Another attic vent thread

[Robert Hronek]

There is a lot of misconception of attic venting, heat flow and moisture. From my perspective in the Midwest with moderately high humidity levels for part of the year and somewhat mild winters with periods of cold and snow.

From my research there is not a great deal of temperature reduction from attic venting, assuming there is some venting. You might be talking about a 10 degree reduction.

In the summer heat transfer is from the sun to the roof via radiant heating. Now many have the misconception that convection is the main means of heat in the attic and into the house. In reality heat transfer is via radiant heat from the roof to the attic floor which is covered with insulation. Some of the radiant heat passes through the fiberglass insulation and some of it heats the fiberglass.

Increasing air flow does little to drop the attic temp and does little to save energy. Spray foam and cellulose are better at stopping the radiant heat.

Venting will also help control moisture in the attic. In the summer moisture is not a major problem.

In the winter venting carries away moisture that is leaking from the house. Cold temps means low outdoor relative humidity. So moisture in the outdoor air is not a problem. Now if attic is leaky and warm moist air is leaking in to the attic the lack of adequate venting would be a problem.
Also we see a lot of builders have vented bathrooms in to the attic space.

Venting can also have a positive effect of reducing/eliminating ice dams. But this can be a band aid approach. If the ice dams are the result of air leaks or inadequate insulation then putting in more vents is covering these deficiencies. The real fix is to seal the air leaks and to insulate to at least the minimum level recommended for you location.

What should really be done is to have the attic floor throughly sealed at all top plates, plumbing stacks, flues, ceiling Leigh fixtures etc. Bathrooms should be vented to the exterior. Doing that will eliminate any moisture coming from living space. Any moisture introduced will be the relative humidity of the season. Building material should be able to accommodate short periods of high relative humidity. Venting will allow the materials to shed the excess stored moisture when weather conditions change.

Combined with the air sealing the insulation will keep the indoor conditions in the house and out of the attic. In the summer the right insulation will block radiant heat from the attic. Fiberglass is not the right insulation in an attic. In the winter the heat is kept in the house and out of the attic. The will prevent most ice dams.

With the air sealing and insulation you have reduced (not eliminated) your need for venting. The home owner benefits from reduced energy consumptions, increased comfort, better air quality and superior durability.

If you are increasing your attic vents or adding powered vents you are not looking at the true needs of the house.

I am sure most of you have stood with your head in a scuttle on a windy day and felt the air being sucked out of a house. This is a prime example of negative pressure in an attic. When the scuttle was closed the air was being sucked in the soffit vents but also through any leaks in the attic floor. Adding a powered vent will make the negative pressure a more common occurrence. It is not really helping the house.

So it comes down to insualtion and air sealing with enouh venting to be effective.

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[Vern Heiler]

I would agree that more insulation and sealing leaks is good, but you will get much more bang for your buck stopping radiant heat with a reflective radiant barrier product. Stopping radiant heat with any other product is like stopping a bullet with bed pillows.

[Robert Hronek]

I would agree that more insulation and sealing leaks is good, but you will get much more bang for your buck stopping radiant heat with a reflective radiant barrier product. Stopping radiant heat with any other product is like stopping a bullet with bed pillows.

Vern

In most cases I would disagree. I am talking about a foil radiant barrier laid in an attic. Installers are getting $0.75 to $1.50 per square foot. A pretty high price to pay for limited benefit and a diminishing benefit as other insulation is increased.

I will say it is hard to get good info on radiant barriers and you have to get a grasp on everything to come to a conclusion.

First off radiant barriers need an air space. But convection decreases the effectiveness of a radiant barrier. In an attic there is convection. Dust also reduces the effectiveness. Radiant barriers only fight one path of heat movement. The more insulation you have the less benefit you get from a barrier.

Radiant barrier are not effective in the winter and other types of insulation are needed to combat heat loss in the winter.

A radiant barrier is most effective in an under insulated attic that has fiberglas insulation. Fiberglass insulation has the most air of any insulation. This air makes fiberglass prone to heat loss from convection through the material. It also allows radiant heat to pass through these air spaces. If you have somewhere around R20 of fiberglass you are likely to see quite a bit a benefit from a barrier. But that benefit is limited. It is not stopping air leaks out of the house and it is not stopping convective heat loss that fiberglass suffers from when it is not in a 6 sided enclosure.

