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Old 07-20-2018, 05:13 PM   #31
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Re: Cathedral Ceiling Redo


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Originally Posted by Nealtw View Post
Behind the chimney there should be cricket or saddle that looks like this.

A box vent or ridge vent might be able to be placed on that and then you would just cut into the cavity from below.
The other question is why did they put solid blocking above the window.
I doubt it was needed other than holding things in place during construction and a few 2" holes thru them would not hurt anything.


There are also vents much like the ridge vent that can go lower on the roof but that is disturbing the roof likely more than you would like.
I will down load that picture and post it.

The whole ceiling was done by a DIY'er who refused all advice, including the whole corner window thing with the blocking above. That's a separate issue but anyway above that block I have access to a soffit and can put a regular rafter vent in, so there's no reason for holes.

The two rightmost wood blocks above the chimney do block access to the soffit but it's possible I could cut a U at the top to get access. The three leftmost blocks have no soffit access. I don't know if those blocks serve any other purpose than keeping the rafters aligned.

I will look into the box vent but I wonder if having 4 out of 7 bays vented would be enough.
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Old 07-20-2018, 05:23 PM   #32
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Re: Cathedral Ceiling Redo


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Originally Posted by arcticranger View Post
The whole ceiling was done by a DIY'er who refused all advice, including the whole corner window thing with the blocking above. That's a separate issue but anyway above that block I have access to a soffit and can put a regular rafter vent in, so there's no reason for holes.

The two rightmost wood blocks above the chimney do block access to the soffit but it's possible I could cut a U at the top to get access. The three leftmost blocks have no soffit access. I don't know if those blocks serve any other purpose than keeping the rafters aligned.

I will look into the box vent but I wonder if having 4 out of 7 bays vented would be enough.
I would be nice to see the cricket if you have one.
Worst case would be a couple box vents each straddling a rafter so they serve 2 bays each..
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Old 07-20-2018, 05:27 PM   #33
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Re: Cathedral Ceiling Redo


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Originally Posted by Bud9051 View Post
Those look like substantial rafters (perhaps larger than necessary) so one might consider many holes along the top edge to allow bay to bay air movement. This approach is common with a hip roof where rafter bays don't go all the way to the top. Not the best and should have local code approval, but it could get some air in there from each side. Also depends upon good venting in those side bays.

Lacking ventilation can be minimized by better air sealing and a good vapor barrier across the bottoms of those rafters.

Bud
I was considering using holes to get bay to bay transfer. But how does bay A transfer air to bay B if bay A is using a rafter vent?

Or could the the 1" gap space between deck and solid foam perform as a rafter vent?
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Old 07-20-2018, 05:31 PM   #34
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Re: Cathedral Ceiling Redo


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Originally Posted by arcticranger View Post
I was considering using holes to get bay to bay transfer. But how does bay A transfer air to bay B if bay A is using a rafter vent?

Or could the the 1" gap space between deck and solid foam perform as a rafter vent?
Even with oversized rafters you are not allowed to drill holes other than the center third as far as I know. But cupping around hoes in the center would not displace much insulation.
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Old 07-20-2018, 06:24 PM   #35
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Re: Cathedral Ceiling Redo


Neal, not sure what you are describing as cupping.
First we would need to know what is up there 2x?. And how long they are.
As for not allowed we all know the final word comes from the local authority. Slim chance but may be worth asking.

As for how the air would move, poorly, but better than nothing.

Another option might be to spray foam the difficult bays and vent the rest.

Bud

Just to note, the expensive spray foam only needs to be thick enough to keep the inside surface above any possible dew poing, reference available. The remainder could be filled with Roxul, keeping in mind what you need for total r-value.

Last edited by Bud9051; 07-20-2018 at 06:26 PM.
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Old 07-20-2018, 06:37 PM   #36
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Re: Cathedral Ceiling Redo


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Originally Posted by Bud9051 View Post
Neal, not sure what you are describing as cupping.
First we would need to know what is up there 2x?. And how long they are.
As for not allowed we all know the final word comes from the local authority. Slim chance but may be worth asking.

As for how the air would move, poorly, but better than nothing.

Another option might be to spray foam the difficult bays and vent the rest.

Bud

Just to note, the expensive spray foam only needs to be thick enough to keep the inside surface above any possible dew poing, reference available. The remainder could be filled with Roxul, keeping in mind what you need for total r-value.
I was looking at drilling joists and I imagine the rules would be the same.
If you drill a few holes down a few inches from the sheeting and then you would need a channel of some sort on both sides to leave room for the air to flow. Something like this cut in half and attach over the hole to hold insulation back.
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Old 07-20-2018, 06:47 PM   #37
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Re: Cathedral Ceiling Redo


rethinking my post and I like the no-vent spray foam for the difficult rafter bays. Saves inventing something that the inspectors would probably not like anyway.

