# Insulating a flat roof with a pitched roof over



## Maintenance 6 (Feb 26, 2008)

Are you saying that this structure used to have a flat roof, but some-one added trusses and now you have a pitched roof? What once was the flat roof area is now enclosed as attic space? Is the waterproof membrane from the flat roof still in place? Yes, you can buy paint with a low permiability rating to act as a vapor barrier. You'll need to go to a regular paint store.


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## joecaption (Nov 30, 2011)

Get out the phone book and look up insulation installers and make some calls to see what they think.
I've had to do this to a home one time and never want to do it again.
We cut out rectanguler shaped holes in the top side and blew in insulation.
We ended up using 1-1/4 PVC duct taped to the hose to be able to push it to the end of the wall to ceiling area.
Your working blind and just hoping for the best as you pull it back when filling.


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## Windows on Wash (Aug 30, 2011)

Where is the home located?


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## Aarone1972 (Jan 9, 2012)

Thanks for the reply guys! Yes, Maintenance 6, you have described the situation perfectly. It appears from my initial inspection that there was no vapor barrier installed originally. But I have only cut into one cavity. I will certainly look into the low permeable paint.

Joe Caption, I like the idea of the PVC pipe. Its a small attic and pretty tight up there, so Im sure moving around a PVC pipe up there will be no easy job, but never the less a great idea. Did you use the type of machine you can rent/borrow from Home Depot/ Lowes?

My home is located in Williamsport, MD just outside of Hagerstown.

Thanks again for the great replies, you have helped me immensely!

Aaron


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## Windows on Wash (Aug 30, 2011)

Aarone1972 said:


> Thanks for the reply guys! Yes, Maintenance 6, you have described the situation perfectly. It appears from my initial inspection that there was no vapor barrier installed originally. But I have only cut into one cavity. I will certainly look into the low permeable paint.
> 
> Joe Caption, I like the idea of the PVC pipe. Its a small attic and pretty tight up there, so Im sure moving around a PVC pipe up there will be no easy job, but never the less a great idea. Did you use the type of machine you can rent/borrow from Home Depot/ Lowes?
> 
> ...


We don't really need a vapor barrier in this climate zone (Zone 4).

Are you saying the the gable ends of the sloped roof are open? If so, that is fine. 

Is the flat roof vented in any capacity? If not, the easiest way to insulation is to make sure the flat roof is sealed against any air leaks and blow insulation over the top of it. That dead air space between the rafters does not really need to be insulated if it is trapped air but it needs to be sealed in order to make sure it is functioning as an insulator at that point.

The DIY insulation blowers will not be sufficient to dense pack fiberglass or cellulose so don't bother with them in that capacity. They will open blow so that is fine.


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## Gary in WA (Mar 11, 2009)

I’m with WW on this, do not insulate the cavity of the flat roof, cell., or f.g.

You have an impermeable layer on the flat roof decking, asphalt coating = a vapor barrier, adding another below (v.b. paint) will trap any moisture in the ceiling cavity. More importantly, you have no ventilation to remove the moisture from the wet cellulose or fiberglass insulation.


Insulate *on* the flat roof to raise the temperature of the cavity to prevent condensation, as per WW.


http://www.buildingscience.com/docu...archterm=attic+fiberglass+batts+in+flat+attic

http://www.buildingscience.com/documents/insights/bsi-028-energy-flow-across-enclosures

Gary


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## Maintenance 6 (Feb 26, 2008)

Regardless of the insulation, you should cut up or remove the old flat roof membrane so that it is not a moisture trap. Any moisture that moves upward from your living space is going to condense on the underside of that old roof when the air temperature is below dew point.


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## Aarone1972 (Jan 9, 2012)

Wow, this was very helpful! 

http://www.buildingscience.com/docu...archterm=attic+fiberglass+batts+in+flat+attic

Ok so I forgot to mention that I do have rolled insulation in the attic on top of the old flat roof. I still have some significant insulation issues due to the fascia and the soffit. I drew a picture to illustrate (ok, so it looks like my 2 year old drew it!)

Anyhow Id love to hear your thoughts.

Thank you

Aaron


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## Maintenance 6 (Feb 26, 2008)

Looks like from your drawing, that the insulation you've got is almost useless. Air getting into the soffit can travel across the space between the old roof deck and your ceiling. You need to get the insulation into that space without blocking off the ventilation into the attic space from the soffit. I would cut up the old roof membrane so it doesn'tcause a moisture issue. Then I would put some blocking at the ends of the joist cavities to prevent closing off your ventilation. Then I would blow cellulose into the cavities and finally, I would coat the ceilings with a low perm paint for a vapor retarder.


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## Windows on Wash (Aug 30, 2011)

Aarone1972 said:


> Wow, this was very helpful!
> 
> http://www.buildingscience.com/docu...archterm=attic+fiberglass+batts+in+flat+attic
> 
> ...


Which space is vented? Is the interstitial space of the older flat roof vented?


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## Aarone1972 (Jan 9, 2012)

The flat roof cavities are not vented. The space in the attic above the rolled insulation is well vented.


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## Windows on Wash (Aug 30, 2011)

Just put insulation over top then like you are doing.

If you wanted to get more technical about it, you could cut and put blockers inside the joist bays to get more R-Value at the outside wall but it is not totally necessary.


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## wkeyau (Oct 25, 2013)

putting a pitched roof over an existing flat roof leaving the asphalt and insulation....is that safe? will the release toxic gas back into the house?

I'm interested in buying a house that has a pitched roof added to a flat roof.
Just not sure about the roof.

The owner got engineering plans and city permit, the addition looks OK. However, the job was not completed. They added insulation on top of the flat roof in some location, but not the entire roof.

Thanks


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## Windows on Wash (Aug 30, 2011)

It is safe as long as the space is vented and your envelope/air barrier layer is complete and intact.


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## PoleCat (Sep 2, 2009)

I am in a similar predicament and initially was intending to blow in fiberglass. After enough research I decided against this and have been dismantling the flat roof. In my case most of the roofing was gone so all I need to do is remove the 1 by decking. Sounds like this will be impractical for you. In your shoes I might consider loose polystyrene balls blown in.


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