Attic insulation for Summer heat?

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Old 09-16-08, 02:48 PM
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Attic insulation for Summer heat?

I live in a small condo building, it is only 2 stories tall and it has a typical pitched roof like a house. I live on the second floor and have access to the attic above me. There are soffit vents running along each side and two vents about 10" in diameter about halfway up the pitched roof. Unfortunately there is no ridge vent, this building was built in the 1950's. The insulation is fiberglass batts with a paper vapor barrier on the bottom. The 2"X8" joists are open other than a few areas where I threw some plywood, the ceiling is made of rock lath, then a layer of concrete, then a thin layer of plaster.

I like it cold, during the Winter I have most of the radiator valves shut off because it gets too hot (my heat comes from a single boiler controlled by the association). However, in the Summer I think some of the intense heat from the attic is heating my living space below.

If I upgraded the insultation in the attic, would it have an effect on my unit underneath? Could I just spray that loose insulation on top of the fiberglass batts that are up there (cause they are DIRTY and it would be a pain to remove it all).

Any opinions? Thanks!
 
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Old 09-16-08, 03:12 PM
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Stepping into the attic moves you from your property to the common property of the condo association, so don't make any changes without the permission of the association. You won't gain anything by adding the insulation, but oh, the difference the building will have if the association is amenable to opening up the ridge and installing a ridge vent. The air must move to let the insulation do its job. I would present it to the association and see what happens.
 
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Old 09-16-08, 03:14 PM
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Spray is probably not the right term, since it usually refers to urethane foam insulation. Blown in mineral or rockwool, added on top of the batts would help considerably. For NJ you need at least 12" total(including batts), and a bit more would not hurt. Another method is to add another layer of fiberglass, but run it perpendicular to the joists. Both methods improve heat/cold problems considerably.

Lowes does, or did, let you have the blower for free with a certain amount of insulation.
 
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Old 09-16-08, 03:22 PM
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Originally Posted by chandler View Post
Stepping into the attic moves you from your property to the common property of the condo association, so don't make any changes without the permission of the association. You won't gain anything by adding the insulation, but oh, the difference the building will have if the association is amenable to opening up the ridge and installing a ridge vent. The air must move to let the insulation do its job. I would present it to the association and see what happens.
I'm on the board, we allow members to do whatever they want up there as far as insulation, flooring, etc. Most people add insulation to retain the heat in the unit below, but I don't care about that since I like it cold.

We had a new roof put on last year so there won't be any ridge vents being added for next few decades, if ever. There are vents on the roof itself as well as soffit vents, as I detailed in the first post.

So you are saying that insulation won't help at all?
 
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Old 09-16-08, 03:29 PM
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Originally Posted by Just Bill View Post
Spray is probably not the right term, since it usually refers to urethane foam insulation. Blown in mineral or rockwool, added on top of the batts would help considerably. For NJ you need at least 12" total(including batts), and a bit more would not hurt. Another method is to add another layer of fiberglass, but run it perpendicular to the joists. Both methods improve heat/cold problems considerably.

Lowes does, or did, let you have the blower for free with a certain amount of insulation.
OK, so you have an opposing view to chandler, you believe insulation will help keep the heat in the attic from heating up my unit below?

I agree that spray isn't the right term, I should have said blown. I've seen what you were saying about renting the machine or getting it for free with enough insulation purchased, that is what made me start thinking about doing this. It will cost a couple hundred dollars at least, so I just want to make sure it will be worth it.

I'm not sure if laying fiberglass batts on top of the joists will help since the batts that are in there now are a few inches under the top of the joist, and if I layed new batts across there would be a large airspace between the batts. What do you think?

I could easily blow in new insulation on top to make a total of 12", altho it would be above the joists and in certain areas I would need to compress it down when I lay some plywood sheets for storage.

Thanks for the help!
 
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Old 09-16-08, 03:45 PM
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Insulation works both ways, both in preventing heat from leaving your home in the winter, and from entering your home in the summer while you attempt to cool the home. It's a misconception that insulation is "invisible" to radiant heat.

Adding insulation will most certainly help slow the transfer of heat from your attic, but once the room gets hot, it will also hold that heat in.

I'd also suggest adding more UNFACED batts over the top of your existing. If the existing has sunk 3" or so, you could first add R-13 then run another layer of insulation (whatever r-value you desire) perpendicular to that.

Compressing insulation reduces its effectiveness, so you should avoid laying plywood around unnecessarily. It would be better to build some sort of elevated storage platform above your insulation.
 
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Old 09-16-08, 06:10 PM
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Guys, It does not work like that. Insulation works buy keeping the heat or cooling into a space. Attic ventilation will(help) keep the heat from transferring to the 2nd floor. The other heat will be radiant and insulation will do 000% at stopping this. Radiant barrier will stop this.
 
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Old 09-16-08, 06:36 PM
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No our opinions don't differ, and I missed the point on the roof vents. Sorry. They are second best to ridge vents, however. If adequate ventilation is afforded the space above the insulation, it can do it's job.
 
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Old 09-16-08, 06:55 PM
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In the original question, the OP seems to indicate that he does not have the recommended r-value in his attic. IMO, if they do consider a radiant barrier, it would best be installed only AFTER the recommended r-value for his climate has been installed.

Radiant barriers do reflect a percentage of radiant heat, making the insulation more effective.

