Attic Insulation Baffle Question

[diogenes]

I have a question that I can't seem to find an answer to about baffling the roof deck for blown in insulation. I suspect the answer is applicable to other types of insulation in the attic.

I am going to blow cellulose into my attic space and need to put baffles in to preserve air flow under the roof deck. But:

1) The joist spacing ranges from 32" to 36" between 4"x 4" joists. (They did things differently back in 1825.)

2) The underside of the roof deck is festooned with nails. It would take days to cut back the nails so I could have a place to install commercial baffles that wouldn't fit very well anyway.

3) The slope of the roof is fairly low so I really don't want to spend a ton of time laying on my belly working with my arms stretched out ahead of me stapling, taping, foaming and fiddling.

The only answer I can think of is to use whole sheets of insulation board or thin plywood or some other like material across the length of the attic screwed to the bottom of the joists and seal the edges of each sheet where they meet. That would create an air channel under the roof deck and seems to me would take very little time and effort.

But I can't seem to find any indication that anyone has done it this way. Either I am a genius or there is some very good reason not to do it this way.

Any thoughts or recommendations would be greatly appreciated.

 

[nh_yota]

Don't overthink it. Take two of the baffles and staple them side by side between the roof joists. The purpose of the baffle is to leave a path for air to rise along the underside of the roof from the soffit up to the ridge vent. It doesn't have to be perfect.

 

[bikesandcars]

What if ya stick the baffle to the nails poking through, push batts against it to seal and support it, blow against batts

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[diogenes]

Don't overthink it. Take two of the baffles and staple them side by side between the roof joists. The purpose of the baffle is to leave a path for air to rise along the underside of the roof from the soffit up to the ridge vent. It doesn't have to be perfect.

I'd still have to clear enough nails between each pair of joists to have a place to staple. I assume this is why some people tack strips of wood to the joists and attach baffles cut to size on site.

I guess the gist of my question is what's wrong with just running a sheet of material along the underside leaving the whole space for airflow and save all the measuring, cutting, re-measuring, cursing, re-cutting, cursing, foaming lots of seams, etc.?

 

[nh_yota]

If you get the foam baffles the nails will pierce them without much effort. You don't need to use a ton of staples because the nails will hold it in place. You only need a few staples to keep it from falling down.

 

[bczygan]

I'd still have to clear enough nails between each pair of joists to have a place to staple. I assume this is why some people tack strips of wood to the joists and attach baffles cut to size on site.

I guess the gist of my question is what's wrong with just running a sheet of material along the underside leaving the whole space for airflow and save all the measuring, cutting, re-measuring, cursing, re-cutting, cursing, foaming lots of seams, etc.?

Nothing at all.

Baffles are used to maximize the amount of insulation you can install close to the outer wall, while still reserving adequate space for air flow.

New construction uses a truss with an energy heel, or fatter heel, allowing for more depth of insulation at the outside wall.

Bill

 

[Kaizen]

that's what I would do. cheapest thinnest material across 8 feet of joists just as high as the thickness you are going for. remember if you are doing 1 foot of insulation because the roof is at an angle you will need more then 1 foot. its really just to keep the insulation out of the eave when you blow it in. if it does get in the eave and gets wet from ice dams it blocks all air flow.
However with that old house do you have any eave vents anyway?
I know its cheaper but man that blown in stuff ***** to have if you EVER have to go up there. when I go up in my attick I always wear a respirator as there are always find particulates floating around. if it were my choice I'd use batts.

 

[bczygan]

that's what I would do. cheapest thinnest material across 8 feet of joists just as high as the thickness you are going for. remember if you are doing 1 foot of insulation because the roof is at an angle you will need more then 1 foot. its really just to keep the insulation out of the eave when you blow it in. if it does get in the eave and gets wet from ice dams it blocks all air flow.
However with that old house do you have any eave vents anyway?
I know its cheaper but man that blown in stuff ***** to have if you EVER have to go up there. when I go up in my attick I always wear a respirator as there are always find particulates floating around. if it were my choice I'd use batts.

You could even use cardboard or that plastic cardboard material.

