# Mansard style roof insulation question



## carverman (Nov 8, 2010)

With one of the coldest winters we have seen in many years, my heating bills have jumped up quite a bit because of poor insulation in the upper part of my 2 story Mansard style roof house.
Some of the friction fit batts between the 2x4s on the upper floor appear to have fallen out and I can hear cars and street noise now as well as drafts. I have contracted a reputable
roofing company to remove the 24 year old mansard roof shingles and the roof sheeting to get access to the 2x4 walls, remove the old insulation (R11? or R14?), and spray in foam insulation at least an inch thick. Then replace the existing batts (if they are still serviceable, although I expect they will need to be replaced) with Roxul R14.

The roofer says we could use Roxul R22 (5.5 inch thick) in place of the old 3.5 inch fiberglass batts, and with the spray foam I would get about R30 or so in the walls.

There are two issues that I need to resolve here.

1. How would a Roxul insulation (unfaced) batt work in a 2x4 cavity on a ( 16 inch 2x4 center)work and not fall out again? The vapour barrier is on the inside of the wall, so I can't
use a second plastic vapour barrier, but what about this Tyvek house wrap? Can I use it to staple it to the outside part of the studs and form a more draft resistant wall?

The builder says we shouldn't use Tyvek to prevent the batts from falling out, (moisture buildup) so I am at a loss as to what we could use to hold the batts in.
The re-insulation and new shingles are quite costly, so there is no way I will be doing this again in the next 25 years so I want it done as best as possible. 

I also think that installing R22 (5.5 inch thick batts) is over kill since some of the thickness of the batt will extend beyond the 3.5 inch stud and there will be at least 1 inch of foam sprayed on the walls before the batts are installed.


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## donald (Apr 18, 2011)

Why not just use spray foam for the entire cavitie(s) carv?
It is not proper to mix batts and spray foam.

You can use new batts in the celing of the mansard but spray the walls of the mansard full with spray foam and new plywood and than new shingles.
*get them to install ice and water shield at the start(bottom)of the mansard and than install a synthetic underlay on the plywood and than the shingles get applied to-the spray foam will set and be shaved of excess so no need to play around with the inferior batts imo-this is what the roofer should be pushing.
carv I own a roofing co(we are one of the best in wpg).....if you want extra protection in ceiling put blow in out of batts-make sure you have your roof venting sorted out though!very important carv.


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## donald (Apr 18, 2011)

Should notebviously cant see your mansard but sometimes,depending-it can be very beneficial to also install foam boards on the plywood(esp if you don't use spray foam in cavities(dampen the noise)different procedures on a case by case basis


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## carverman (Nov 8, 2010)

rikk said:


> If it was me, I'd look into spraying in 3 1/2" for ~ R22. I'd also, which you've probably done, is look into the attic insulation ... the ceiling loses more heat than the walls is my understanding ... maybe a new home for the old batts ... bats ??? :hopelessness:.


I upgraded the attic crawl space with at least 12 inches? of celluose blown in over the existing 3.5 inch? fiberglass batts ( about 10 years ago. The contractor estimate indicated existing insulation
(batts = R20) but that may have been the old R rating..3.5 inch fiberglass now is R14. Anyway, he blew in enough to raise it to R42 (at least that is the claim).



> If you use new batts in the walls, you might consider using horizontal 1x2 strapping nailed to the studs to hold them in place. Even more worth while insulating these days, what with the gas/hydro rate increases in the 40% range :cower:


That is one option, I suppose.
I don't want any more fiberglass batts coming loose to the breeze air flow inside the mansard. The wall that gets a lot of wind is the west wall. Last night with the wind howling and -20c
windchills, the furnace was running almost every 5 minutes and I could feel the draft coming right through the west wall. A few years ago, I worked on the porch and saw loose batts that were
coming loose on the 2x4 walls. The roofing contractor (who will install the new insulation at the same time, since the old mansard roof sheeting has to be removed to get at the insulation batts
is telling me that he recommends R22 Roxul batts, but these are 5.5 inches thick and the 2x4s that form the upper walls wont accomodate these thicker batts without some kind of support
structure and to use 1x2 strapping at 3 places (top/mid/bot) means that these batts will be compressed by 3.5 to 3 inches where the strapping is tied to the 2x4s. The builder is suggesting
using twine to hold them in in a zigzag fashion staple the twine to the 2x4s.