The department of Energy makes the following statement regarding the benefits of radiant barriers:

"Radiant barriers are more effective in hot climates than in cool climates. Some studies show that radiant barriers can lower cooling costs between 5%–10% when used in a warm, sunny climate. The reduced heat gain may even allow for a smaller air conditioning system. But in cool climates, it's usually more cost effective to install more than the minimum recommended level of insulation rather than a radiant barrier."

Which is why I'd suggest getting more insulation in the attic first. Once radiant heat reaches the insulation it will still travel through the fibers by conduction. So it's best to have thicker insulation to reduce this effect. And then add a radiant barrier if you desire it's beneficial properties.

The June 2008 Journal of Light Construction, pages 62, 63 contains a discussion of r-value, and how it reflects resistance to conduction, convection and radiation. It also disproves the idea that "radiant heat passes right through conventional insulation", a claim often suggested by marketers of radiant barriers.

In actuality, both products have their place.
 
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Old 09-20-08, 05:12 PM
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Thanks for all the info guys!

If I go with the blown in type, it will be going over the joists (I've got 2X8's and 12" minimum was recommended). I assume it would be bad if it falls into the soffit on each side since it might block airflow, correct? Should I screw some small sheets of plywood to the lower part of the roof beams to keep the insulation out of the soffit?
 
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Old 09-27-08, 04:45 PM
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One more question, what should I do about the opening into the attic? Currently it's just a 30" X 30" piece of 5/8" plywood. Should I just glue a couple pieces of insulation to the top of it? Since I am going to be insulation the entire attic, I assume I should also insulate this opening so the heat doesn't radiate thru it into my condo.
 
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Old 09-27-08, 07:01 PM
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Only closed cell foam will keep the heat from radiateing into the home all other insulation it will let it in.
 
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Old 09-27-08, 07:06 PM
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So what are you saying, the millions of new homes with fiberglass batts laid in the attic don't stop that 120 degree heat from entering the house? I'm not understanding...
 
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Old 09-28-08, 06:52 AM
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Regular insulation will not stop radiant heat
 
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Old 09-28-08, 07:04 AM
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Fiberglass, like most other materials known to man, can absorb heat and re-radiate it. One fiber aborbs heat, conducts it to nearby fibers, and when there is a space between fibers, it can be re-radiated to them. This would continue through the fiberglass until it reaches the interior of the home. With fiberglass, only a small percentage of heat (some studies show only 5%) actually is re-radiated into the home as compared to if there were no insulation at all. If fiberglass did nothing to stop radiant heat, your ceiling would be hot to the touch just like your roof is. So it's not that fiberglass is ineffective.

Fiberglass is not as effective as foam, but it is not to say that it is worthless. All things being equal in ASTM testing, R-38 of fiberglass is equal to R-38 of foam. The advantage closed cell foam has is that you don't have air movement through foam... whereas fiberglass only SLOWS air movement.
 
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Old 09-28-08, 10:58 AM
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Originally Posted by airman.1994 View Post
Regular insulation will not stop radiant heat
You keep saying that, yet you refuse to explain.

I'll ask once more:

"So what are you saying, the millions of new homes with fiberglass batts laid in the attic don't stop that 120 degree heat from entering the house? I'm not understanding..."
 
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Old 09-28-08, 11:02 AM
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Originally Posted by XSleeper View Post
Fiberglass, like most other materials known to man, can absorb heat and re-radiate it. One fiber aborbs heat, conducts it to nearby fibers, and when there is a space between fibers, it can be re-radiated to them. This would continue through the fiberglass until it reaches the interior of the home. With fiberglass, only a small percentage of heat (some studies show only 5%) actually is re-radiated into the home as compared to if there were no insulation at all. If fiberglass did nothing to stop radiant heat, your ceiling would be hot to the touch just like your roof is. So it's not that fiberglass is ineffective.

Fiberglass is not as effective as foam, but it is not to say that it is worthless. All things being equal in ASTM testing, R-38 of fiberglass is equal to R-38 of foam. The advantage closed cell foam has is that you don't have air movement through foam... whereas fiberglass only SLOWS air movement.
Thanks for the reply!

How about the blown in type of insulation (which I would prefer to use). My plan was to just blow this on top of the fiberglass batts I have up there now, maybe 6" above the top of the 2"X8" joist. That should work fine, correct? I assume there are different types of blown in insulation, all of them will work about the same or is one type better?
 
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Old 09-28-08, 11:38 AM
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Cellulose (usually blown) will be similar to fiberglass in it's ability to insulate / slow the flow of heat toward an area of lesser heat. The bottom line is the R-value per inch so you'll have to compare.
 
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Old 09-28-08, 12:24 PM
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Originally Posted by XSleeper View Post
Cellulose (usually blown) will be similar to fiberglass in it's ability to insulate / slow the flow of heat toward an area of lesser heat. The bottom line is the R-value per inch so you'll have to compare.
Great, thank you very much!
 
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Old 09-28-08, 02:14 PM
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Your attic opening would work best with some foam glued to the lid, like Thermax, r-board, or similar.
 
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Old 09-28-08, 02:29 PM
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Originally Posted by XSleeper View Post
Your attic opening would work best with some foam glued to the lid, like Thermax, r-board, or similar.
Are these things I could find at Lowes/Home Depot?
 
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Old 09-28-08, 03:15 PM
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Yes, I believe so.














 
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