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Questions about attic insulation and vapor barriers.


GettingItDone's Avatar
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02-13-13, 07:57 AM   #1  
Questions about attic insulation and vapor barriers.

I have a cape cod with finished space in the 2nd floor. The insulation is inadequate so I want to redo it. I would like to treat it as a cathedral ceiling outside of the knee walls. I also want to add a radiant barrier as the roof has dark shingles.The rafters are 2x6s 24" OC.
My thought is to use 1" polyiso board with foil spaced two inches from the underside of the roof deck (taking the place of Smartvent or a similar product). My thought on this is that it will stop/slow the radiant heating from the roof before it gets to the insulation. To keep costs down I would then use R13 batting to fill the space. This should be about R 21.5 plus the radiant barrier. My question is, since the polyiso board is a vapor barrier ( I think), should I use unfaced batting so that I am not trapping moisture? If I wanted additional insulation I would most likely use a rigid foam board product on the inside of the batting. I know that the pink XPS is vapor permeable so would that be ok to use?
My concern here is making sure that I don't end up with two vapor barriers and trapping moisture in the insulation layers. I appreciate any thoughts. Thanks.

 
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02-13-13, 08:38 AM   #2  
It is always hard to project from the various opinions and research out there an exact answer for every proposed solution, that's my excuse for maybe not getting this right .

First, your numbers don't add up as given. The 2" gap plus 1" PIC leaves just 2.5" for fiberglass and you can't get r-13 fg in that space. If your 2" gap was reduced to 1" the numbers work, if 1" is acceptable. Also, where the RB may provide some summer benefit, it will not add to the winder insulation value.

Your bottom line is, that 5.5" is just not enough space for all of the R-value you want.

Another also. Capes are a pain, I live in one, and the details of how the roof rests in the walls below is important. Do the rafters rest directly on the wall below, or was a deck built and covered with the rafters resting on the deck. In the first case, air flowing in through the soffits has easy access to the insulation that extends under/over the living space. In the second method, that space is somewhat sealed. There are other methods and we need to understand how yours is constructed to advise on air sealing and insulation placement.

Also, you are addressing just the slope of the kneewall space. How is the slope above that area where it is finished on the inside, insulation, vent space? Air flowing up through a vent space directly above fiberglass insulation will greatly reduce the R-value of that insulation.

I have avoided the vapor barrier question for the moment as I need to decide what role the 2" X 6"s are playing in the drying process. The wood may be a vapor short circuit, but I haven't seen a reference to that.

Air flow is far more critical in the movement of moisture so attention to sealing all leakage paths will be important.

And, if you super insulate that side attic, but don't heat it, it will still get cold. And if there is any moist air available, condensation and mold may follow. Closets located on exterior walls suffer from the same issues due to a lack of heat.

I'll let you catch up.
Bud

PS welcome to the forum


Last edited by Bud9051; 02-13-13 at 08:39 AM. Reason: Welcome
 
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02-13-13, 11:31 AM   #3  
Right!

I obviously forgot about it polyiso thickness. I suspect that above the sloped knee walls is a batt of some sort - I don't know the thickness. The finished space is wood paneling and not drywall so I expect to deconstruct and redo that space. I'm in Ohio so heating and cooling are both an issue. The knee walls are only 11/2" inches thick. There has been an 8" duct run to the 2nd floor with two vents. The living space is about 300 sq. ft. There is currently insulation on the back side of the vertical knee walls but obviously, not much. The space outside the knee walls are floored with boards. There is some insulation under at least some of the flooring. The space is a T shape and there are closets and built in drawers along both sides of the T. So, it's basically a big mess. Even with an additional window A/C units it is very hot in the summer. I think that a radiant barrier would help with this. Also, since the space outside the knee walls is fairly small and there's minimal insulation in the floor there, I thought insulating the rafters rather than the outside of the knee walls might be beneficial. I have also been looking at Roxul for the rafter spaces and their site Ceilings and Roofs
shows just leaving a 2" space above the Roxul and not using baffles. I guess since it is much denser that fiberglass the R value won't be degraded by the moving air in the channel. I would also consider adding 2x4s to the existing rafters in order to get space for more insulation. I definitely want to use a radiant barrier somewhere. I am not sure what the rafters are resting on as I haven't pulled out the insulation that was stuffed in there. The house was built in 1946 if that helps. I suspect the rafters are resting on the side wall. I also will be installing venting at the soffit/facia. There are large triangular vents at the front and rear as well as a turbine and ridge venting. Since there are no vents at soffit level these are marginally effective. I will be adding soffit vents. Hope this gives you a better idea of what I'm dealing with. Looks like you may have similar issues.

 
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02-13-13, 12:51 PM   #4  
Don't expect a huge improvement from the radiant barrier. Insulation is number one for summer heat. The discussion can get rather technical, but basically the solar energy that falls on the shingles has to go somewhere. A RB located where you are proposing will reflect the radiant energy back to the bottom of the roof, increasing its temperature. Some of that increase will radiate back into space. Some will blow away with the wind. But some will convect and conduct right back where it was headed in the first place. In addition, dust and such will reduce the effectiveness of the radiant surface over time. Lots of articles if you want me to dig.

Here is a link that pictures some of the issues related to how the rafters are positioned over the walls below.
http://www.dos.ny.gov/DCEA/pdf/Energ...insulation.pdf
If you have something like fig 12.4 then sealing the air flow path can be difficult.

In Ohio, your minimum ceiling R-Value is R-49, walls R-21. The question becomes what do you use on a slope? Is it a wall or ceiling. It is actually a ceiling and the walls would have been specified as R-49 if it was practical to add that much. Pushing homes to be built or modified to 12" walls was not practical. It gets worse. Building codes are considered absolute minimums, so more is better. I know, you are running out of space and money, but controlling heat loss or heat gain will benefit from something more than R-21.

You mentioned re-constructing the inside. If you go that route, some major improvements can be achieved.

My current thinking is that the polyiso sealed between the rafters would not constitute a second vapor barrier as the wood would provide a path. If I locate anything different I will post.

Hot off the press, an article on using Roxul. Just interesting reading. Installing Roxul Mineral Wool on Exterior Walls | GreenBuildingAdvisor.com

Bud

 
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02-16-13, 10:14 AM   #5  
Thanks for the input.

The information about RB is helpful. I realize that anything I can do is going to be a compromise. Without just doing away with the finished 2[SUP]nd[/SUP] floor space there is no way I can meet the recommended minimum R Values as there isnít enough head room to add much, if any, additional sisters to the sloped finished area. I am hoping that good ventilation and as much insulation as I can put in will make enough difference so that the heating and cooling that I can provide will at least make the area livable.
Thanks again for the assistance.
`

 
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02-16-13, 12:08 PM   #6  
A quick list of what you might do. Remove the current paneling and all insulation. Nice and open and easier to do right. One inch polyiso with the foil and spaced 1" away from the roof deck. Fill with 3.5" Roxul. Cover with another one inch of pink or blue and then drywall.

With built-in drawers and such, insulating the rafters makes sense. Just do a very good job of sealing all air leaks from the conditioned space into the rafters. Air leaks carry far more moisture than what can pass through the drywall.

With the cold weather this is not a good time to open things up. You could be faced with instant condensation problems.

Other options would be to apply a thin RB directly to the bottom of the roof deck. In the south they use a plywood with a foil film already on the bottom.

And last would be to consider spray foam to fill the entire space, 5.5". That would eliminate the need for the RB, as insulation does a better job. Wait for warm weather, and DIY or hire someone. Not cheap, but more r-v for the space. Search "hot roof with spray foam".

Bud

 
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