# Adding insulation to a low-slope roof



## redline (Mar 5, 2006)

How thick are the joists?


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## cbeingessner (Sep 20, 2007)

I think they're either 6" or 8". My neighbour's house had 8 inch joists, but I suspect mine are only 6". The home inspector hypothesized they'd have about 4" of insulation in them, regardless of size. The house was built in 1961.


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## redline (Mar 5, 2006)

Does your neighbor also have a low sloped roof?


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## cbeingessner (Sep 20, 2007)

Yeah, his is similarly sloped, though it's a split level, and ours is a bungalow. When he had his roof done, he had the roofers life the plywood and cram insulation in to fill the space. But, that doesn't seem overly efficient to me -- compacted insulation isn't as effective, and it would still only probably bring it up to R-20 at most, which, in a province where -40 is common, doesn't help the old gas bill that much


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## Ed the Roofer (Jan 27, 2007)

I was just reading about packed insulation which is fit too tightly in the rafter space.

True, it does not offer the rated r-value, but it will offer more r-value than a thinner version that properly fits the space.

A 2/12 is too shallow of a pitch for a shingle application anyways though, unless special provisions are made.

Ed


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## the roofing god (Aug 2, 2007)

it`s recommended to cister 2 x 12`s w/cats between them if you want to be able to insulate properly


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## cbeingessner (Sep 20, 2007)

What do you mean by cister? Sorry, I'm a roofing rookie. Will it be hard to get fascia that is tall enough once the roof height is extended? Also, should I worry about the additional weight on the structure of the building? It was built in 1961. Thanks so much!


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## Ed the Roofer (Jan 27, 2007)

Just for clarification, he probably meant to say, Sister the rafters, where you naul an additional rafter side by side to the existing one and the other word cats, was probably supposed to be batts, as in batts of insulation.

Ed


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## the roofing god (Aug 2, 2007)

cister is the right word for securing an additional joist to the side of the one existing,cats are sommetimes called cripplers,basically they go horizontally between the added joists and keep them secure so they don`t twist,you would leave the cats about one inch shy of the sheathing for airflow puirposes,then you can add whatever batts or rolls of insulation to meet code-if there is existing insulation ,it is important to use unfaced insulation above it so you don`t trap condensation between the insulation layers


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## taxlin (Aug 1, 2010)

:thumbsup::thumbsup::thumbsup::thumbsup::thumbsup:


cbeingessner said:


> I live in Canada, and have a low slope (2-12) roof that is shingled. The r-value of the existing insulation would be somewhere between 4 and 7, I think, though there's no way to tell for sure without lifting the roof off, because we have vaulted ceilings inside. The shingles are poor, and I'm looking to replace them, and add insulation at the same time. If I can get the R-value up to at least 28 (and I'd prefer 40 or 60) I can get up to $1800 in grants from the government. If I lift the plywood off the roof, I can add in some insulation, but with only 6" of room, I'd be short of the target. Any thoughts on how to best do this? I've thought of strapping the existing rafters with 2X6s on end, and adding insulation that way. Any better ideas?


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## taxlin (Aug 1, 2010)

I give up where is the reply to the low slope insulation?:thumbsup::thumbsup::thumbsup::thumbsup:


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## beenthere (Oct 11, 2008)

Please don't use the report button, unless you are reporting a post or thread that violates the rules. Thank you.


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## the roofing god (Aug 2, 2007)

taxlin said:


> I give up where is the reply to the low slope insulation?:thumbsup::thumbsup::thumbsup::thumbsup:


:no: the question was regarding insulating an attic area,on a home with a low slope roof,not adding insulation to a low slope roof system,for that search out topics regarding "tapered insulation" :thumbsup:


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## xxPaulCPxx (Dec 2, 2006)

Normally what is done is to remove the plywood, cover with 4" to 6" of rigid foam board (polyurethane is R6.5 per inch) over the rafters, and blow in cellulose underneith the foam to fill any remaining space. Tape the seams in the foam, then cover with 2x sleepers fastened through the foam down to the rafters. The gaps between the sleepers are you new air vent channels, put screen at the eave to keep bugs out, and a ridge vent at the top. Since this is so low, you have to be concerned with snow loads... you may need to sheath with 3/4" plywood.


