# blown insulation in cathedral ceiling?



## xtal_01 (Oct 17, 2014)

I am still trying to research this on the web.

Just to make things more confusing, here is a link to an article I just read.

In it, it ways not to use baffles to create an air space!

It goes on to talk a lot about air control rather than insulation.

Am I getting into something I should not with this cathedral ceiling? I have seen lots and not every house is falling down. Is this article just being too cautious or am I making the problem too simple?

http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

Help!

Mike


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## Bud9051 (Nov 11, 2015)

Hi Mike, that is one article I was going to reference for you. Those folks see a lot problems and get an opportunity to evaluate what happened. In my opinion, cathedral ceilings look great but limit insulation, ventilation and eliminate all of that nice storage.

I built my 4,000 shop with flat bottom trusses but had 16' walls so height wasn't needed or useful. In fact I have rented space with a high ceiling that was horrible during summer, that is where the heat collects.

If you don't need the height consider walk-through trusses. I helped a relative build a nice garage and the walk-through trusses provided a complete upstairs for storage. Normal trusses are usually limited as to how much you can put up there and if you are like most of us, that load will grow.

Enjoy
Bud


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## xtal_01 (Oct 17, 2014)

Thanks!

Funny you mention high ceilings ... this shop will have 16'.

I really only need that in one bay so I can park our RV. I am just making the other two bays 16 ft as it is easier to build it all the same height.

The pad is 50 x 60 so I am partitioning off the bays bay to be 20 x 50.

That leaves me with a very area above the center bay .... 15 ft at the center. I was planning an attic storage area above the center bay. Even with 24" cord flat trusses (that are rated for 100 lb/sq ft), I will have 7 ft at the edge and 11 in the center (allowing for 24" of roof thickness).

I am 55 and plan on this being the last shop I build (just took 5 years to build the house). I can't afford the best but I want to build it as correct as possible.

Thanks .... Mike


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## Windows on Wash (Aug 30, 2011)

I have pulled apart more than a few roofs that are rotted out from interior moisture as a pertains to a blown in vaulted ceiling


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## user_12345a (Nov 23, 2014)

You MUST have a vapor barrier when insulating a ceiling with a roof right on the exterior side.

...and it has to be a really well done vapor barrier - sealed with acoustic sealant and tuck taped everywhere, not just stapled.

OR, you have to use insulation that permits absolutely zero air and moisture movement, like spray foam or foam boards with the gaps spray foamed.

The reason why blown in works on exterior walls is that the moisture that gets in has a way out.

If the wall is air tight enough you don't get much moisture into the wall, and any moisture that gets in can get out. The density of blown insulation stops most of the air/moisture movement.

Conventional attic floors with a ventilated space above have good drying potential. So a lot of old houses do fine with blown insulation, no vapour barrier in the attic provided the ceiling isn't leaking badly and there's sufficient ventilation.

*When you have one side with tar, ash-vault or some other membrane, dense blown in insulation will restrict air movement enough so a little moisture will get in and not be able to get out.

There is no drying potential through the roof, so the little moisture that gets into the roof assembly gets trapped, you get condensation and the assembly rots.

*Cutting corners on the building envelope to save money, like using drugs, feels good in the moment, but will come back to bite you in the ass - so just say no to shortcuts such as blowing insulation in to a cathedral ceiling. 

you'll be cursing yourself having to redo the roof assembly years later.


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## xtal_01 (Oct 17, 2014)

So, just to make sure I have it right ...

No matter what I put on the bottom of the truss for a ceiling (drywall, wood, paneling, ...) there must be a "good" vapor / air barrier on the bottom of the truss (warm side). It must be completely sealed (not just stapled with holes, and such) ... all seams taped.

*** several people have suggested not using a vapor / air barrier and just using the drywall (caulked to seal it ) primed. On the other hand I have gotten several e-mails saying "don't do this" ... it's not that it won't work but it is very hard to ensure a good seal as compared to using poly *****

I can use blow in insulation ... try to do a decent job to get say 20" ... packed around trusses and such.

