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Old 12-01-11, 10:29 AM   #11
Daox
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All the r-values you see should be age corrected ratings. Initial r-values shortly after installation may be higher for some materials. Same goes for cellulose that settles in attics.

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Old 12-01-11, 11:20 AM   #12
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is one brand of isoboard better than another?
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Old 12-01-11, 11:35 AM   #13
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Its really hard to say. If I see two r-values I normally just use the lower one to be on the safe side. If its actually higher, its just that much better.
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Old 12-01-11, 03:14 PM   #14
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Quote:
Originally Posted by scottorious View Post
...I have thought about using the foam board up there. thats a very diy kind of project. what are the cons of layering foam board in the rafters?...
scottorious,

This is an approach that I have a good bit of experience with.

I decided to go this route because I wanted to get the most R-value in the least space. I have the same house configuration that you have.

When you work on your walls/ceiling, you will want to be mindful of two things:
  • Infiltration
  • Insulation

My first efforts were to use fiberglass and put it in the wall and then sheet rock over it. That method was better than no insulation at all, but it didn't get at infiltration, and because of this, the insulation in the wall was not as effective as it could have been. Also, the insulation I put in at the time was rated to be R-13, not so high, and the flow of air through the wall degraded that R-value considerably.

My second effort was to fill the wall cavity recycled shipping peanuts, because it was free, and testing showed it to be on par with fiberglass. Subsequent testing on a plastic peanut/pearlite mix proved much better, because the pearlite filled in voids in the peanuts, and yielded an R-value that was higher than cellulose. I used this combined method on one side of the house and it worked well, but not as well as subsequent methods...

My third effort was to use recycled insulation board chunks in the wall, but I didn't seal between layers or pieces, so the efforts were little better than nothing.

My forth effort was to use new rigid foam board (expnaded ploy-styrene, AKA: EPS), carefully cut exactly to size and layered in the wall until the cavity was filled 100%. This worked so well that I was shocked at the difference between this method and all previous methods. The difference was that I had completely eliminated all air movement in the wall (infiltration), along with using an R-4 to 4.5 insulation. Using this method, I was actually getting minimum R16 in the walls, instead of using fiberglass reated at R-13 in an unsealed cavity and having that R-value degraded to some unknown value that was significantly less than R-13, by air flow through the wall.

I was so impressed with this method that I thickened the walls and roof an extra two inches to recieve another 2" layer of EPS, in the walls and ceiling.

I am continuing to work with this method, with the exception that instead of cutting exactly to fit, I now cut 1/4" all around and run a bead of canned foam in the 1/4" gap to seal it. This has proven to be easier than cutting exactly to shape, and makes a better seal at the edges. Care must be taken to not allow the canned foam to get behind the EPS, because it will expand and push the EPS out.

When I was selecting foam board to use, I had the following choices:
Expanded Polystrene (EPS) - (R-4 to 4.5 per inch)
Extruded Polystyrene Board (XPS) - (R- 5 per inch)
Polyisocyanurate Board - (R-6.5 per inch)

In making my selection, I tried to weigh cost vs. R-value and decided on EPS. Subsequently, I have read about the possibility of degraded R-value in Polyisocyanurate, due to the gradual absorption of moisture. Here is a link on that issue, you should do further research.

Also the cost of energy has gone up since I started the project, I might lean more towards XPS now, especially important in the ceiling.

Another issue for you to consider is that using this insulation method will radically reduce the infiltration of air into your house, and you will require mechanical ventilation in the winter, I would recommend some kind of heat exchange ventilator (AKA: HRV). The mechanical ventilation will be required both for humidity control and also for air quality.

Also, for this method to be completely effective, you need to concentrate on the junction where the roof-slope meets the wall, and make sure that this area is totally sealed. Same goes at the details around the foundatoin and the junction where floors meet walls.

This method is not easy or cheap, but given that you are working on an older house, it is amazingly effective.

Regards,

-AC_Hacker

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