, and so it the probability of it getting disturbed during attic work. I think this could be in addition to a radiant barrier foil stapled to the underside of the roof but cannot replace it. The one under the roof keeps outside heat from coming in whereas the one over the insulation keeps outside cold from getting in. Does anyone have any thoughts on this theory? Anyone tried to verify it?

sooooo, are you able to add more insulation or not?

You appear to be spinning your wheels. The air barrier belongs at the ceiling. If there's air movement in the attic strong enough to cause a problem with the floor insulation, you need to stop it.

How is the attic vented; how thick is the insulation; what size rafters and attic floor joists?

The solution might be as simple as adding foil-faced insulation between the rafters but it might require modifying the existing venting.

@User yes I should be able to add more insulation batts

@User The attic is vented with ridge vents, the insulation is R30 (not R19 as I stated earlier). rafters are 18 inches apart and ~8in thick. What I thought the problem the video is addressing is - Any roof need vents for moisture control and for heat to escape. However during the winter those same vents would allow cold air to come in. This cold air would seep through the insulation easily and reach the sheet rock on the ceiling. The foil placed over insulation batting would prevent that. Did I understand that incorrectly?

that video you posted is misleading on many levels

energy moves from hot to cold so that foil will do nothing to stop that.

it claims to stop convective currents . but has many tiny holes - thats bs . it will not stop convective currents at all, and well installed fiberglass batts should not have convective currents.

if you dont like your energy bill, add insulation. dont waste your money on radiant barrier

Air is not going to enter from the ridge vent, drop down to the floor, penetrate the insulation on the floor and reach the drywall. As DE suggests, if there is air movement, it will be in the opposite direction.

A ridge vent by itself will not provide adequate roof ventilation; there must be eave vents to allow air to enter and rise to the ridge and escape. So tell us: are there eave vents? If so, you might consider putting baffles against the roof sheathing between the rafters to allow air to rise from the eave vents and escape from the ridge vents and then covering the baffles with 6" of foil-faced fiberglass insulation. I say foil-faced just to make you happy; I doubt it will have a measurable effect on your energy use but its fairly cheap and it'll look great.

Sorry, there are eave vents too. I was too focused on following how the hot air escapes !

@User I have the same suspicion about the holes in the foil letting in cold air. Is there anything I need to pay attention to when adding in insulation properly? Does paper backed insulation work better?

@User the baffles is an interesting idea. The foils will create a party atmosphere :-) but if it just to look great, I will pass :-)

Maybe experiment with a dehumidifier in the attic and reduce the vents to keep cold air out in winter

Dehumidifiers belong in basements and unvented crawlspaces. Maybe an unvented attic, but not a "sometimes or sorta" unvented attic. Winter is most important time for cold climate vented attics to vent.

https://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting/view

For better roof performance, peel back existing roof insulation to airseal, eliminate recessed cans, and add more insulation. Radiant barrier in retrofit scenario makes little sense unless you can find cheap foil faced Batts as suggested by 3d. Add ventilation chutes to achieve enough insulation to fully cover tops of framing at attic floor.

https://www.finehomebuilding.com/2015/04/09/insulation-and-attics

If you install the insulation in the rafters, the air from the eave vents will move upward along the sheathing on the cold side of the insulation and exit at the ridge vent so there is no need to close the vents in the winter.

You must have read somewhere that cold air penetrates fiberglass insulation reducing its R value; its a common internet claim from organizations that represent foam insulation manufacturers. I would be skeptical of any insulation opinions on the internet other than from Green Building Advisor and then be careful that you understand what they are discussing.

At any rate, air penetration of fiberglass shouldn't be a serious issue in your attic unless you have a big fan up there.

The one under the roof keeps outside heat from coming in whereas the one over the insulation keeps outside cold from getting in. Does anyone have any thoughts on this theory? Anyone tried to verify it?

It's radiant barrier, it doesn't keep the cold out it keeps the heat in the home OR the heat out in the summer. Heat moves from hot to cold, not cold to hot. Heat is energy, cold is the lack of energy. Energy moves around. Shades of hot or in terms of radiation moves from hot to surfaces that are less hot. (or you could say from warm to cool).

The problem is: this is more complex than what the video goes in to talking about. That's inherently the problem with DIY products... most of the time they play on the ignorance of people. (This sort of thing is often times presented to me as something to sell.... but because I know better they then turn this kind of stuff into a DIY product.)