I might need some kind of baffle to maintain an air gap between the top of the insulation and the roof sheathing. If I use a baffle it must "breath". Cardboard or plastic baffles if there is a bit of a gap between them or maybe better yet house wrap as it will breath.

Make sure I leave an air gap ... I am planning 4" ... right from the soffit right to the ridge vent to allow air flow to dry any moister that may get in.

Have I got it right?

Thanks again ...... Mike


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## user_12345a (Nov 23, 2014)

To leave an air gap, i guess you would have to use plastic baffles and put roof vents to vent the moisture.

But if the plastic baffles aren't permeable, how is the moisture that gets in going to be vented out?

House wrap like you mentioned allows moisture in and out but functions as an air barrier which could in theory work, but how would you maintain the gap between the wrap and the roof deck? 

In general, loose fill and batt insulation are always done with a vapour barrier.

In a new house where i am, they'll always vapour barrier the exposed ceiling before blowing insulation into an attic.

It's only in retrofit applications where they blow it in without vapour barrier - mostly into walls and conventional attics, and if not done properly - ie low drying potential, low leakage, dense packing it (walls only) - doing it can create problems.

I assume you're getting permits.

Maybe do some research (even calling the agency that deals with permits/codes in your area) and see what "best practices" are where you live.

Best practices will vary depending on climate. Like in a hot, humid climate you're not going to have a vapour barrier on the interior side.

I'm in a cold climate and everything i know is based on that.


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## xtal_01 (Oct 17, 2014)

The problem is, I have found lots of people who have done it wrong ... maybe lots have done it right in our area but the houses have not had the roofs taken off to find out.

The baffle on the top to maintain the air gap is what I am most worried about.

I agree, foam or plastic baffles will not let the insulation breath. The only thing I read was that since they are not "sealed" where they join together, you get air at that gap.

If it were not so expensive, I would think window screen would work great. If I could nail (staple) it down 4" from the top of the truss, then it would hold the insulation from the roof sheathing when I blow it in.

Again, I think house wrap may work ... I wish it let even more air through but I think it would work. I also worry it would not let the moister though to dry the insulation should it get wet.

Mike


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## user_12345a (Nov 23, 2014)

House wrap is a unique product - it's an air barrier but allows moisture to flow through it.

And that's ideal because air movement reduces the r-value of the insulation. 

If you can make a path for flow between the soffit vents and roof vents somehow, lined with house wrap it could work.

But it's far better to just use non-permeable insulation to begin with like foam.

blown in with no vapor barrier is the absolute worst.

i assume to blow in, you would blow in from the roof after the drywall is put up, then put the roof sheathing.

how else would it be done?


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## user_12345a (Nov 23, 2014)

the article you linked to is really good - i would heed the advice given there..


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## Nealtw (Jun 22, 2017)

You won't know you have water in the insulation until it shows up in the drywall so worrying about it drying may be just worth the time.
Using house wrap would allow breathing and move water to the outside of the house.


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## xtal_01 (Oct 17, 2014)

Thanks for all the advice!

It is great to bounce ideas off you guys.

I wish I had he money for foam ... I know it would be the best way to go.

But blown in insulation (done myself) is less than $1 a sq ft for R-60 ... hard to beat that.

I drew a picture of how envision the roof section.

Is this what you guys are thinking is my best option?

As to how to actually achieve this ... I am thinking put up the trusses, house wrap, roof sheathing and roof first.

I can then cover about 8 ft of ceiling (start from the out side wall) all along the roof (gable end to gable end ... 50 ft) ... blow in the insulation ... do another 8 ft .... blow in insulation ....

I have blown insulation over a ceiling but not into a cavity. I think the hose will reach in 8 ft and since it is loose pack I think this will work.

Any thoughts on this?

Thanks again .... Mike


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