My house is very poor construction.

What about using plastic netting stapled to the studs to keep the batts in place?
http://www.uline.ca/Product/Detail/S-9964/Netting/Stretch-Netting-Roll-20-x-3000


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## carverman (Nov 8, 2010)

donald said:


> Why not just use spray foam for the entire cavitie(s) carv?
> It is not proper to mix batts and spray foam.


Cost mainly. The roofing company is suggesting Tigerfoam (two 20lb propane size bottles with a hose coming off each one. The applicator gun sprays a wide pattern that sets up in about 10 minutes for about 1 inch thickness once the foam cures. To do this 1 inch around 3 sides of the mansard (windows exclued) is approximately 640sq feet. One double bottle application of Tiger foam for 600 sq feet is already $772 and that won't be enough for 650sq ft of wall space (on 3 walls exposed to the elements(20 x 8H = 160sqft x 2 = 320 sq ft. 40 ft L x 8 feet h = 320 sq ft) .
This spray foam is expensive..to apply one layer (1 inch thick) will cost about $1406 (taxes in). *This is an extra expense from the roofing quote. *

This means that the wall cavity between the 2x4 will be reduced from 3.5 inches (which accommodates a R14 batt to about 2.5 inches. He is suggesting that we discard the old insulation and use new R22 Roxul (*extra cost at approx $840)*



> You can use new batts in the celing of the mansard but spray the walls of the mansard full with spray foam and new plywood and than new shingles.


They are not doing the ceiling/attic, that has already been upgraded to R40 about 10 years ago.


> *get them to install ice and water shield at the start(bottom)of the mansard and than install a synthetic underlay on the plywood and than the shingles get applied to-the spray foam will set and be shaved of excess so no need to play around with the inferior batts imo-this is what the roofer should be pushing.


They are installing water and ice shield (certainteed Winterguard) in the quote, new metal flashings around the window wells, so I don't have to keep painting them, an ice dam drain at the top of the roof, new plywood (bluewood), and DiamondDeck instead of traditional tarpaper under the new shingles. The insulation and application labour is extra. 
new "50 year guarantee" Certainteed Landmark shingles 



> carv I own a roofing co(we are one of the best in wpg).....if you want extra protection in ceiling put blow in out of batts-make sure you have your roof venting sorted out though!very important carv.


Ceiling has been done already (insulating company claims it's R40 now)...my main concern and the BIG EXPENSE now is the mansard part of the roof on 3 sides (since my house is a semi-detached). 
Current estimate to replace roof wood, shingles, and apply foam is$6972,50 (Taxes in) New bluewood plywood (7/16) for roof sheeting labour included is $2622 (labour included) and is factored in to the estimate for a total cost.... (minus actual insulation cost, if orginal batts are too compressed or damaged to be used).... is $10,841 (taxes in).


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## carverman (Nov 8, 2010)

View attachment 472


donald said:


> Should notebviously cant see your mansard but sometimes,depending-it can be very beneficial to also install foam boards on the plywood(esp if you don't use spray foam in cavities(dampen the noise)different procedures on a case by case basis


See attached pics Donald. Front of my semi with mansard style roof and a flat membrane roof which replaced the old tar and gravel in 2003. Extra blown in insulation in roof "attic"
in 2003 to R40. It's the poor style of insulating the upper walls in the mansard cavity that I am trying to address now.


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## hboy43 (May 10, 2009)

carverman said:


> View attachment 474
> View attachment 472
> 
> 
> ...


OK, not knowing the meaning of "Mansard" the picture helps. I still do not have a picture in my head of what is going on, as to my thinking a 2/4 wall cannot hold a R20 batt, so I am missing something. Is there a vertical wall adjacent to the mansard sloped roof and you are insulating this vertical wall and not the sloped roof structure?