Do you agree that an R20 attic should have more insulation? If you know the benefits of air sealing do you agree that at least some air sealing should be done? Particularly for can lights, flues, plumbing stacks and chimneys.

Even is you have an R40 of fiberglass adding cellulose on top acts as a cap and traps the air inside the fiberglass and stops the radiant heat from getting to the fiberglass. You have now improved the existing insualtion and eleiminated the need for a barrier.

Fiberglass is the weakest insulation on the market and I am not a fan of it. Cellulose has smaller air pockets and more material that helps block radiant heat. Spray foams also do a good job of blocking radiant heat. Cellulose and spray foam also do a good job of stopping convection and reducing or eliminating air leaks. If you insulate with either of these materials to over an R 40 there is little benefit to a radiant barrier, to the point it will not pay for its self.

In an attic and with cellulose there is no reason not to shoot for an R value of R60 and up to R80. At this point you will not need a radiant barrier.

So if you insulation is in the range of R 20 the first buck you spend should be air sealing and insulation. Once you bring it up to reasonable levels you have cut down on the need for a barrier but if you think you still need more than spend that money on regular insulation. You will come out ahead with this strategy.

[Vern Heiler]

My comment was directed at the best material to stop radiant heat transfer. The three types of heat transfer, convection, conduction and radiant, have three modes of transfer. Convection requires the movement of air and is best stopped by stopping air flow. Conduction requires a solid to move from one place to another and is best stopped by breaking the path. Radiation is high frequency energy that travels through most material and is best redirected by reflection.

If the radiant barriers are being installed on the attic floor, dust will settle on the surface and reduce the reflective property of the surface. That is why most of the radiant barrier I have seen is installed with the reflective side down, attached to the under side of the rafters.

As far as radiant barrier not working in winter months, I would disagree. My father owned a garage in Gaylord Michigan when I was growing up. The garage was block wall with a 2:12 roof. The winters are brutal in that part of Michigan, believe me. Somewhere around 1958, as I remember, he had a foil insulation (no fiberglass) installed on the bottom of the open rafter ceilings. The difference was dramatic!

What I am saying is that each type of heat transfer has its own mode of transportation and each is best stopped in a different way.

[Robert Hronek]

My comment was directed at the best material to stop radiant heat transfer. The three types of heat transfer, convection, conduction and radiant, have three modes of transfer. Convection requires the movement of air and is best stopped by stopping air flow. Conduction requires a solid to move from one place to another and is best stopped by breaking the path. Radiation is high frequency energy that travels through most material and is best redirected by reflection.

If the radiant barriers are being installed on the attic floor, dust will settle on the surface and reduce the reflective property of the surface. That is why most of the radiant barrier I have seen is installed with the reflective side down, attached to the under side of the rafters.

As far as radiant barrier not working in winter months, I would disagree. My father owned a garage in Gaylord Michigan when I was growing up. The garage was block wall with a 2:12 roof. The winters are brutal in that part of Michigan, believe me. Somewhere around 1958, as I remember, he had a foil insulation (no fiberglass) installed on the bottom of the open rafter ceilings. The difference was dramatic!

What I am saying is that each type of heat transfer has its own mode of transportation and each is best stopped in a different way.

Vern


A garage in the 1950s would not really be insulated. I am envisioning block walls with no insulation and very minimal insulation for the roof. In such a situation anything would be an improvement. Back then energy was cheap and things were not insulated well. As you know many homes built then did not even include sidewall insulation and maybe an R10 for the attic.

In the garage setting the heat was directly reflected back to the person instead of the being absorb by the ceiling. In an insulated attic the primary heat loss is not radiant and the reflected heat is not directed at a person. In comparing the garage to the attic is not apples to apples. In the garage you were being directly affected by a cold surface the same as standing in front of a cold wall or windows. You could be in a warm room but still feel cold due to the radiant robbing effect of the cold surface. Place a foil barrier between you and the wall/windows you increase your comfort but would have little if any effect on saving energy.

In a cold attic the difference in temps is not great between the roof and the top of the insulation. Radiant barriers are the most effective when there are large temperature differences. My comment on they don't work well in the cold was in reference to the application in a cold attic.

I stated that as a building becomes better insulated the less need there is for a radiant barrier. I have seen studies that point to about a 5-10% decrease in cooling costs from adding a radiant barrier. This was in homes that had r18 to r 30 insulation. Hardly what I would call well insulated. Also the homes had fiberglass insulation. Two things about fiberglass. First it is the least resistant to radiant heat of any of the insulation common in the market today. Secondly, in an attic fiberglass can lose 40% of its R value from convection. So an R 30 is an R24.