Bud
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Old 07-20-2018, 09:25 PM   #38
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Re: Cathedral Ceiling Redo


Going back to an earlier question. Besides the window leak was there any indication that you were having issues with the ventilation/insulation that existed before you opened the ceiling?

Did you already mention that you have a ridge vent or some other type of high venting?
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Old 07-21-2018, 11:34 AM   #39
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Re: Cathedral Ceiling Redo


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Originally Posted by Bud9051 View Post
First we would need to know what is up there 2x?. And how long they are.

Another option might be to spray foam the difficult bays and vent the rest.

Bud

Just to note, the expensive spray foam only needs to be thick enough to keep the inside surface above any possible dew poing, reference available. The remainder could be filled with Roxul, keeping in mind what you need for total r-value.
The rafters are stacked 2x6's, length is only 12ft. I don't like the idea of drilling holes now that I consider these aren't 2x12's.

This is a 700 sq ft house with a normal sloped roof. It's an open floor plan with 9 ft ceilings in 3/4 of the space and a half-cathedral in 1/4 of the space. That is the space shown in the pic.

So 3/4 of it has soffit vents. 4 of the rafter bays in the cathedral do not. I'm describing this because maybe the totality of it changes the venting requirements picture.

I like the idea of venting the bays with soffit vents and spraying the other 4. Any reason not to use cut and cobble for all the bays whether sprayed or vented? I'd just skip the air gap for the sprayed bays.
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Old 07-21-2018, 11:45 AM   #40
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Re: Cathedral Ceiling Redo


If it is only 12 ft, one 2x6 would have carried the weight the extra only for insulation depth. If the lower one is doing the work notching the upper one might be OK?
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Old 07-21-2018, 12:06 PM   #41
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Re: Cathedral Ceiling Redo


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Originally Posted by ryansdiydad View Post
Going back to an earlier question. Besides the window leak was there any indication that you were having issues with the ventilation/insulation that existed before you opened the ceiling?

Did you already mention that you have a ridge vent or some other type of high venting?
There is mold on the sheathing around the skylight but nowhere else. And I found a thriving ant colony in the same bay which is now history.

Venting: I have no ridge vents at all and only a tiny gable vent. Which means most air coming in via soffit vents is going nowhere. The attic is crazy hot in the summer but I would expect that anyway.

Insulation: none of the rafters have insulation but the floor of the attic is R30 Roxul with sanded plywood over it to make for storage space. The open floor plan below has 4 baseboard electric heaters and the only room that can't be heated is the cathedral part. I had to partition it off with plastic last winter which sucks as it would be nice to sit by the fireplace. Also, there are five 6x3' Low-E windows across two of the walls giving it that Star Trek look. So that's a heating challenge as well.


ps I am a software developer and don't know *** about all this. But at least I ask questions!
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Last edited by arcticranger; 07-21-2018 at 12:14 PM.
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Old 08-20-2018, 12:54 PM   #42
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Re: Cathedral Ceiling Redo


So update (sort of): I've removed the bad skylight and re-roofed it. It rained like crazy for 3 days afterward and not a drop got in. Done. I also discovered from the previous owner that the house already had an existing aluminum ridge vent. Sure enough in my attic I saw perfect ridge vent cuts in the sheathing. So the ridge vent was obviously removed and the 20 foot 3" aperture shingled over when my new roof was put on. Last week I had new ridge vents installed and capped. So done on that too. Lastly, soffits on the entire South side of the house were blocked by "decorative" rough cut lumber, I've torn that down and reopened those soffits.

The evil that men do.


I got estimates on commercial closed cell for the cathedral ceiling bays and they are in the range 1200-1500. So I am going with cut and cobble as we discussed but I am still unclear on the correct type of rigid foam to use. It seems I need a layer to air seal the sheathing and additional layers to add R value while allowing drying to the inside.

Does any rigid foam actually create an air seal (with sprayed edges) or was @Bud9051 referring to an initial layer of spray closed cell?

Here's that article again which states that EPS or fiber-faced polyiso are better than XPS because they allow drying to the interior.

https://www.greenbuildingadvisor.com...ble-insulation

I am very confused between the air sealing and drying elements of this project.

Last edited by arcticranger; 08-20-2018 at 01:01 PM.
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Old 08-20-2018, 03:13 PM   #43
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Re: Cathedral Ceiling Redo


First, are those cathedral bays isolated so they do not have air low access to the ridge vent. I'll send this while I go back and read.

Bud
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Old 08-20-2018, 03:23 PM   #44
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Re: Cathedral Ceiling Redo


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First, are those cathedral bays isolated so they do not have air low access to the ridge vent. I'll send this while I go back and read.

Bud

hi Bud,

These bays have no soffit vents due to a chimney in the way, but they do terminate at the same 20 foot long ridge vent which services the rest of the house. They have no dedicated airflow.
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Old 08-20-2018, 03:31 PM   #45
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Re: Cathedral Ceiling Redo


If you have the proper cricket behind the chimney you can put a box vent there and open a hole in the roof sheeting to supply venting behind the chimney
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