At any rate, almost anything will hold in a batt. I would suggest 6 inch wide strips of regular 6 or 10 thou plastic stapled in place, probably 3 strips per 4 foot high piece will do. Unlike string, they cannot rot or likely suffer insect or rodent damage. And as only strips, won't be a vapour barrier.

hboy43


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## carverman (Nov 8, 2010)

hboy43 said:


> OK, not knowing the meaning of "Mansard" the picture helps. I still do not have a picture in my head of what is going on, as to my thinking a 2/4 wall cannot hold a R20 batt, so I am missing something. Is there a vertical wall adjacent to the mansard sloped roof and y*ou are insulating this vertical wall and not the sloped roof structure?*


Look at the blueprint in this link and you will see a mansard style roof with the slope which forms the outside walls of the house structure. There is a free air circulating cavity
between the inside (insulated walls) and the actual sloped roof.

http://www.bluegreengroup.ca/site/w.../Gambrel-or-Mansard-Roofs-and-Ice-daming1.pdf
Yes, I am insulating the upstairs vertical walls (2x4 studs) which on the warm side (inside have a plastic vapour barrier and drywall). On the outside of these walls is friction fit fiberglass batts that are fitted into the 2x4 cavity spaces, and* there is nothing to hold them in..except the friction of the sides of the insulation.* Some have come loose over the
years and there is a draft that comes through that area when there is a wind blowing. 



> At any rate, almost anything will hold in a batt. I would suggest 6 inch wide strips of regular 6 or 10 thou plastic stapled in place, probably 3 strips per 4 foot high piece will do. Unlike string, they cannot rot or likely suffer insect or rodent damage. And as only strips, won't be a vapour barrier.


I was thiinking along the same lines. Strips of plastic (4 inches wide) stapled to the studs to hold the batts in. But with 8 foot walls, it would require 3 strips because insulation bundlles
normally come in 4 ft lengths.


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## OhGreatGuru (May 24, 2009)

I wouldn't recommend either strips of plastic sheeting or plastic netting to retain the insulation. The plastic will age from the high summer temperatures, become brittle, and break. So it won't last long enough.


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## OhGreatGuru (May 24, 2009)

The layer of sprayed foam insulation is good idea to give you a very good vapour barrier, because the existing is probably not that great. 

You are right that compressing the 5" R22 insulation would reduce its insulation value. 

Beyond that, it's hard to comment without a drawing detail. I think you should sit down with your contractor and ask him to sketch a detail of how he proposes to retain the batts without compressing them, because you don't want to have them falling out the way the old ones are. Maybe he is planning to add 2" blocks under the furring strips?


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## carverman (Nov 8, 2010)

OhGreatGuru said:


> I wouldn't recommend either strips of plastic sheeting or plastic netting to retain the insulation. The plastic will age from the high summer temperatures, become brittle, and break. So it won't last long enough.


That is true. Polyethelene plastic (even if it's 6 mil) will eventually get brittle over many years and that west exposure mansard roof does get very hot in the summer sun.

Ok, so now I'm back to either 1x2 wood strips, or some kine of twine as Polypropelene rope (the yellow braided kind by the meter is far too expensive .
I will need lengths about 1000 ft or more. 
Twine that you get in a roll, is the most economical so far. This is what the roofer suggested.


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## carverman (Nov 8, 2010)

OhGreatGuru said:


> The layer of sprayed foam insulation is good idea to give you a very good vapour barrier, because the existing probably not that great.
> 
> Beyond that, it's hard to comment with a drawing detail. I think you should sit down with your contractor and ask him to sketch a detail of how he proposes to retain the batts without compressing them, because you don't want to have them falling out the way the old ones are. Maybe he is planning to add 2" blocks under the furring strips?


It's a stupid design with loose friction fit bats. I was thinking at first to have them staple TypeR to cover the batts, the way new home construction is these days, 
but he is telling me that that is not a good idea , because if there is any moisture that accumulated in the insulation from the warm side (even if a poly vapour barrier..
if it even has one), it won't dry out.


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## RBull (Jan 20, 2013)

I think I understand what you want to do but have some questions. I work with contractors doing renovations. 

Why are you having drafts if you have a vapour barrier on the inside of your walls? The fiberglass insulation won't do a lot to stop this. If it has fallen down I can understand how you are hearing more noise and may feel colder since there is no insulation but this shouldn't impact drafts a lot. Is the purpose of the spray foam to address air leakage? If so how will work with the existing vapour barrier? ie is it a closed cell formula that acts as a vapour barrier and is there possibility of issues from this? Is there a reason you can't use spray foam in the full depth of the wall/roof cavity, even if that was increase as per below?

