Insulating a narrow depth

[Nate R]

Have a sloped ceiling in my bathroom that follows the roofline. The house is only 20 feet wide, and has 2X4 rafters at about an 11/12 slope. I've pulled most of the plaster ceiling out of the bathroom, and the rock wool insulation. Real, brand name Rock Wool.

Anyway, once I'm done, I want to reinsulate of course. Because I only have a 4" cavity (Actually 4", not 3.5) I'm thinking of 2 layers of 2" rigid foam board, giving me an R20 insulation value in the cavity. Otherwise with fiberglass I'm looking at R13, plus R3 1/2" foam board as well.

So,
Drywall
Vapor Barrier
Rigid Foam board (2 layers)
Roof sheathing

1. Anything wrong with that assembly?

2. Is there any reason I should not or can not use the rigid foam board on the interior side? I'm thinking of possibly using it in the walls in some spots, too.

3. How would you fasten it to the exterior sheathing? Or would you just Glue it to the rafters on each side? I'm worried if I use it in a wall, glue it to the sheathing, and someone pulls off a piece of sheathing that they'll be ripping the insulation out with it. Or am I worrying too much?

 

[Markts30]

You still need an air gap between the insulation and the roof sheeting...

 

[Sponsor]

 

[Terry]

With only 4", foam board may be a good idea.
You do leave some gap between the insulation and the roof for air circulation.

I think that once the drywall is installed, it will be there for a long time.

 

[BigLou]

Rock wool is a great insulator its also fire proof something foam is not. if at all possible I would have left it there. I see a lot of people on these forums who want to remove the old perfectly good insulation to upgrade, I just don't get it.

I would spray foam insulation and forget about the air gap. I know it may void the shingle warranty but most shingle warranties are pro rated and worth very little in the last half of thier life $10,000 worth of installed 30 year shingles is worth about $29.00 on the end of the 29th year . Voiding the warranty is no gurantee they will fail either its just an out for the manufacturer.

If you were to foam/caulk the edges of the faom boards to prevent air movement and its moisture from getting to the underside of the roof deck there is really no need for the air space

Lou

 

[hj]

insulation

I don't see anything in this discussion about a vapor barrier.

 

[Nate R]

So,
Drywall
Vapor Barrier
Rigid Foam board (2 layers)
Roof sheathing

I mentioned it there.

The Rockwool had to come out in the areas I've removed it. Where a window was getting moved to, where I had to add some new framing, etc in the walls. In the bathroom ceiling, I'm adding a skylight among other things. Other sloped ceilings and most of the walls in the house are insulated with Rock Wool. No intention of disturbing it if I don't have to. From what I've found whoever blew the stuff in did a good job not leaving any large air spaces.

Shingle warranty? Really don't expect the roof to last too long anyway. Flashings weren't replaced that should have been when the roof was redone in 05. (Before I owned it.) It was reroofed as quickly and cheaply as possible. So the shingles won't be the first to go. House is only 20X20 outside dimensions with a 90 SF addition, so there's not much roofing anyway.


I'm thinking I may go ahead and use foam board. Turns out the rafters are a tad under 4", so I'd have to use 3.5" of foam board, which means I could leave an air gap.

Thanks for the responses, all!

 

[LiamM]

what are you using for a vapor retarder? 4- or 6-mil poly? this is generally not recommended since it's almost TOO good a barrier...it blocks air and water vapor, and can lead to condensation in the framing cavity.

for the foam board, are you using XPS (extruded polystyrene, like Owens Corning Foamular), or expanded polystyrene (looks like styrofoam)?

2" thick XPS has a perm rating of 0.55, so it qualifies as a vapor retarder on it's own. seems like you're using a thicker layer, which would lower the perm rating further.

for your local climate, do they recommend the vapor retarder face in or out? i'm not sure how this is handled when the vapor retarder is the multi-inch foam board itself, instead of a single layer like kraft facing.

just a coupla questions you might want to look into...wouldn't want you to have to change things later, or accidentally end up with a double vapor retarder.

