Attic insulation and plastic baffles for ventilation

[Gordon Maney]

You can buy pieces of plastic that go in your attic, attach to the rafters, and keep a channel for air open, so that the presence of insulation does not cause that channel to be blocked and hinder proper ventilation.

I don't know the official name for these things, but I bought some that were for 24 inch spacing. The problem is that they are too narrow, but obviously too wide for rafters on 16 inch centers.

The overall width of this plastic piece is roughly 22", and the inside to inside dimension on the rafters is 22-1/2"

What am I missing here?

[MoparNorm]

That is weird,they should be 14.5 or 22.5???
We don't use them much out here, except in the mountains. They separate the cool exterior surface from the insulated side of the roof and prevent or lessen "icing" at the drip line.

Is there by any chance a clip or channel that they slid into to make up the difference?

[Gordon Maney]

That is weird,they should be 14.5 or 22.5???
We don't use them much out here, except in the mountains. They separate the cool exterior surface from the insulated side of the roof and prevent or lessen "icing" at the drip line.

Is there by any chance a clip or channel that they slid into to make up the difference?

How are these things supposed to work, they extend 3/4" onto the lower face of the rafter? You would have to notch for the joist, too, wouldn't you?

They are not wide enough, I don't think, unless I stretched them out, but they don't seem to like that much. There was another kind made of a sheet foam, but they seemed really fragile to me.

[Steve G]

The baffles are used in a full size attic to keep from blowing insulation in the soffit and blocking the air flow. If you have a story and 1/2 type house you don't want the air blocked between the knee wall and the attic space. I have seen these used in that application but a better way is to nail a spacer to the side of the joist tight to the underside of the roof on both sides of the space then cut a piece of thin plywood to fit in the space. When you insulate the baffles wont be pushed shut and you know there is maximum air flow.

[JimmieD]

Is the length correct to lay them in the other direction somehow, or are they square 22" X 22"?

[Gordon Maney]

Is the length correct to lay them in the other direction somehow, or are they square 22" X 22"?

They are 48 inches long. The cross section of them would not lend very well to turning them the other way. I did consider that, but I don't think they intended for them to be used that way.

[MoparNorm]

No guys, you can't lay the baffles across the rafters, that defeats the entire purpose of the baffle and the insulation.
Two scenarios, one is a finished vaulted ceiling or cathedral type ceiling which gets a continuous baffle from eave vent to ridge vent, like this:



It is important to maintain the 1.5 to 2" air space the entire rafter run AND still have room for the roof insulation.

The other scenario is an unfinished attic, with the only requirement is to keep the soffit vents clear, like this:



This photo show cardboard, that will quickly mold and rot with any amount of moisture, the plastic is the wise choice.

[Gordon Maney]

I did find some photos of insulation baffles and saw that you don't fasten them to the rafters, instead you staple them to the underside of the roof deck.

I guess I need to get a stapler and some short staples. I had been imagining that you fastened them to the underside of the rafters. Wrong again....

[Steve G]

What I said is in Norm's pictures. Gordon, are you doing the catheral type or the other type?

[Gordon Maney]

The ceiling is level, under a pitched or sloped roof. Not a cathedral.

[OLD DODGE]

Good Info

Perfect timing! I had a pole barn with attic trusses put up recently and am at the point where I need to insulate. I was wondering about not blocking the airflow. Very helpful info. Thanks guys!

Kevin

[MoparNorm]

I did find some photos of insulation baffles and saw that you don't fasten them to the rafters, instead you staple them to the underside of the roof deck.

I guess I need to get a stapler and some short staples. I had been imagining that you fastened them to the underside of the rafters. Wrong again....

Yeah but....if you staple them to the underside of the roof deck, and not the underside of the rafter (or mid-way as in the photo) how do they allow air flow, up from the soffit???
It would seem that they serve no useful purpose there, unless they are ribbed?????

[Cheyenne Dave]

Yeah but....if you staple them to the underside of the roof deck, and not the underside of the rafter (or mid-way as in the photo) how do they allow air flow, up from the soffit???
It would seem that they serve no useful purpose there, unless they are ribbed?????

The units I use are made from the same polystyrene (SP?) that is used in egg cartons. They are shaped like a slightly flattend hat channel, similar in cross section to those used under the bed of a Power Wagon. They are attached by stapleing through the brim detail of the hat, leaveing a nice long trough for fresh air to travel up each individual rafter bay from the soffit. This system works best in concert with continuous soffit and ridge vent, thereby venting each individual rafter bay straight up to the peak.

They are stapled to the underside of the roof deck, as Gordon mentioned. They slide over the top plate and should just barely extend into the soffit space. The main objective here is to keep both sides of the roof deck as close to the same temperature as possible, year-round. This is not only to prevent ice damming, as Norm pointed out, but to prevent premature failure of the roof's deck and asphalt shingles.

When you see shingles with the corners of the tabs curling down, that is caused by poor ventilation. Notice that this condition never occurs over the unheated area over the eves. Also, you will find that a roof with inadequate ventilation is sufficiant grounds for voiding the shingle's manufacturers warrentee. This is usually printed right on the package.

C.D.

[MoparNorm]

That sounds reasonable and make sense, if the rafter bay is also insulated as in the drawing.
I insulated all of my roofs as well as the attics, a cool roof lowers the attic temperature by at least 20 degrees in Summer and keeps it about 10 degrees warmer in Winter, the baffles work well to inhibit ice dams.

[BobbyMike]

C.D. has got it right. They don't need to be tight to the rafters. They make them a bit small so that they're easy to install. If you're not insulating the roof of the attic, but the "floor" then they only need to go from the soffit over the level of insulation.
We use the polystyrene (egg carton) version's out here.
If you install them all the way to the top, start at the top and work down, overlapping each one slightly. That way any moisture that collects will drip down to the soffits and not on your insulation.