thoughts on radiant barrier in attic

[heehaw]

are you trying to keep your attic cooler or your house? could you not lay the reflective stuff on top of the attic insulation an keep the house cooler that way? i do know that some of that stuff that folks are selling, is made of the same stuff the Forest Service fire fighters use for their protective tents, in case they get over run by the fire, an it has saved a lot of lives. i haven't looked at his site in a long time, but Doug Rye is a big time energy consultant, an use to have a really nice an informative web site about tech shield an other energy related stuff. dougrye.com i think?
heehaw

 

[Alan L.]

are you trying to keep your attic cooler or your house? could you not lay the reflective stuff on top of the attic insulation an keep the house cooler that way? i do know that some of that stuff that folks are selling, is made of the same stuff the Forest Service fire fighters use for their protective tents, in case they get over run by the fire, an it has saved a lot of lives. i haven't looked at his site in a long time, but Doug Rye is a big time energy consultant, an use to have a really nice an informative web site about tech shield an other energy related stuff. dougrye.com i think?
heehaw

I am no expert, but my understanding is that putting the barrier up on the rafters is better. One reason is that reflective material placed on top of your attic insulation quickly becomes dusty and loses its reflective properties. Not sure of all the physics that make it better otherwise, but I am sure there are others who can explain.

 

[Alan L.]

"This was for a 2000 sq ft house. Here we keep it between 78 and 81, we are quite comfortable at those temperatures"

I envy anyone who is comfortable at 78 to 81. That would save some money. I'm not even close. I can't sleep unless its about 60 and dry (bad allergies). Sitting around the house 69-70 is about right. With temps of 78 I would be wet with sweat and totally uncomfortable. Shouldn't live in Texas I guess, but I didn't choose my birthplace.

If the government starts telling us how cool we can set our thermostats, then I'll have to move I guess.

With the insulation in my house if I set my thermostat on 80 I wonder if the a/c would run 10 minutes a day.

 

[dmccarty]

I read up on this stuff around 5-10 years ago and was not convinced enough to put it in our house. I was really worried about the effect on the shingles.

For the most part I don't care about the heat in the attic. I don't go up there. If the ceiling over your living space is correctly insulated it should not matter how hot or how cold the air is in the attic. Would it make more money sense to better insulate the ceiling space to help out in winter AND summer?

I read quite a few discussions, some of them quite heated( no pun intended ) on the subject. I think the conversations were in this discussion area on Journal of Light Construction. Building Science - JLC-Online Forums- Building Science Though it could have been in another area on the website. Building Science Consulting is also a good resource, Building Science Consulting. Lstiburek has written books about building houses in different zones of the US. Our house is built with his specifications for walls, floors, and ceilings. Though I wish we had built a vent less attic. Sounds strange but go read what he has to say. I already had enough out of the main stream ideas spec'ed on our house I figured a vent less attic was just too much for the builder and the inspectors.

I just looked at some of Lstiburek's attic drawings and radiant barriers were not used. Lots of insulation though.

The link are two good starting places to read up on the subject. Have fun.

Our house is 2400ish square feet with 10 foot ceilings and huge amount of windows aka holes in the walls. Our AC cost is worst case about $70 a month. Other power is about $100-120 of which a good percentage is from the clothes washer and dryer since we have two young kids. If we run the heat pump which is seldom, the heating bill will be $70ish extra. We ran the heat pump for a month or so this winter since we ran out of firewood.

The house we had in the city was 1200 sqaure feet with maybe an average of 8 foot ceilings and very few windows. Though the windows were metal framed. The power bill in the old house was more than what we pay today but for much smaller building.

I decided that more insulation was better than the barrier. We have our AC system in the attic which is why I think we should have built a vent less attic which tries to keep the heat and humidity out of the attic in the first place. I'm sure our AC bill is higher than it should be due to the R6 insulation on the AC system. I did put in more insulation this spring over the ducts and handlers. Not sure it helps but what the heck I had a coupon.

Later,
Dan

 

[thatguy]

are you trying to keep your attic cooler or your house? could you not lay the reflective stuff on top of the attic insulation an keep the house cooler that way? i do know that some of that stuff that folks are selling, is made of the same stuff the Forest Service fire fighters use for their protective tents, in case they get over run by the fire, an it has saved a lot of lives. i haven't looked at his site in a long time, but Doug Rye is a big time energy consultant, an use to have a really nice an informative web site about tech shield an other energy related stuff. dougrye.com i think?
heehaw

I cant put the radiant barrier on the insulation (attic floor/main living area ceiling) because the attic has a plywood floor installed..