 

[Nate R]

I was going to use 6 mil poly for the vapor barrier.

And I also was thinking of using XPS for the foam board.

Thanks for the input, I'll look into it.

 

[BigLou]

Econguy,

Its very unlikely that he will end up with a double vapor barrier if he tighty installs XPS or EPS. If he caulks or foams the edges and the seams there will be no air movement to carry the water vapor. Its equally resistant to vapor dispersion in both direction and it does not absorb water vapor so there would be no place to the vapor to get trapped by a double vapor barrier. the only way he could have have a double vapor barrier is if he leaves a gap between two layers of the foam and allows for a lot of air movement

Lou

 

[Frenchie]

I agree with Lou, that the "local climate" question becomes irrelevant when you use foam - there's no cavity to protect from condensation. So the inside/outside vapor retarder question doesn't apply anymore.

But Econguy's right that another, seperate vapor retarder isn't called for - the foam is the vapor retarder, so there's no good reason to put plastic in there.

I'd leave it off. Unless you're in the midwest, right next to Canada... plastic is usually a bad idea, no matter what insulation you're using.

 

[Nate R]

Let's elaborate on vapor barriers a bit.

I am in Milwaukee. So it is fairly far north.

Not sure what I should do any more as far as vapor barriers and where.

The exterior walls, when complete, were planned to be:

Vinyl Siding
Tyvek
1/4" Fanfold board
1" Sheathing
1/2" XPS Foam Board
3 1/2" Fiberglass insulation
6 Mil Poly
5/8" Drywall

But I'm not at all sure that's the way to go. Suggestions on what order to do what or not to do and why would be greatly appreciated.


Now, in the roof cavities where I'm installed the XPS, I would have

Shingles
Roof boards
2" XPS
1.5" XPS
3/8" airspace
5/8" Drywall

Does that work if I seal the edges of the XPS to use it as the vapor barrier?


Suggested resources on Vapor Barrier theory? It seems like it changes often.

 

[BigLou]

Nate,

Sealing the edges is the only way to go. 99% of all moisture moves on air currents not through vapor dispersion. The dywall and the paint you apply to is also act as a vapor barrier. So very little vapor will actually get to the insulation layer unless its on air currents.

In your order for the roof below if you are going to put an airspace in I would put it underneath the roof sheathing not between the drywall and the insulation. If it under the roof sheathing it "may" meet the code/shingles requirement for venting.

As for your wall assemblies.
I would do it in this order based on your list of materials. I am unsure of how you plan on flashing things or where you water barrier is ?

vinyl siding
Fanfold insulation
Tyvek ( I don't like tyvek but its an effective air barrier)
Sheathing
XPS (make sure to use foam and caulk to seal the edges)
Fiberglass, (probably the worst choice on the market due to difficulty of actually getting marked R value)
poly vapor barrier
Drywall (not sure why you feel the need to use 5/8)

My wall assembly would go like this.
Vinly siding
1/2" or lager shiplap XPS insulation board. (flashing details will change based on thickness)
optional Tvyek (this would be overkill just to make sure that wind got through)
#30 felt as water barrier ( things may be flashed to this layer depending on thickness of XPS)
Sheathing
Spray foam or blown in dense pack cellulose (both will prevent air movement in the wall)
1/2" drywall, unless there is compelling reason to use 5/8 like fire code why deal with the extra weight and cost ? primed and painted. perhaps with a vapor barrier paint

Lou

 

[Frenchie]

Vinyl Siding
Tyvek
1/4" Fanfold board
1" Sheathing
1/2" XPS Foam Board
3 1/2" Fiberglass insulation
6 Mil Poly
5/8" Drywall

foam on both sides of the sheathing? I'm curious what the thinking was, there; never seen that detail before.

the poly... well, I already commented on that. Here, I'd use kraft-faced FG if i was using FG. I'm not sure if Milwaukee qualifies as okay for plastic, but it probably does.