What is the space you have from the 2x4" interior walls and the exterior walls/mansard roof? How much room for insulation is there and how room is there for circulating air?
If there is a lot of space you could have the studs furred out further to the depth of the maximum depth of insulation you want to install. ie another 2 x4 depth. You could use any number of methods to keep these in place from nylon string to blocking boards at bottom and face of the insulation


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## donald (Apr 18, 2011)

If you must(because of expense)more labour but 100% better than messing with twine and other such things that are ''homemade'' just cut the batts to length and use the factory width for the vertical length,just ensure that each batt is carefully tucked!
If not that than fur out the 2x4's with lumber so you make the cavities smaller thus making the batts fit vertical,1x4 swo be good for one cavity each!(on the flat)or use vertical strips of insulation as fillers to compress cavity(alternating each side)
I would not do homemade twine.
I could come up with half a dozen ways to insulate that project(and have it preform),your roofer should not be talking about rope and strips of this and that.


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## carverman (Nov 8, 2010)

RBull said:


> Why are you having drafts if you have a vapour barrier on the inside of your walls?
> 
> I don't know..it just feels like drafts when the wind is blowing from the west on the large wall (40 ft).
> The fiberglass insulation won't do a lot to stop this. If it has fallen down I can understand how you are hearing more noise


I definitely hear more street noise...like the crunching of car tires on the street.


> and may feel colder since there is no insulation but this shouldn't impact drafts a lot. Is the purpose of the spray foam to address air leakage? If so how will work with the existing vapour barrier? ie is it a closed cell formula that acts as a vapour barrier and is there possibility of issues from this? Is there a reason you can't use spray foam in the full depth of the wall/roof cavity, even if that was increase as per below?


Don't know how this spray foam will work with the existing poly vapour barrier..it also may have the older styled (tarpaper) faced batts. so that will be discovered when all the original fiberglass
insulation is removed to apply the spray foam. There is no reason why they couldn't apply 3 layers of spray foam..it's just that the cost to do 1 inch on my place is $1245 (taxes in) and
the R14 or R22 to fill the cavity (after the spray foam is applied is) $760..so there is a cost saving there. Three 1 inch applications of foam (Tigerfoam) will cost $772 (600sqft) and $473 (200sq ft kit)
so that's $1245 x 3 kits = $3735 + taxes...pricey!



> What is the space you have from the 2x4" interior walls and the exterior walls/mansard roof? How much room for insulation is there and how room is there for circulating air?


Lots of room..at the bottom the soffit is 14 inches...much less at the top because of the slope of the roof.



> If there is a lot of space you could have the studs furred out further to the depth of the maximum depth of insulation you want to install. ie another 2 x4 depth. You could use any number of methods to keep these in place from nylon string to blocking boards at bottom and face of the insulation


Not at the top..which is pretty tight to the 2x4 frame...at the bottom yes..but that means only furring out about 1/2 of the 2x4 wall and that is going to involve extra carpentry labour as well,
which has not been costed, and I'm sure will come out almost as expensive as spraying in 3 layers of foam in the existing cavity.

I'm trying to do this to get the most for the cost of the job..it's already at nearly $11,000 and that doesn't include the tigerfoam or any Roxul insulation.


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## carverman (Nov 8, 2010)

rikk said:


> About the 2x3s ... if you have the space you can make them into 2x5s by fastening 2x2s to them, or 2x3s even better ... drill 1/4" holes about half way through to countersink say 3" drywall screws ... or nails ... I did that on my first house but from the inside after removing the paneling that was used ... and someone suggested just use blocks every so far along the studs. Twine is an idea ... perhaps better would be something rodent proof ... maybe bailing wire if it's still available ... if something can go wrong, it usually does ... 1000 ft ... that would be a lot of 1x2s. I'm sure it will all work out in the end ... nice project ... all the best with it :encouragement:


It's a 2x4 inside wall (3.5 inch studs). There isn't enough room to build out the 2x4s at the top (near the flat roof) in this design, so that is already a problem. 
I can't do this myself anymore.
The roofing contractor is willing to apply the insulation, but I don't think he's too eager to start messing around with 2x4s to build them out..with the labour costs to do this and the cutting on a table saw, etc, (we are talking about 84 linear feet at 16 centers = 64 2x4s that would need to be built out at least. 
it becomes a more involved project than the 2 days he's set aside to do the complete job.