 

[diogenes]

bczygan,

I figure if I use insulation board, say 7.5 or 10 R, and backfill against it with cellulose to stop air moving between the board and the cellulose I can make up for the loss of space going outside the joists. And since the joists are only about 4" anyway it is probably a wash R wise.

Kaizen,

When I had the house sided they took care of venting the soffits. The openings over the top beams are narrow but long. The attic is not used for storage, the only thing up there is a TV antenna and I have already finished the rewiring that needs to be done. I shouldn't need to go in there for a very long time. The access is a hatch, so it isn't very attractive for any future uses.

Here is a picture for your amusement:

Mortise and tenon FTW.

 

[CJ7VFR]

Sweet old house you have! They don't make things with that type of craftsmanship anymore!

You can use the Raft-R-Mate baffles from Home Depot to make things easy. They are made by Owens Corning, and are extremely easy to install. As others have said, the nails in your roof would just poke right thru them so you could place them anywhere you needed. Then all you would have to do is put a few staples at the top, middle and bottom of each one to hold it place.

These things are shaped like a giant "W" and provides the barrier you need to keep insulation from closing off the air access in your attic. I have used these things in the two homes I have owned in recent years, and they are easy to put up, easy to work with, come in boxes of 10 or more depending your needs, and require no tools other than a staple gun to put up. And each one can be separated into two pieces (length wise) so they can fit between framing that is 16 inches on center or larger.

Here is a picture from the HD website of the ones I am talking about. This is a box of 10 for less than $18 bucks.



Jim

 

[bczygan]

bczygan,

I figure if I use insulation board, say 7.5 or 10 R, and backfill against it with cellulose to stop air moving between the board and the cellulose I can make up for the loss of space going outside the joists. And since the joists are only about 4" anyway it is probably a wash R wise.

Kaizen,

When I had the house sided they took care of venting the soffits. The openings over the top beams are narrow but long. The attic is not used for storage, the only thing up there is a TV antenna and I have already finished the rewiring that needs to be done. I shouldn't need to go in there for a very long time. The access is a hatch, so it isn't very attractive for any future uses.

Here is a picture for your amusement:

Mortise and tenon FTW.

It will work.

 

[nh_yota]

Sweet old house you have! They don't make things with that type of craftsmanship anymore!

You can use the Raft-R-Mate baffles from Home Depot to make things easy. They are made by Owens Corning, and are extremely easy to install. As others have said, the nails in your roof would just poke right thru them so you could place them anywhere you needed. Then all you would have to do is put a few staples at the top, middle and bottom of each one to hold it place.

These things are shaped like a giant "W" and provides the barrier you need to keep insulation from closing off the air access in your attic. I have used these things in the two homes I have owned in recent years, and they are easy to put up, easy to work with, come in boxes of 10 or more depending your needs, and require no tools other than a staple gun to put up. And each one can be separated into two pieces (length wise) so they can fit between framing that is 16 inches on center or larger.

Here is a picture from the HD website of the ones I am talking about. This is a box of 10 for less than $18 bucks.



Jim

Yep that's the baffle I was talking about.

 

[diogenes]

Here is what I don't get about the baffles:

If the point of them is to promote keeping the roof deck as close to ambient temps as possible (Stop ice damming and keep your shingles from cooking), while keeping insulation out of the way, how can this kind of install work effectively? Only a minority of the roof deck has air flow.

This style seems more likely to provide air flow and keep insulation from leaking into the eaves and/or getting washed by winds. But it is fit to modern construction dimensions.

That is why I am leaning to the sheet of barrier material across the whole space idea. What am I missing?

 

[Kaizen]

Here is what I don't get about the baffles:



If the point of them is to promote keeping the roof deck as close to ambient temps as possible (Stop ice damming and keep your shingles from cooking), while keeping insulation out of the way, how can this kind of install work effectively? Only a minority of the roof deck has air flow.

This style seems more likely to provide air flow and keep insulation from leaking into the eaves and/or getting washed by winds. But it is fit to modern construction dimensions.

That is why I am leaning to the sheet of barrier material across the whole space idea. What am I missing?

The ones without ribs **** imo as often the insulation installation collapses it. For standard bays they work great. My house also has no standard so I have some gaps.
So yea just a barrier is fine. If you were making a cathedral ceiling then you would use them

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