# air gap below roof decking



## ryansdiydad (Aug 16, 2015)

Hey.. I have some hard to access low slope roof ceiling joist bays that I need to re-insulate.. The old bays had fiberglass in them with no air gap between the insulation and the roof.. As might be expected this caused us some problems with moisture being trapped in this space..

It would really hard to replace the fiberglass at this point so figuring I would put some loose fill in its place.. Is there a type of loose fill that does not require the air gap? The roof is vented... poorly vented but vented...


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## Bud9051 (Nov 11, 2015)

Your climate zone will tell us if you are dealing with winter and/or summer conditions. Also, what is under that roof? If conditioned living space, heated and cooler then we look at moisture issues.

Are you removing the old fiberglass for a reason. For a minimal air gap, some will slide one of the baffles in above the existing.

What were those problems and what is your objective.

Basically, no there isn't a fibrous insulation that can be installed against the bottom of the roof deck. Moisture issues as you have apparently discovered. But let's make sure we are addressing the right problem.

Bud


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## ryansdiydad (Aug 16, 2015)

Yeah sorry.. I am in Northern Illinois... 

There are bedrooms under this low slope roof. The only part of the roof that showed any moisture problems are the areas where the fiberglass was pressed up against the roof.. There is also some mold in the worst areas that I need to treat which is why I am removing the fiberglass... I may be able to find a way to install more fiberglass in its place... but it would require sliding it a good 6-7ft into the ceiling joist gaps.. 

I will post a picture when I feel adventurous enough to get up there...


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## ryansdiydad (Aug 16, 2015)

here are some pics of the space I am working with... i practically have to crawl on my hands and knees just to get thru this space... and avoid taking too many roofing nails in the head.. have to tilt my head sideways to seven see up into the full cavity..

If I blow in some loose fill.. can I rely on the settling of the fill to create the necessary air gap?


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## Bud9051 (Nov 11, 2015)

If you have 5.5" of depth to work with (2x6) then 3.5" Roxul would work nice. Very dense and will hold up to being pushed into those spaces. Now, trade secret , my insulation fork. I use something like a 2x2 with a 13" "T" on the end with nails (heads cut off) forced into holes so they are facing away and slightly up. Then I attach a triangular piece of flashing, thin and smooth on top of my "T" and pole. This stabilizes the assembly and makes it easy to retract without snagging on the insulation. But you stab the bottom 4 or 5" area of the batt and push it out into the cavity. Wiggle it into place and slide the tool back out. I've used this method on many occasions where I don't fit out into those long arm places and it works great.

One option, back later.

Bud


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## Bud9051 (Nov 11, 2015)

You got those pictures in before I posted. Ya, I see the moisture issue. What do you have for soffit vents? And high vent openings?

Bud


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## ryansdiydad (Aug 16, 2015)

There are 5 roof vents above the area with the moisture issues.. that sit 1-2ft below the ridge.. there is no ridge vent here... A roofer suggested potentially putting in a one sided ridge vent and taking out the 5 other vents... like the one in picture.. no clue if that makes any sense.. so the only vent opening would be on the side with the higher slope 4/12 I believe.. 

The soffit vents were just recently expanded to allow for more ventilation..

dont know why the picture is upside down...


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## Bud9051 (Nov 11, 2015)

Air flow for attic ventilation is directly related to the difference in height between the high and low vents. A low slope roof is a challenge right from the start. If you have improved the soffit venting, and it should supply each rafter channel, then , along with providing a better air path, you have made some improvement. Adding the ridge vent along with the existing roof vents can be a future improvement if needed. Adding the one sided ridge vent does not require removing those roof vents, they both become high vents.

Bud


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## ryansdiydad (Aug 16, 2015)

Ok.. I thought the ridge vent would just end up pulling some air from the existing roof vents.. which would negatively impact the air flow through the soffit vents... 

I should also have been clearer... the areas with the issues are actually on the 4/12 side of the roof.. right as it approaches the slower slope section... I'll take more pictures of the entire roof... 

on the side thou would roxul still be a good choice to refill those spaces?


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## Bud9051 (Nov 11, 2015)

It would take more space than we have here to go over the pressures involved, but in your case leaving the roof vents in place would increase your total high vent area and result in more pressure across the soffits. That increases your total ventilation. The old "short circuit theory" that the roofing industry has been repeating for decades is embarrassingly wrong. Their explanation is slowly changing, but they prefer to sell the extra ridge venting that could otherwise be reduced.

If the low slope is open to the 4/12 slope that is good as it will help the total airflow.

The use of Roxul has increased significantly as it is far denser and a better product for many applications where old fiber was often used.

Bud


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## ryansdiydad (Aug 16, 2015)

cool.. thanks... right after I posted.. I read a 5 sided debate on that whole issues.. 

The ridge vent seemed to make sense because with the insulation in place there really isnt a lot of opportunity for air to flow between bays to get from one bay to the bay that has the vent...

The low slope section of the roof also has vents.. kind of like these.. and from a visual inspection he low slope area doesnt have moisture issues..

is that ridge vent a reasonable DIY job?


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## Bud9051 (Nov 11, 2015)

If there is an actual peak and not a slope ending against a wall, then you are looking at a full ridge vent, UNLESS, one side of that peak is too low of a slope. Then a single sided vent can be used as when it is too low it is more subject to rain and snow blowing in.

Who was winning in that 5-way debate?

If there is no common space in either the soffit area or at the peak, then you need ventilation at top and bottom of every channel. As you said, the ridge vent accomplishes that better.

What was installed for soffit area venting?

Bud


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## ryansdiydad (Aug 16, 2015)

There is an actual peak... but one side is too low of a slope... 

I didn't finish reading it... It was like. Gable vents are bad, gable vents are great cause that other guy doesnt get it.. ridge vents are good.. ridge vents are bad... bla bla bla..

I cannot verify what is going on with the low slope venting without taking a few soffit panels down... I popped a hole in the ceiling in one of the rooms to check for issues... I could not see the soffit from the inside.. which makes me question how the heck its getting any ventilation at all.. The end of the joist bay is a 2x6 that pretty much boxes it in... 

Is it easy to remove and replace those panels or would I be likely doing damage that would require new ones?


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## Bud9051 (Nov 11, 2015)

Not sure what you have for panels. Maybe some more upside down picture .
BTW, you are not the only one with flipped pictures. I have no ides why that happens, but it does a lot.

Yes, somehow you will need to verify what you have for venting, if any.

Bud


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## ryansdiydad (Aug 16, 2015)

by the way.. thanks for stickin with me on this thread Bud... appreciate the help.. 

More pictures.. The soffits are aluminum but based on what I saw on one side of the house I expect that the aluminum is covering the original wood soffit... house was built in 1955... 

Here are more pictures... 2 inside the ceiling towards the edge of the house that shows its pretty much just boxed in except for a small notch..


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## Bud9051 (Nov 11, 2015)

Don't I hate that and it happens all too often. need to confirm, but they could have just covered up what was there.

I also hate the aluminum, not because it is bad, but i don't know how to work with it. From another thread some time ago, they said to start at one end and slide them out. Some of the others here can advise much better on this.

That board we are seeing, is the soffit on the other side?

Bud


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## ryansdiydad (Aug 16, 2015)

Yep.. the soffit is on the other side of that board... I can see into two of they bays and they are both blocked off like that... 

I am limiting my ceiling damage to the closet in that room so the wife doesnt totally flip out on me...


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