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drewdin 11-25-2012 08:23 PM

Ceiling Insulation Question
I am insulating the ceiling in my sunroom. The rafters are 2x6 so I only have 5.5" of space for insulation, I planned on getting an R23 for the 2x6 and I also planned on putting a 1" Rigid foam under that to try and get to an R value of 30.

My question is with the many different types of foam insulation, which one should I choose? I found one at Lowes and one at Home Depot but i am not sure which is better, both have an R value of 6.5 but i am not sure which is better for an interior ceiling?

Any help or suggestions would be greatly appreciated, thanks!

Home Depot Insulation

Lowes Insulation

nealtw 11-26-2012 11:22 AM

You need to leave a gap above the insulation so air can travel from the soffet to the house roof for proper venting of the house roof.

AndyGump 11-26-2012 10:42 PM

Well the foam should go directly under the roof deck sheathing, tight up against it too.
Then fill the rest of the bay with the fiberglass insulation.
This is just a very general description of how the code requires insulation in an un-vented attic space. I do not know your location and your local jurisdiction may require more insulation than what you have described here.


drewdin 11-27-2012 07:45 AM

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Thanks for the info, the ceiling im trying to insulate is a porch that was turned into a sunroom. There are no soffits or attic, its more of a slanted roof off the back of my house.

@andyGump, I though the foam insulation acted like a vapor barrier and it should be under the insulation between the warm and cold zone?

Although its not the best picture, this is what I am in the process of insulating. My plans were to add insulation and then the foam. Do you recommend that I put the foam first and then the insulation? I live in Boston and its 20 degrees and snowing today, im trying to seal this up quick, thanks!

nealtw 11-27-2012 10:45 AM

drewdin; Are we not looking at the bottom of the house roof at the top of this roof?

drewdin 11-27-2012 11:30 AM

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@nealtw, no the house roof is above and off to the side of this roof. Here is an outside side picture of the sunroom.

nealtw 11-27-2012 11:58 AM

So we are looking at the original soffet inside. That is what I was talking about. If you close this up, you will have no soffet vents for that part of the main roof. The common way to deal with that is to allow air to travel from the outside of the shed above the insulation and into the attic of the main roof. Have I missed something?

drewdin 11-27-2012 12:54 PM

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Hi Neal, this roof buts up against the side of my house, the air would have no where to travel if I had soffits and a path for the incoming air from the soffit to go other than the inside of the house.

maybe this picture is better. my soffits, ridge-vent and everything else are on the top roof, not the sunroom roof. Since the house is a colonial gambrel, those side panels you see are parts of the interior of the house.

nealtw 11-27-2012 01:18 PM

Now I understand that two don't meet. I would still look at venting this roof. This low slope and snow will sit here and the outside wall will still be a place to have the most heat loss. That heat loss is what causes ice dams and free flowing cool air above the insulation is your only defence. And yes you can vent the roof where it meets the wall.

drewdin 11-29-2012 07:54 AM

the ceiling joists are only a 2x6, i dont have much space. I planned on putting Roxul R23 which is 5.5" thick for insulation there. My thoughts are that the insulation would help keep the heat in vs losing valuable space to try and vent the ceiling. What are your thoughts

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