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-   -   insulating 2x6 rafter bays- not enough room! (http://www.diychatroom.com/f19/insulating-2x6-rafter-bays-not-enough-room-27201/)

roadk 09-22-2008 09:37 AM

insulating 2x6 rafter bays- not enough room!
 
so i am thinking of finishing off my 1926 colonial attic.
the rafter bays are 24" OC and are 2x6 rafters.
not enough room for a good amount of insulation...

so, i need rafter bay vents/baffles, then i can fit r-21.

should i extend the rafters out a bit with ferring or 2x3 to give me a slightly deeper rafter bay- and allow me to get r-30 batts int hem?

or can i use rigid foam OVER the r-21.....
and if you think rigid, do i apply that directly to the rafters, and then the sheetrock directly screwed to the rafters, touching the ridgid foam?


thanks for the help.
-thomas

Wildie 09-23-2008 01:51 PM

At one time, I made an attic into a loft. The rafters were 2X6's so I screwed 2X2's to allow an air space between the R20 fibreglass bats and the roof sheeting.
This allowed air to move from the soffit vents into the space above the ceiling and out the roof vents!
2X3's would even be be better!
Foam insulation is expensive, so I would assume that it would be more economical to use fibreglass with 2X3 furring strips.

white29 09-23-2008 03:08 PM

Don't use baffles.When I finished off my 1929 colonial attic I placed 1x material ripped to 1 inch as spacers at the 90 degree joint where the top of the rafter meets the bottom of the roof deck. To this I stapled foamboard insulation,as thick as the space would allow (dont remember the R-value) and roll insulation over this. Using this method for vaulted ceilings works well when space is limited because the spacers/foamboard are your baffle.

Big Bob 09-25-2008 09:58 PM

White29,

when I read your post a couple of days ago...I thought what a great..great... idea.. then the + & - stuff crept in.

Heat transfer: conduction, convection, radiation.

You have created a good positive area for convection air currents to help reduce the the heat gain transfer from the roof. This will help your shingles to not cook and hopefully give them long life. The lower K factor of the foam also may suggest reduced heat transfer via radiation ( possibly offset by increased surface area contact and conduction).

Unless provision has been made for the escape of humidity in the fiberglass insulation the fiberglass can not work as well.

R value = K/ thickness. K is a value at a given humidity and temperature.. ( A bell curve... ) standard R values are provided with K factor at 75 degrees F at 50% RH most K # are between .24 & .26 for fiberglass insulation at the mentioned temperature and humidity.

Higher humidity and temperature raise the K factor or value. SO the higher the temp & humidity the higher the K factor. The higher the K factor the lower the R value at a given thickness for an insulation.

The typical (W) pattern of the available thin foam baffles allows the fiberglass to expel RH and heat via convection with minimal conduction transfer.

I'm sure your system is working fine for you. If your system works better than the (W) shaped foam baffle is a ?

If installed properly the (W) foam baffle allows the roof sheathing and the fiber glass to work properly.

white29 09-29-2008 06:20 AM

Big Bob, forgive my ignorance but,HUH..........? I've done two ceilings this way,one 20 years ago and the other is close to ten years. The first was a flat ceiling ,nine feet tall which I removed and replaced as I stated.I then finished with 1X6 t&g pine. The second was an unfinished attic which I vaulted to eight feet and then it is flat and drywalled. Both roofs(hips) have no issues and neither has the interior. Where I live is high spring ,summer and even fall humidity. Precipatation is rampant,lots of rain and snow. My method may not work on all applications,but it has for me.

Big Bob 09-29-2008 07:31 AM

Quote:

Originally Posted by white29 (Post 166237)
Big Bob, forgive my ignorance but,HUH..........? I've done two ceilings this way,one 20 years ago and the other is close to ten years. The first was a flat ceiling ,nine feet tall which I removed and replaced as I stated.I then finished with 1X6 t&g pine. The second was an unfinished attic which I vaulted to eight feet and then it is flat and drywalled. Both roofs(hips) have no issues and neither has the interior. Where I live is high spring ,summer and even fall humidity. Precipatation is rampant,lots of rain and snow. My method may not work on all applications,but it has for me.

white29,

Again, I'm sure this method is working for you. I see nothing really wrong with it.

I'm sorry I didn't make it clear above. Fiber glass works best (performs better) when it can breathe. My post above was trying to explain why.

Maintenance 6 09-29-2008 07:32 AM

To the OP: You have at most 5-1/2 inches of depth to put insulation into. You need to maintain 1-1/2inches for ventilation top to bottom in the space. Compressing 6" of fiberglass into 4" of space will not give you R-21. It will be less. Fiberglass does not like to be compressed into a space smaller than it is rated for. Installing foam on the undersides of the rafters will increase the insulation value of course, but you need to plan on something solid to place your drywall against. Screwing it through the foam is not a good practice as the foam will collapse and the screws will poke through the finish.


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