Is it okay to insulate against the underside of roof?

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PabloCruise

PabloCruise

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My question is about the unfinished interior of the garage.

I am wondering if it is okay to hang insulation up against the inside of the roof in the garage?

The unfinished garage ceiling has 2x4 joists on 24" centers. The portion of interior ceiling I am concerned with faces West, so I thin it would make a difference to the garage temps if I did this.

I am not a builder, so I don't want to screw anything up with my efforts. I would eventually like to build some shelves up there, so the time to insulate would be coming up soon.

Thanks,
 
45Kevin said:
Sure it is. Treat it like a wall, with insulation and then a vapour barrier.
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No, roofs are different than walls as they are more susceptible to water penetration thus mold growth if not given enough mitigating ventilation. Insulation suffocates the roof sheathing thus preventing any convective currents from drying it out.

And you certainly do NOT want a vapor barrier, as while you live in a cold climate which is typically OK for vapor barriers your roof is more susceptible to solar gains and moisture always travels from hot to cold, in the summer months you will have solar drive through your roof assembly and the last thing you want is a vapor barrier for moisture to condense on.


KLF said:
Does the garage have a cold roof design? Vented soffits and a ridge vent?

Kinda need to see some pics.
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^this...

regardless though, air sealing is your best bet for energy conservation. After that I'm guessing some rigid foam along the bottom cords of your trusses allowing for air flow beneath the roof deck or sheet rocking the ceiling and blowing in insulation would be the best bet. I hate hot roofs, I see enough moldy cold roofs to never trust it.
 
90FJ said:
No, roofs are different than walls as they are more susceptible to water penetration thus mold growth if not given enough mitigating ventilation. Insulation suffocates the roof sheathing thus preventing any convective currents from drying it out.
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Can't say that I agree, but I've been wrong before.

1. A roof is designed to be water tight and properly constructed and maintained it will be.

2. A roof does get hotter than the walls in the summer, but doesn't matter, at least in colder climates.

3. If the cavity between the rafters/trusses is filled with insulation and is air tight, like the walls, you will need a vapour barrier.

4. Flat roofs are built this way, as well as some sloped roofs with vaulted ceilings.

5. Soffit and ridge vents are needed in unheated attics.

Since the OP is in Colorado I would think that the buiding practices are similar to where I live.
 
You guys really have me thinking when you mention things like vapor barrier and mold - I do not want any problems with mold.
From my amateur perspective it does not look like I have a vented roof cap or vented soffits...
See pictures
What to do if I want to insulate?

image-1866580875.webp


image-1340704727.webp
 
If you are using batt insulation, you will need an airspace between the roof deck and the insulation. If you are using a closed cell foam, you do not need the airspace. But I have found that even then, with an airspace, proper venting, and a radiant barrier between the airspace and the insulation that you can keep interior temperatures down considerably.

Take a look through this article on cathedral ceilings - basically what you are creating: http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

As noted there, the issue is not moisture from the outside but from the inside. You can build an airtight ceiling and use batt insulation, but you really have to pay attention to what you are doing. Using a non-permeable insulation (like closed cell foam) greatly increases the chances that you are not going to have vapor transmission.
 
The main difference between a roof and a wall is that warm, moist air rises. The stack effect means that such air will be pushed/pulled through any available openings and into/through the roof.

Positive pressure can do the same through a wall, but it is a bit more difficult for it to happen there. Through a roof has too many laws of physics working for it - just too easy for it to happen if the path is available.
 
PAToyota said:
Using a non-permeable insulation (like closed cell foam) greatly increases the chances that you are not going to have vapor transmission.
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Sounds pretty concise to me... :confused:

[edit] The problem with a "simple" yes/no answer is that you have to have some understanding of the factors at work - the "fluff" as you put it. Which is why I went to the trouble to explain the "why" of what you want to do.
 
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To me that means vapor is an issue with closed cell foam. In the climate hear in Wyoming and N. Colorado, water vapor is not an issue if you have some sort of roof vent.

I have become sold on closed cell foam insulation. I used it in my cabin, which is at about 10,000 feet and has about 12 feet of snow around it now. I have 2 vents on the eaves only and I have never had a problem with vapor. It stays nice and toasty and dry in there for the 5 years it's been up.
 
snobdds said:
To me that means vapor is an issue with closed cell foam.
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...closed cell foam) greatly increases the chances that you are not going to have vapor transmission.
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I don't know how to make it any clearer...
 
PabloCruise said:
My question is about the unfinished interior of the garage.

I am wondering if it is okay to hang insulation up against the inside of the roof in the garage?

The unfinished garage ceiling has 2x4 joists on 24" centers. The portion of interior ceiling I am concerned with faces West, so I thin it would make a difference to the garage temps if I did this.

I am not a builder, so I don't want to screw anything up with my efforts. I would eventually like to build some shelves up there, so the time to insulate would be coming up soon.

Thanks,
Click to expand...



No problem, that article below shows the standard in europe.

http://www.icopal.co.uk/Products/Bi...roofing-design/safeseal-warm-roof-design.aspx


safeseal_warm_roof_550.jpg
 
My house had a fully vented attic. I installed ductwork so I went and had it foamed with closed cell to make an unvented attic. It works great in my cold climate.
The biggest thing is that it is in an unused truss attic. If I were to use the space below, I'd have to put in a fire barrier. I assume thats the reg no matter how far the foam is over your head.
If the roof is within 5 years of replacing, I'd go with adding foam over it with a new roof on top.
 
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