I realize the radiant barrier will mainly help in the summer but I am not sure how much, if any, since the house is pretty well insulated already. I need to try and see if I can calculate what heat is coming into the living area from the attic currently to see if the savings are worth it.

thanks for the Doug Rye site.. I will check it out..

brian

 

[txdon]

At a seminar in a home show, an analogy that the radiant barrier was like foil and a house is like a potato, and if you put the potato in the microwave the foil would not let the potato cook.

My question: If you have a metal roof do you NOT need a radiant barrier?

 

[RonMar]

Good demonstration with the IR camera Dex3361. That is exactly what the IR barrier helps to prevent. Radiant heat from the inside of the roof sheething to the other absorbtive surfaces in the attic. All that surface area absorb the heat from the sheething and in turn heats the attic air thru conduction/contact. More surface area = more hot air and an attic that is like an oven. The radiant barrier cuts off this path to all the other surface area in the attic, but like the parts of the pan that show cool on the IR photo, is still hot and will still heat air thru conduction/contact. This is still a lot less surface area though. To further stop this, insulation could be used, or better still would be to remove the heat with airflow from behind the IR barrier(between barrier and roof sheething). This would also help to cool the roof sheething/shingles.

IF the IR sheething could be sealed over the top like a inner roof, so that all the cavities formed by the joices are connected to the peak vent, this would make that peak vent more effective at emoving the heat from the back side of the sheething. A given ammount of airflow applied to a smaller space means a greater percentage of the air volume in that space is moved/exchanged in a given ammount of time compared to a space with a larger volume. The volume of just the space between the joices is far less than the entire attic volume. You of course need to insure that enough supply air is available at the bottom eaves/soffetts.

Brian. If you know the material type, thicknesses and surface area between the attic air space and the living air space, you can probably come up with a co-efficient number(total thermal conductivity) that allows you to take a guess at how many BTU are being transfered for a given attic and living space temperature. Once you have the coefficient, it is multiplied by the SQ/FT of surface area and again by the temp difference. Temperature coefficients are usually expressed as BTU, per SQ/FT, per degree F of temp difference, per hour. Say the coefficient was 10 and you had a 10' square ceiling(100 SQ/FT). And it was 80F in the living space and it was 120F at the attic floor(40F difference). That would be 10 x 100 X 40 or 40,000 BTU/HR from that one ceiling space... You might note, that if you cut the temp difference in half, you cut the heat transfer in half. So dropping the temp at the attic floor from 120F to 100F would half the ammount of heat conducted into the living space. Ideally, at least in the summer, you want the temps in the attic as close to outside ambient temp as possible...

Txdon, metal roofs suffer from the same issues. It is the inside sheething temp that heats the rest of the attic space. Of course metal roofs typically being lighter in color will absorb less IR from the sun than a black shingle roof...

 

[BobRip]

are you trying to keep your attic cooler or your house? could you not lay the reflective stuff on top of the attic insulation an keep the house cooler that way? i do know that some of that stuff that folks are selling, is made of the same stuff the Forest Service fire fighters use for their protective tents, in case they get over run by the fire, an it has saved a lot of lives. i haven't looked at his site in a long time, but Doug Rye is a big time energy consultant, an use to have a really nice an informative web site about tech shield an other energy related stuff. dougrye.com i think?
heehaw

If you did this the reflective stuff would trap moisture in the insulation and cause all sorts of problems. (less insulation, wood rot, mildew).

 

[Kfbeal]

If you were to put that under the rafters, would that cause moisture to build up on the rafters?

I would think if you blew the adhesive rockwool on the ceiling and rafters might benefit more?

 

[RonMar]

In order to get moisture to build up, you need a surface cooler than the surrounding air in order to condense the moisture out of the air. I don't think you are going to find this typically on the inside of a roof, particularly in summer, as just about any daylight at all is going to make the roof warmer than it's surroundings. IF the attic space is cool, there is not much airflow thru it to draw in much moist air.

The moisture Bob was refering to being trapped, comes from inside the living spaces(showers, cooking, breathing ect...). There are some different ways of dealing with moisture, but it is generally accepted that it is best to have the insulation exposed to the air outside so it stays at least at the ambient relative humidity. Just like heat always flows to cold, so does moist flow to dry. Any moisture that finds it's way into the insulation from the inside will eventually evaporate into the air it is exposed to. Remove that ability to evaporate, and you will have rot and mold issues inside the walls. I think it is generally accepted as best to do all the sealing and trapping of moisture inside the living space(vapor barrier right behind drywall/plaster/paneling). The insulation keeps the inner wall warm and helps keep the moisture from condensing on the walls. The moisture content inside the home is controlled with ventilation typically...