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Thread: roof ventilation

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    Member akfirefighter's Avatar
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    Default roof ventilation

    When insulating a roof what is the best way to ventilate? I have 12" rafters and was thinking about using 8" batts for my insulation. Would the 4" provide enough ventilation? Also, I didnt put a ridge vent on my roof, would I need to put one on for the heat to escape properly? How thick of a vapor barrior do you guys recommend.

    Thanks for the help.

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    I would go with 10" batts to leave a 2" air gap (which is plenty) or install durovents from the eave to the peak and use 12" batts. If you use durovents, be sure the insulation doesn't cover the ends of the vents so that you maintain a free air path all the way to the peak. Yes, you will need to install a ridge vent to properly vent the space.

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    Member Vince's Avatar
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    Quote Originally Posted by akfirefighter View Post
    When insulating a roof what is the best way to ventilate? I have 12" rafters and was thinking about using 8" batts for my insulation. Would the 4" provide enough ventilation? Also, I didnt put a ridge vent on my roof, would I need to put one on for the heat to escape properly? How thick of a vapor barrior do you guys recommend.

    Thanks for the help.
    yes you can get R 30 in 8" bat. install the card board or foam rafter spacers inbewteen the rafters to hold the insulation down lower in the rafter and force the air space, remember a good 6 mil vapor barrior, and tape or mastic each joint and tape the staple lines.

    you will have to place roll blocks between the rafters over your walls, and drill and screen vent holes in each one... the screen is to keep BEES out. if your using a vented soffet then a shorter roll block can be used, like 2x8
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    Member fullbush's Avatar
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    Quote Originally Posted by Vince View Post
    yes you can get R 30 in 8" bat. install the card board or foam rafter spacers inbewteen the rafters to hold the insulation down lower in the rafter and force the air space, remember a good 6 mil vapor barrior, and tape or mastic each joint and tape the staple lines.

    you will have to place roll blocks between the rafters over your walls, and drill and screen vent holes in each one... the screen is to keep BEES out. if your using a vented soffet then a shorter roll block can be used, like 2x8
    Good info Vince but I don't see the necessity to tape the staples in the vapor barrier. Its a waste of time if you have an attic access because thats far from airtight

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    Member Vince's Avatar
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    Quote Originally Posted by fullbush View Post
    Good info Vince but I don't see the necessity to tape the staples in the vapor barrier. Its a waste of time if you have an attic access because thats far from airtight
    if your insulating the floor of the Attic, it remains the COLD air space above the warm. and that is part of the insulation factor. the tape on the staple is to seal the HOLE made by the staple that will allow moisture ( vapor) to escape into the insulation and cause icing, in Loose or less then air tight conditions it is not so important, however many people
    are building tighter and this causes pressure to force unwanted vapor into any void it can get into.. hence the need for Air -air exchange systems or wall vents to allow it to escape to the out doors. icing on thermal pain windows is a key in identifying lack of ventilation.

    however when insulating between rafters for a Loft type installation, there is no Attic access, and that cold air space above the insulation is diminished, and every possible stopping point of the vapor is really needed, to prevent icing on the inside of the roof and condensate from forming in the warm times to prevent moisture in the insulation. if there is not enough ventaltion... it will act as a hot roof rather a cold roof and cause eve icing faster then you can say go...
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    Member Vince's Avatar
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    BTW a good weather strip and a latch on the attic hatch will make it air tight for about 30 bucks. if sheet rock back it with a light ply wood and a peice of blue board. for a few more $$
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    Member alaskachuck's Avatar
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    Well put Vince I could not agree more...Plus use 6mil vapor barrier....It is not that much more and holds up much better......Holds the staples, wont tear and is what we use commerical.

    Plus you can always caulk the hatch shut....cut it clean with a utility knife when you need access and recaulk.
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    Member akfirefighter's Avatar
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    Thanks for the replys. What do you guys do for the floor of your cabins? Do you insulate them with batts too? Should the floor get a vapor parrior too?

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    I never liked the idea of shoving your batts up under a tightly wedged piece of foam in the rafter bays; sure, it adds r-value, but how does the bay vent? Cardboard baffles work, but you don't need 'em if you properly size everything. For floors, it really depends on your foundation. Posts? Skirting? Poured wall?
    Generally what the engineers are saying nowadays is insulate the perimeter, vapor barrier on grade, and don't insulate the floor, don't vent. I don't have skirting, but I put R-19 in the floor and ply up underneath to keep the squirrels out. Don't need vapor barrier with T&G subfloor.
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    My ceiling is R34.... couldn't fit R38 in without losing the airgap. I also used the cardboard to keep the eaves open. 6 mil vapor barrier fully taped. I missed a couple staples and left a small hole. Withon a couple months, I had to tear out a couple pieces of insulation due to mold. I HIGHLY SUGGEST taping, especially when you are looking at propane as a heat source.

    My floor is R34 also. I used JCIs and after stuffing the insulation, I cut blueboard and sit it on the ledge to keep it from sagging and creating cold spots. I then put OSB over the bottom. Day 2 at subzero and my floor is warm. Last year the floor leaked REALLY bad. The bottom of my front loft was brutal last winter. It is framed with 2X10s, I put R34 in it, the covered it with 1/2 plywood so I can clear coat it next spring. It is working very well this year. I plan to put wood looking linoleum on the floor as a vapor barrier and area rugs where the floor is not exposed to snow or high traffic.
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