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Latest dumb question-radiant barriers


Edinpitt's Avatar
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04-17-02, 02:25 PM   #1  
Edinpitt
Latest dumb question-radiant barriers

I'll bet this has been asked before, but ...
With the latest hot spell we are getting in my neck of the woods (Pittsburgh), I am discovering that our newly bought Cape story and a half bakes the second floor. After all, the second floor is surrounded on three sides by hot attic. Now I have plans to add some small amounts of insulation and ventilation to the second floor part of the attic, currently stymied by a jammed attic door leading the top part of the attic (another story). But I have a feeling that R-13 on the knee wall and sloped ceiling ("raked roof"? - I saw that somewhere) is not going to keep out all that heat. An attic fan is a possibility, though they all seem to be more powerful than I probably need or want. But here (after all this) is my question.
What about a low rent radiant barrier, using household aluminum foil? Why not get a more permanent one? Well, for one thing it wouldn't work well with the sloped ceiling (no room except for the space between the rafters - I would have to cut slices). For another, I am fairly convinced that having a radiant barrier in the winter in our climate would be a bad thing. I am looking for a sytem that works in the summer, and can be discarded in the winter (I want heat gain in the attic in the winter). A disposable barrier fits the bill.
So what am I over looking ...
thanks,
Ed

 
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04-17-02, 06:20 PM   #2  
You can go to http://www.ornl.gov/roofs+walls/radiant/rb_02.html this is a fact sheet on radiant barriers by the Dept. of Energy and is written by Oak Ridge National Laboratory.

As far as your question concerning aluminum foil, yes you could use it but you're going to find it difficult to apply, if not impossible without tearing the foil. You might be better off getting a radiant barrier. As far as removing it for the winter, it's not necessary if installed correctly. If you read that DOE site, it explains some of the problems with installation of radiant barriers.

The 3 types of heat transfer are conduction, convection and radiant. In cooling the dominant heat transfer is radiant but diffusion still plays a factor but is not dominant. In heating the dominant heat transfer is diffusion, which is conduction, but radiant still plays a factor. What this means is the radiant barrier applies to both heating and cooling, more so in cooling than in heating. Once you apply a radiant barrier you don't have to remove it.

 
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04-18-02, 10:19 AM   #3  
Edinpitt
Thanks

Thanks for the response. I think I might try using regular aluminum foil first, with a "I only have a little to lose" attitude. Actually, I suspect I will use a mix of things because of the nature of a story and a half house. I know I did read somewhere that radient barriers are not recommended in cold climates, I thinik it was a different DOE website. Not surprising that even two DOE sites would disagree, it seems like there is not a whole lot of agreement about the value of types of insulation or ventilation (everyone has their favorite gimmick). Me, I just want the secod floor to be more comfortable.
Ed

 
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04-18-02, 12:30 PM   #4  
You apparently have a Cape Cod house. One of the problems with insulation is that it prohibits heat flow by retaining the heat. Which is good for both heating and cooling because it prohibits the flow of heat. The bad part is that this retention increases the ability for the heat from the attic and roof to radiate into the rooms below. There are some other ways that you can alleviate the problem you're experiencing. At the bottom of this message is an icon with "www", click on it and it will bring you to my site. Read topics, "Insulation, Air Boundary, Thermal Boundary and Ventilation".

 
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04-18-02, 02:46 PM   #5  
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Well, I did take your suggestion and glanced at those topics, I have looked at information on your site before and I will study the matter in a bit more depth. But, if I might continue a bit of the topic here... Yes, as I understand it my house type is a Cape Cod. I sort of assumed that part of the reason the second floor is warm is that is is surrounded on three sides by hot attic. Now, I have not been to the top part of the attic yet (jammed attic door that I do not want to take a sledge to yet), but based on what I have seen in the side attics, there are 2 inches of rockwool all around. There are 2x4 joists (?) on the knee walls, and 2X6 rafters, including the sloped ceiling. As I said, I have not been to the top part yet, but there are 2x8 joists over the first floor in the side attics (into which I stuffed 6 inch fiberglass over the existing 2 inches of rook wool), so I am hoping that is what is up top too. I want to put another couple of inches of fiberglass on the knee walls (when I can find a vendor that sells only 2 inch fiberglass, or when I chop up some R 19), and something into the sloped ceiling (I am really thinking about some permanent radiant barrier strips there), and whatever the top will take. The ventilation consists of gable vents (which I have not inspected) and 7 soffit vents made by knocking out the panel between the rafters and placing wire mesh there. The vents are at three corners and in mid points along the soffit area. The fourth corner has a finished (tiny) room, which apparently made it impossible to add the last soffit vent. I have not closely inspected these vents yet, when it cools down I plan to take a look at the vents for blockage, and maybe use compressed air to clean them.

So, I guess, good insulation and airflow ... by 1970's standards, when I am guessing a retrofit was done. Not so hot now. Well, too hot really. I am looking for temporary fixes and long term solutions I can implement in a few years. Hence the radiant barrier idea. I know we will need a new roof, so I would consider a cold roof at that time, but I am praying we can wait a few years. Are there other things we can do, particularly less invasive (not cutting into the exiting roof)? I was thinking of hijaking Solar Attic's idea of a duct with an inline duct fan pulling about 250 CFM from the center of the attic to cool the attic (part of the reason has to do with the odd split gable vents: a regular gable fan would likely have an air short circuit). I am also thinking about soffit vent plugs. They may be tiny and nearly useless, but I own a drill with a 3"hole saw, not a jig saw (and I would be nervous about making the cut anyway), and I could put a plug vent at every soffit.

Uh, any thoughts, suggestions (apart from telling me to adjust my medication).
Thanks,
Ed

Oh, I will throw in that I used "Insuladd" ceramic paint additive some on the first floor and all over on the second. The review is mixed, the house has been comfortable during the heating season, though I don't feel it retained heat as well as I would have liked, but the stuff has not seemed to help as much during this hot spell. Mind you, we painted immediately, so I do not have a baseline to compare with.

 
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04-18-02, 06:51 PM   #6  
A cold roof would resolve many of the problems associated with a Cape Cod style house. Besides making the upstairs cooler, it allows you to add more insulation if you want. I would also recommend you have a light colored shingle installed. Some studies have shown that a light colored roof does more than some radiant barriers, when it comes to cooling the house.

The research on radiant barriers is incomplete. What makes them even more difficult to deal with is that they double as a vapor barrier. Which creates some controversy in some climate areas. I usually don't recommend or discuss them as a result. I wish Oak Ridge or Tennesee Valley would finish their research on it, not because I would like to use it in my work, but because I'm ask questions on it all the time.

If there isn't any blockage on your slant wall upstairs and there is full access from your soffits to your gable vents, then I would recommend a gable vent fan. They usually operate at 1850 cfm and judging by your description, it should be fine. The way that I would install one in your situation is to cut the sheet rock between the ceiling joists below one of your gables. I would make it wide enough for you to get access to the gable without cutting the joists. I would save the sheet rock and glue the insulation to the back of it. Then use molding around the hole I just cut open and make sure there's at least a 1/8th inch inner lip in the opening from the molding. Then insert the sheet rock diagonally to get it above the lip created by the molding and lay it on the lip. It should cost you around $40. to do it.

 
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