So I have two choices here..do the full 3 inch application of styrofoam (3 layers) and forget about the batts at a cost of nearly $4,000 with tax.
or
Do 1 inch application of foam to seal the cracks, drafts etc and apply either the existing batts (if still ok)... or new R14 roxul batts at an additional cost of $760
for a total additional cost of about $1245 to $2000..a $2000 saving for me.


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## RBull (Jan 20, 2013)

carverman said:


> It's a 2x4 inside wall (3.5 inch studs). There isn't enough room to build out the 2x4s at the top (near the flat roof) in this design, so that is already a problem.
> I can't do this myself anymore.
> The roofing contractor is willing to apply the insulation, but I don't think he's too eager to start messing around with 2x4s to build them out..with the labour costs to do this and the cutting on a table saw, etc, (we are talking about 84 linear feet at 16 centers = 64 2x4s that would need to be built out at least.
> it becomes a more involved project than the 2 days he's set aside to do the complete job.
> ...


I suggest this option. Should be able to reuse the batts. There's more labour involved with the handling but should still save money. Your existing batts will be R12 so a little less R value. 
Not worth it to furr out with what you describe with only 1/2 wall available. 

Sealing is definitely an excellent idea as long as you can confirm this will work fine with your existing set up. I think it will. This will give you the most energy savings and reduce sound. 

Good luck.


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## hboy43 (May 10, 2009)

OhGreatGuru said:


> I wouldn't recommend either strips of plastic sheeting or plastic netting to retain the insulation. The plastic will age from the high summer temperatures, become brittle, and break. So it won't last long enough.


Good catch. I retract the suggestion.

hboy43


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## carverman (Nov 8, 2010)

hboy43 said:


> Good catch. I retract the suggestion.
> 
> hboy43


We are going to use sisal twine to hold in the batts and staple the twine to the 2x4s. I would like to treat the twine against rot at least with fence post preserver, (green solution)
which I already have. Just soak the ball of twine in the preserver and let it dry out before using. 

There is only one thing that worries me..the roofer said that they can't use Pressure Treated plywood under the shingles because the preservative has been known to rust the
roofing nails. 
http://www.lowes.ca/packaged-rope/l...632&cse=3632&gclid=CKnvz627qb0CFchZ7AodtUIAxQ

In my case, it's whether the PT solution will eventually dissolve the staples hold the twine down onto the 2x4s.

This mansard roof design is a curse!


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## RBull (Jan 20, 2013)

^pt plywood is overkill. If you're worried about water leakage/rot do the whole roof with ice/water shield over the plywood. 

Why not use nylon twine?


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## hboy43 (May 10, 2009)

Hi:

I agree that there should be no need of PT wood inside the structure, the correct solution is to not let it get wet.

However, if PT does need to be used for some reason, stainless steel works with PT. Low grade 18-8 SS small round head screw will run you about 5 - 10 cents a piece IIRC from Pacific Fasteners. Although if one doesn't need many, SS is available at the building stores for about 5 times that price. 

hboy43 (whose SS inventory is extensive on account of fixing boats)


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## carverman (Nov 8, 2010)

RBull said:


> ^pt plywood is overkill. If you're worried about water leakage/rot do the whole roof with ice/water shield over the plywood.
> 
> Why not use nylon twine?


Will nylon twine disintergrate over the years? When this re-roof is done, it should last at least 25 years. 
BTW, He is using ice/water shield over the whole roof area, something called DiamondDeck underlayment which is far superior to standard tar paper.


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## donald (Apr 18, 2011)

The coil roofing nails themselves these day are 100% coated galvanized(ie rust proof)not sure why that would be a issue?You don't need to worry about the nails rusting.
Save the money regardless on PT plywood-mansard roof as we know is a design that is similar than a wall,meaning no snow will sit on it/it is not as prone to ice damming and the weather elements are quickly repelled off the surface(if you really want you can put it on the top of the mansard where there is a low slope but even still no need(this is for sure one area to not upgrade if you are looking to save money)but use the synthetic underlay on the spruce plywood
Guys,some of the suggestions on this thread is not right.
It don't matter to me but just to state one more time-you should not be needing twine carv for the insulation mate!
I earn a living directly doing this kind of work,of course you don't need to listen to me makes no dif but I find this thread funny lol


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## carverman (Nov 8, 2010)

hboy43 said:


> Hi:
> 
> However, if PT does need to be used for some reason, stainless steel works with PT.


