Insulation in roof rafters???

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  #1  
Old 01-23-13, 07:53 AM
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Insulation in roof rafters???

Hello,

I have a workshop that I am trying to insulate. The roof pitch is 12x12 and the rafters are 2x8s 16"oc. It is a cathedral ceiling, so I need to tuck in some insulation between the roof rafters.

My question is this....I know I need ventilation and I have soffit vents and a ridge vent. I know I need to keep a 1"-2" air gap between the insulation and the bottom of the roof decking plywood- so air can circulate between the soffit vents and the roof vent. They make these plastic 4' sections that you tack in between the rafters to make sure that gap is kept there.

Do I really need these? Or can I just roll in R-19 insulation and tack to the bottom side of the 2x8 rafters. The R-19 will leave a 1.5-2" gap for the air to circulate (assuming the 2x8s are 7-7.5" deep or so. I know that if I put in R-30, it would not- and would therefor require these plastic spacers to keep the gap. I am wondering if these are entirely necessary (cost about $2 per 4' section and I need about 250 of them. Or are they made for sprayed in insulation primarily.

Thanks very much for your advice.
 
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  #2  
Old 01-23-13, 08:07 AM
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Fiberglass insulation performs poorly in the presence of air movement, since the incoming cold air would simply flow down through it and steal the heat from your drywall. A mineral wool like Roxul is very dense and would perform better, at an added price. A thin strip of foam (1.5 - 2") on each side of each rafter cavity with 1/2" rigid foam the full length creating the gap is an alternative, but I haven't compared the cost.

Your number one objective should be to eliminate any air leakage from the house to these cavities. The drywall does the majority of that job, but electrical boxes or can lights act like open holes and transport moisture and heat to where you don't want it.

They make a fiberglass insulation with a perforated plastic covering. Whether that would reduce the degradation from the air movement I'm not sure. I have never used it.

Bud
 
  #3  
Old 01-23-13, 12:39 PM
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thanks- that makes sense.

I doubt warm air would escaping too much from the room area, through the drywall on the ceiling and into the cold air area. No can lights in the ceiling- so that will help.

I guess the big question is...does installing these plastic trays to maintain the air gap (they are 16" wide and 4' long, and fit between the rafters) - are these a necessity?

Or would the fiberglass perform poorly with these installed, just as it normally would.

Thanks
 
  #4  
Old 01-23-13, 02:30 PM
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They will definitely help the fiberglass insulation perform better, but not at its best. Although "best" for fiberglass is an ongoing debate.

You suggested R-19. What if you installed R-13, you would expect your heating costs to increase. The room will still be warm, it will just cost more to heat. Omitting the baffles or other vent chute configuration or not switching to mineral wool will simply cost more to heat that space. Does the extra cost of adding the baffles save enough in heating costs to justify using them? That's the most asked question.

Give me your nearest large city with similar weather conditions and the surface area of those slopes and I will create some numbers to help you decide. Include your method of heating and fuel source and cost per therm or gallon. You also said this is a workshop, so will it be heater 24/7 or intermittently and how many hours per day on average?

Bud
 
  #5  
Old 01-26-13, 06:34 AM
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Hi and thanks for your help.

This is a question about insulating the rafters in a workshop - near Detroit, Michigan.

I don't use the space often. I do have a wood burning stove in the shop- and when I plan to work there in colder months, I will generally start a fire in the morning and wait a while for it to warm up. I was hoping that I could devise a way to keep the warm air in there- but it is not a huge cost issue with me since it is wood and I have plenty.

The rafters are a above a lofted area that is used for storage mainly, but I wanted to do it right in case it turns into more of a sitting area, or even guest sleeping area some day down the line. I will plan to sheetrock directly to the rafters once insulated. I also have a ceiling fan in this area, which I thought would help keep the warm air from the woodstove down instead of rising and escaping through the roof/ceiling area.

Thanks for your suggestions.

RE- R-13 vs R-19. - I can safely fit the R-19 in there with adequate ventilation space I believe- so I would just assume get that in there instead of R-13. My main question is are these plastic trays really necessary/helpful or are they mainly for sprayed in foam insulation.
 
  #6  
Old 01-26-13, 09:32 AM
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Actually, they are more important for fiberglass. Because cold air flows up through that cavity, something is necessary to keep the air out of the FG. Cold air sinks and would wash right through the insulation and accelerate the heat loss. Plus, FB batts tend to fluff up over time. But since you have 7.5" to start with, a space should always be there. Consider the "Poly-Encapsulated Insulation", I believe there are different brands, but this is one. Poly-Encapsulated Insulation - Building Insulation | Johns Manville
The encapsulation is vapor permeable to allow moisture to pass (necessary), yet it should reduce the air infiltration and the fluffing effect and provide a better air path. And it should cost a lot less than the extra $2 per baffle.

Bud
 
  #7  
Old 01-26-13, 12:50 PM
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great thanks

So to clarify....this poly-encapsulated from Johns Manville would NOT require the plastic trays to be installed correct?

Thanks again Bud
 
  #8  
Old 01-26-13, 01:55 PM
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You have 2 x 8's which are kind of inbetween for insulation depths. If you had 2 x 6's I would say the baffles are required or you would not have an effective air gap. With the extra 2", if you use R-19 insulation, typically 5.5" deep, there should always be a gap. However, air flow over exposed fiberglass would be throwing away half or the R-value you paid for. Your choices are to construct an air channel, baffles or other, above the fiberglass or as a compromise I have suggested the ploy-encapsulated FG to perhaps lessen the infiltration and preserve more of that R-value.

If you realistically plan to improve this area and fully heat it, there wouldn't be a better time than now to add the baffles and use the max insulation that can fit in there. If that isn't going to happen and codes allow, it's your choice.

If you look at baffles, you will see that there is a starter baffle for the low position. That one differs in that it hangs down over the end of the fiberglass to protect it from the incoming air. Just wanted to mention it.

Bud
 
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