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## OldNBroken (Jun 11, 2008)

Wood nailer around entire perimeter, infill with staggered multiple layers of polyiso, resheet over polyiso with minimum 5/8 cdx, install new roofing system. (preferably a standing seam)

2 layers of 2" approx R25
3 layers of 2" approx R32

All depends on your roof design. if its too cut up its not a very feasible option.


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## Hexar (Mar 5, 2010)

I have a similar house: 1300 sq.ft. single floor bunglow, with a flat roof, currently it has tar and gravel roof, and truss is 8" tall, the current insullation about 3-4" tall butt insulation stuff. 

How much, roughly of course, will it cost to:
1. remove the the tar and gravel
2. blow in the air space between ply wood and top of the current but insulation
3. add 4-6" rigid foam insulation
4. add new roofing, single ply or similar.

?

Thanks!


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## beenthere (Oct 11, 2008)

Hexar said:


> I have a similar house: 1300 sq.ft. single floor bunglow, with a flat roof, currently it has tar and gravel roof, and truss is 8" tall, the current insullation about 3-4" tall butt insulation stuff.
> 
> How much, roughly of course, will it cost to:
> 1. remove the the tar and gravel
> ...



This is a DIY site. Not what does or will it cost site.


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## federer (Aug 20, 2010)

Hexar said:


> I have a similar house: 1300 sq.ft. single floor bunglow, with a flat roof, currently it has tar and gravel roof, and truss is 8" tall, the current insullation about 3-4" tall butt insulation stuff.
> 
> How much, roughly of course, will it cost to:
> 1. remove the the tar and gravel
> ...


i would suggest contacting some roofers/ insulators to get estimates. they usually do free estimates. i would say at least 7grand


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## Hexar (Mar 5, 2010)

beenthere said:


> This is a DIY site. Not what does or will it cost site.


yeah I know, I usually do everything DIY, but roofing seems hard.


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## xxPaulCPxx (Dec 2, 2006)

Roofing is not hard.

Do some research and you will be able to easily pull it off.


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## Hexar (Mar 5, 2010)

federer said:


> i would suggest contacting some roofers/ insulators to get estimates. they usually do free estimates. i would say at least 7grand





xxPaulCPxx said:


> Roofing is not hard.
> 
> Do some research and you will be able to easily pull it off.



Thanks guys, I will ask for a quote first and see if I can do it myself, next year, this year it has been raining so hard.


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## federer (Aug 20, 2010)

xxPaulCPxx said:


> Roofing is not hard.
> 
> Do some research and you will be able to easily pull it off.


not everyones equally handy. i dont think i can do roofing on my own


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## xxPaulCPxx (Dec 2, 2006)

That's just fear of the unknown talking.

Here is whats involved:

Tear off old roof - this requires the skill of shoveling and nail pulling. You may occasionally have to replace a rotted board.

Lay down underlayment - requires the skill of hammering a nail and unrolling rolled up material.

Lay down starter roll - see above

Snap lines and nail down shingles - requires following spacing instructions on package of shingles, as well as being able to hammer a nail.

The rest is just variations of the above!


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## Hexar (Mar 5, 2010)

xxPaulCPxx said:


> That's just fear of the unknown talking.
> 
> Here is whats involved:
> 
> ...


Paul,
This is a low pitch roof, mine is 1/12, so I probably cannot use shingles. If my roof is like 5/12 or whatever high pitch and just re-shingle, I am on the roof already.


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## federer (Aug 20, 2010)

my roof is flat too. no shingles


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