We will be using 3/8" T50 Arrow SS staples to staple the twine to the 2x4s. That way I can be assured there will be no rusting of the staples due to any humidity inside the air space.


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## carverman (Nov 8, 2010)

donald said:


> It don't matter to me but just to state one more time-you should not be needing twine carv for the insulation mate!
> I earn a living directly doing this kind of work,of course you don't need to listen to me makes no dif but I find this thread funny lol


Donald, the issue here is not the roofing material but the loose fit batts. I already have some of them that have come loose over the last ..ok 42 years since the place was built.
The thing I want to make sure is that when the roof sheeting is off and they can get at the insulation, whether they can reuse the existing fiberglass batts (after applying spray foam)(
or new batts, I want to ensure that these will never come loose again to any kind of air pressure disturbances inside the air space between the actual roof and the 2x4 stud walls.
This mansard roof is a stupid design, but unfortunately I have to live with it now, but make sure it's done right.

The roofer is suggesting that for the same price per bundle, *he could install new R22 instead of R14*...but not sure if these would stay in without some kind of 
string supports as the cavity will be reduced from 3.5 inches to 2.5 inches, after the foam is applied..
...and the 5.5 inch R22 batts would be sticking out past the 2x4s by 2.0 inches.


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## donald (Apr 18, 2011)

I understand what your saying.
You can do what makes sense to you(and I suppose your contractor)
If I was your contractor ''we'' wound not be stringing twine is all.


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## carverman (Nov 8, 2010)

donald said:


> I understand what your saying.
> You can do what makes sense to you(and I suppose your contractor)
> If I was your contractor ''we'' wound not be stringing twine is all.


Donald; 
I just got an email from the roofer. He is suggesting that instead of doing this loose batt stuff and stringing twine and time consuming stuff like that...

1. Get the 2 inch thick Owens-Corning foam board (48x96x2) and cut that into 3 pieces to fit into the 2x4 cavities. Tight fit those into the cavities. That equals R10 (r5 per inch of thickness)
2. Spray Tigerfoam over top to secure the foam board, (1 inch of Tiger foam = R7 approx)..That will give me an overall R=17. Slightly better than the R14 batts.
What do you think?


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## Mortgage u/w (Feb 6, 2014)

I apologize in advance if I may have missed something in your thread or simply don't understand.
If your wall is built with 2x4's and have a vapour barrier on the inside and wood on the outside, then where would the batt 'fall' onto?? Isn't the batt already sandwiched between the vapor barrier and plywood? Or is there a doubled wall on the inside which allows for more room......cause I don't see how you would fit spray foam AND a 5.5 inch batt in a 3.5 inch space.


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## gardner (Feb 13, 2014)

Mortgage u/w said:


> I apologize in advance if I may have missed something


I think I missed the point where this became a DIY home improvement forum.

I gather the O/P has one of these: https://en.wikipedia.org/wiki/Mansard_roof










I take it that the insulation inside the knee-walls is falling out, since there's nothing much to hold it in place.

Personally, I would go with rigid foam, even the one with foil on it, because the vapour barrier on that knee-wall is probably crappy and the rigid foam will act as a VB.


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## carverman (Nov 8, 2010)

gardner said:


> I take it that the insulation inside the knee-walls is falling out, since there's nothing much to hold it in place.
> 
> Personally, I would go with rigid foam, even the one with foil on it, because the vapour barrier on that knee-wall is probably crappy and the rigid foam will act as a VB.


This is exactly what I am doing. Will be ordering 21 sheets of that pink Owens-Corning 2 inch thick (R10) 4 x 8 foam board.
Roofers will cut the sheets up, and friction fit into the cavities between the 2 x 4s, then secure them (and seal any cracks) with a 1 inch application of tigerfoam OVERTOP of
the foam board for a overall R value of about R17 (R10 + R7 for the foam). 

This is the only effective solution to fix the heat leakage and the batts falling out..it is a STUPID ROOF DESIGN! 

The roofing job is just too expensive to take a chance on any other method over the years.


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