Insulating a small roof

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  #1  
Old 01-14-09, 04:23 PM
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Insulating a small roof

Hey everyone,
I am hoping that I can get some advice here on how to insulate a small roof I have. I'm a carpenter so I know the basics. What I have is a 2 story house. On the front of it there is an addition that is just the one story with about a 4/12 shed roof on it. It is un-vented space and I really need to get it insulated. I have a few thoughts. One would be to get r-30 fiberglass between the rafters and then put 1/2" rigid foam board over that and tape the seams for an air barrier. Or maybe just plastic as the air barrier. Another idea is to use the fiberglass between the collar ties the put the vapor barrier below that. Either way is basically the same except one would insulate at the roof and the other would insulate at the ceiling. I am thinking the ceiling might be easier. Now what about that vapor barrier? Should I put it up? I plan on putting sheetrock in later on. I am trying to do this cheaply. I'd get one of those do it yourself 2 part foam kits but $400 is a little steep for me and I have the fiberglass already. Let me know if you need any more information. I am planning on doing this project this coming weekend.
Thanks in advance,
Jason
 
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  #2  
Old 01-14-09, 08:07 PM
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Hi Jason, once you heat the space below, assuming you are in cold country, you should add the VB, especially if you are not going to put up the drywall right off. Seal it well. Make sure the wall to the main house that will become part of this small attic is well insulated and add a layer of house wrap if just OSB or plywood is there. Probably already done. Add side vents and soffit vents. If soffit venting is not possible, then consider the hot roof option. If you add any recessed lights make sure they are IC and air sealed. Seal any electrical wiring holes from walls that can leak air into the attic. Add a bead of caulking or construction adhesive around the edges to seal the VB air tight. Same around your walls. Of course foam in around all of the windows and doors. With soffit vents maintain an air space to allow cold air to flow under the roof deck. Usually those foam chutes. The ĹĒ rigid or plastic will work, again sealed.

I just checked your post and you stated unvented space. Is it going to remain unvented?

Iíll check back in the am
Bud
 
  #3  
Old 01-15-09, 03:28 AM
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Thanks for the reply!! I live in VT and yes it will remain un-vented.

Thanks,
J
 
  #4  
Old 01-15-09, 05:36 AM
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Un-vented, ok, now you are in the unproven zone. The ones I see that work, are the ones that place the finished ceiling directly on the bottom of the rafters, as in a v-panel pine ceiling on a log home. If you insulate and finish the ceiling, then you must perfectly air seal the attic space from any warm moist air. Wall cavities, electrical boxes, even the main house. I just worked on one this summer that had two small 2Ē vents in the upper side walls, but her husband had never finished the drywall, thus an exposed kraft covered ceiling. Each winter, when she gets a warm spell she floods as all of the frost and ice above melts.

If you insulate the rafters and then close off the ceiling, then you have isolated the rafters from their heat source below and the inside surface of the rafters/insulation will get very cold and potentially create condensation.

Spray in foam is a good option, but doesnít work in the cold. Let us know which way you go. Sorry I canít be of more help.

Bud
 
  #5  
Old 01-15-09, 01:23 PM
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Smile

I want to keep the flat ceiling so I am thinking that you mean I should insulate the ceiling ties. The cold air would be above the insulation and below the insulation would be a vapr barrier and then strapping for the drywall. Do you think this will work? Where would it condense in a situation like that. When you say seal everything you must mean between the warm space and the cold space. I shouldn't need to seal the roof right? As long as I keep the warm and cold spaces separated with insulation and a vapor/air barrier then it should be ok I am guessing. Also an r-value of 30 would seem to be enough to me, what is your take?

thanks so much for the advise,
Jason
 
  #6  
Old 01-15-09, 02:06 PM
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The ceiling ties is the easiest and yes just seal the warm from the cold. Remember, fiberglass does not stop air, so seal with caulk or can foam. Now, even though there will be no intentional venting, the space up there will breath. Just keep out the moisture so it doesnít have to breath too hard.

R-30 would be a minimum, ceilings are now using up to r-50.

You donít need to seal the roof, other than keeping the rain out. In fact, any extra air leakage you can create up there will work like venting.

GL
Bud
 
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