Shingles
Roof boards
2" XPS
1.5" XPS
3/8" airspace
5/8" Drywall

the airspace, if you're going to have one, belongs outside the insulation, not inside.

Suggested resources on Vapor Barrier theory? It seems like it changes often.

That's a VERY easy question to answer: buildingscience.com

I find the site hard to navigate, but the search feature works pretty well. Lots of good, clear articles for download.

Their climate-specific Builder's Guides are well worth the $45; excellent explanations of the principles, clearly explained & illustrated construction details, and all of it's been put to the test.

It'll answer the question of plastic in your climate, anyways.



Lou - I prefer 5/8, myself. It's heavy, but I like the rigidity of it when I'm finishing. Never used it until I moved here (where it's code), but the few times I've done 1/2", since then, confirmed it. It also 'smooths out' wonky old framing (a bit, anyways), whereas I find the 1/2" follows every irregularity... but that's just my personal preference. Could just be that I'm so used to 5/8, now, so 1/2 feels floppy & flimsy to me? Could be.

 

[Nate R]

I'm using 5/8" Drywall because my stud/rafter spacing is anywhere between 20-24", so it's necessary.


Foam on both sides of the sheathing?
The 1/4" fanfold board was what I've always seen/used under vinyl but over sheathing. My studs are over 100 years old, and are a full 4" depth. So 1/2" of foil backed XPS rated at R 3.3 as my first interior layer seemed to be a great way to make use of the extra space in the cavities.

Does that make sense?

 

[Jadnashua]

With 4", I'd be tempted to put two layers of the 2". Have you looked at having someone spray foam? Or, the cellulose? When going in walls, the cellulose is mixed with a little glue to hold it in place. Either one fills the voids and stops air infiltration if done right.

 

[Bob NH]

Suggested resources on Vapor Barrier theory? It seems like it changes often.

"Vapor barrier theory" is pretty simple.

Air contains water vapor.

The amount of water vapor in the air, measured as a percentage of the amount of water that it will hold before condensing, is called relative humidity.

The water vapor wants to move from places where there is a lot of it to places wher there is less of it as it tries to reach a condition of equilibrium where the rate of water moving TO is equal to the rate of water moving FRO.

Because the air can't hold as much water when it is cold, the movement of water vapor is generally from warm spaces to colder spaces, as long as the warmer spaces have a source of water vapor; which they usually do have.

As the water vapor moves from the warmer space, through permeable walls, it will condense in the colder spaces, causing the insulation in those spaces to become wet and therefore less effective as insulation.

That is why the rule on vapor barriers is that they should be installed ON THE WINTER-HEATED SIDE OF THE INSULATION in cold climates. Otherwise you can get ICE in your insulation.

That is also why you want to ventilate the unheated spaces outside the insulation so that water vapor that gets past the vapor barrier can escape.

I am not familiar with the practice in the south where the predominant climate-control is air conditioning. However, there will generally not be long periods where the outdoor dew-point is below the controlled indoor temperature so the insulation will tend to dry out. Therefore, I suspect that location of the vapor barrier is not as critical in those climates.

 

[Toolaholic]

1' st ,I'm a Gen. Contr. ,This is My Music. If You can drop the ceiling. cut plywood gussets nailed into the side of rafters and secure another 2x4 or 2x6
Buy plastic baffels,installed up against roof for air channel. vent eves and ridge. Sleep well.knowing You did it right, not hacked.

 

[Nate R]

Don't have the headroom to drop down another 2X4. Part of this roofline drops over the stairway and I would hit my head if another 2X4 was there.

jadnashua: My studs are right around 4" but my rafters are more like 3 3/4 to 3 7/8, so the 2nd layer of 2" wouldn't fit.

 

[Lakee911]

Rip some strips out of 1/4" ply and fur it down slightly to get the extra space?

 

[BigLou]

I still don't see why everybody here is so hell bent on an air space thats just really not necessary