Because this guy is selling a product, he's only going to tell you how great and simple it is. He wants to sell this product to as many people as possible. You have to do the prep work as I suggested earlier up in the thread... but once you do the prep work reinsulating properly, proper ventilation etc. the need for radiant barrier is greatly reduced.

The reason I know this: I own a home that doesn't have any radiant barrier on it and it resides in a hot and sticky climate. Ventilation + Proper Insulation and that's really all you need. Radiant barrier can help... but without proper insulation + ventilation it probably won't make that much difference.

A ridge vent by itself will not provide adequate roof ventilation; there must be eave vents to allow air to enter and rise to the ridge and escape.

yep, too many people focus on getting the heat to escape and don't realize they have to replace that air that is leaving with cooler air. If the cooler air has no way in, well that hot air isn't going anywhere either... just sit in there and get hotter still.

I know we'll put a roof mounted fan in there and force that heat out. LOL.

"Since cold air tends to settle downwards, it will go right through the insulation. So even a thicker insulation won't help."

This is an incorrect assumption based on specious self-serving internet propaganda.

There will be very little air movement in your attic unless there's an open window and the wind is blowing hard outside.

In the winter heat will slowly move from the living space through the attic floor insulation and then through the roof to the outside creating vertically stratified air temperatures. In other words, the air in the lower part will be warmer than the air in the upper part and along the sloping roof. That will set up a convection current with warmer air rising at the center and cooler air dropping at the sides where it would warm and join the convection current upward.

The idea that cold air would reach the insulation and sink to the ceiling is a fantasy that is causing you a lot of unnecessary concern.

Just add some insulation on the floor or at the rafters and make sure the attic is properly enclosed and vented. If you think that would involve a dehumidifier and closed vent in the winter you should hire someone to take over this project and do it right..

Fiberglass insulation is problematic as ceiling/attic floor insulation because it doesn't stop air movement. The predominant airflow path is interior air flowing into attic, not other way around though it's possible from negative pressure caused by exhaust fans or leaky HVAC ductwork.

Some high performance builders use flashcoat of spray foam on attic floor for better airsealing and blown cellulose, fiberglass or mineral wool stops airflow better than Batts. With a perfectly airsealed ceiling/attic floor, Batts performs comparably but most new and existing homes have very leaky air barriers at ceiling/attic floor. Airseal, airseal airseal, then insulate.

https://www.energyvanguard.com/blog/are-you-confusing-insulation-with-air-barriers

I agree; the critical issue is to seal penetrations at the ceiling below the attic rather than installing a barrier on top of the attic insulation or by closing attic vents.

Penetrations include: light fixtures, recessed can lights including ICAT rated, interior partition walls, and attic access hatches. These airleaks have a much bigger effect than radiant barriers. Airleakage could have a bigger effect than more insulation and thermal bridging of exposed ceiling/attic floor framing.

Fiberglass insulation, like any other kind of insulation, slows down the transfer of heat from the warm interior to the cold exterior of a house. It does this by trapping air and preventing it from moving in a convection pattern. If air could pass through it like the foam manufacturers claim on the internet, it would have little or no R value. Comparing high density fiberglass insulation in an enclosed cavity to a low density fiberglass furnace filter in a forced air system is specious beyond description.

I agree those comparisons are just marketing but think it's easy to draw the wrong conclusions when comparing batts (of any type) to other forms of insulation. I think Res prefers blown-in dense pack to batts which is another thing I agree with.

Air does flow through fiberglass batts better than most insulation alternatives which means it tends to underperform in comparison. Yes, in a completely airtight cavity with well installed batts there is little air movement but that's not reality in most field conditions.

Even in cavity sealed on all 6 sides, air flow loops will form in typical batt installations because batts do not completely fill the cavity.

A visual guide to why batt insulation underperforms

Most cavities are NOT air-sealed on all six sides with the worst cases being airleaks from exterior to interior. Unless your build team is addressing airtightness at exterior sheathing or interior drywall (or vapor barrier) then spray foam or dense pack will perform much better than batts when looking at rated R-values alone. This is due to the effects of stopping air leakage.

If build team is addressing air leakage at those locations and it's decided to use batts, then the installation should be as close to Grade I as possible. This is another good reason to use Energy Star or reputable third party energy rater.