I agree the foil is the best at reflecting the radiant heat but its properties are limited to just that. No air sealing effect, does not stop conduction and convective heat transfer.

Spray foam and cellulose insulation don't reflect radiant heat but they do stop it.

In the study with the 5-10% savings on under insulated houses. If it was my house I would spend my first $ on air sealing and beefing up the insulation. In this case we are talking of an existing house. First off I would seal up any major air leaks such as around plumbing stacks, soffits, can lights etc. I would even consider sealing all the top plates from the attic side. Then I would cap off the fiberglass with cellulose in the range of an additional R40.

As its currently installed the fiberglass is losing up to 40% of its R value. The cap of cellulose has been shown to increase the effectiveness of fiberglass by stemming convection loss.

A Colorado study of test structures has demonstrated that 2 identical buildings, one with fiberglass and one with cellulose. The cellulose insulated structure used 26% less energy with the same R value.

Dollar for dollar you get more benefit from air sealing and more insulation, as long as its not fiberglass, than you would from a radiant barrier.

I think if the reflective barrier is part of another product and there is not installation cost associated with a barrier then is it probably worth doing. I am talking about things like foil faced rigid insulation, foil faced sheathing, house wrap with reflective properties. The cost of install is built into the primary material you just have the added function of a radiant barrier.

My comments are limited to installations in attics. There are other uses where a radiant barrier is a cost effective and important strategy for steeming heat flow. One of the best applications is reflective coatings on windows.

[Michael Thomas]

With reference to "radiant barriers":

Radiant Barriers: A Solution in Search of a Problem | GreenBuildingAdvisor.com

[Robert Hronek]

With reference to "radiant barriers":

Radiant Barriers: A Solution in Search of a Problem | GreenBuildingAdvisor.com

Mike

Thanks, Great article.

After reserching energy efficiency I found out about radiant barriers. I even considered selling/installing. I found credible reserch hard to come by. But I learned what insualtions block radiant heat and what ones dont. I eventually came to the conculsion that it isnt worth it.

[Ted Menelly]

No matter which way you look at it the best route is radiant barrier. Before anyone states there is no back up to the affects of radiant barrier just go into an attic with radiant barrier and then go next door to one without, Same insulation, heating and cooling systems, same pull down stairs that need insulation and sealing, etc etc etc.

With out any doubt what so ever the home with radiant barrier wins hands down. The attic is upwards and beyond of 20 degrees cooler. No matter what climate you live in radiant barrier is the single most item to reduce the heat in the attic. Now add proper ventilation, more and or better insulation into the upper 30s, (I happen to like cellulose) seal the ducts and duct connections up, pull down stairs sealed and insulated and you just saved a fortune on heating and cooling the entire year...and every year form then on with proper maintenance. Your savings will not be immediate and will take time to pay back on the investment but not that long at all. The best part is that you reduce your cooling bill instantly and forever.

I also prefer the spray on but many installers balk at that. Most of the complaints stem from the dollars that can be had from the material and installation of the radiant barrier. I have seen the spray on that has been there sometime and the radiant applied over the rafters as well as laid on the floor. I hate the radiant on the floor and believe it is not doing half the job as the barrier under the rafters. In saying that I really do not like the rafter radiant. Not because it is not performing it's intended job but you have that pocket between it and the roof sheathing that is difficult to vent properly and if there is a leak in the roof the water will run down into the softness and may not be seen for some time. Then you have the factor of finding the leak by cutting the material away to find the leak.

Enough of the rant. Study after study after study. Who cares. It reduces the heat in the attic by a serious amount which keeps the heat off of the insulation and of course all the duct work and Air handlers the is extremely prevalent in the south to about mid country.

They say spray on may void the warranty of shingles.....well, how about the sheathing with the foil already on the back....same thing as far as I am concerned. As far as heating the roof shingles....I have even checked the temp on several occasions ans the temp variation was practical not there at all.As far as the less need for radiant barrier the higher the insulation.....go for it. You still won't save as much (in the heat) with the exception of harsher climate winters but we are talking of *radfiant barrier* which is for blocking the heat.

All just my opinion but a well tested, on site, home after home resolve on my behalf.

I like radiant barrier.