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I have a flat roof house and don't want to tear out my ceilings or roof but am considering installing some sort of insulation because my house temp is so uncomfortable. I've seen that there is spray foam that can go over existing roofing, roofing systems made of foam board, spray in cellulose which goes in the space between the ceiling and roof (though i'm not sure how to get it in there w/o removing roof or ceiling), and i think that's it. What is the least expensive and what would the cost be approx. for a 1100 sq ft home? eh?
 

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Poly-Iso board insulation (preferably at least 5") on top the roof deck topped with EPDM is a good method that's commonly done in commercial construction.
 

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If I could do it over again, I'd put at least 8" of ISOboard. If you ask an engineer/architect, theyd ecommend more R. A contractor will tell you less R is OK.

Is this flat roof vented? If not, the insulation options will be more limited. Cellulose should not be used in unvented flat roof assemblies.
 

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Your talking about installing 5 inches of ISO right?? lol I was thinking about how much that would be to install.. Or what about 8-inches lol.. thats funny..
 

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If you don't want to demo anything, then you're looking at the most expensive option of polyisocyanurate panels that have been spoken about above.

Filling the joist bays will require either a new ceiling or new roof.

If this is a must-do project for you then my recommendation would be to drill the roof and blow in insulation into the cavity from above. Then install polyiso panels to the thickest you can afford and then put on a new roof system. Polyiso is about R-6.5 per inch.
 

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My opinion is that you get far more insulating values from getting the insulation to fill the ceiling joists. To equal those insulating values takes a lot of ISO installed on the roof.
If the uncomfortable inside temperature is heat, then maybe consider a reflective coating. Or wait till you do your roof, take up some decking and spray in insulation and just put a a standard layer of insulation down.
 

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My opinion is that you get far more insulating values from getting the insulation to fill the ceiling joists. To equal those insulating values takes a lot of ISO installed on the roof.
If the uncomfortable inside temperature is heat, then maybe consider a reflective coating. Or wait till you do your roof, take up some decking and spray in insulation and just put a a standard layer of insulation down.
Actually polyisocyanurate foam boards is one of the best insulation on the market today when looking strictly at insulative value. There is not thermal bridging and the R-value per inch exceeds almost every other kind of insulation readily available today.

Filling the ceiling joists is definately an option, but as far as being the most energy efficient it is not as the joist themselves create thermal bridges. The lastest energy star regulations require a minimum R-5 foam sheathing because of this.

A reflective coating is a great idea!
 
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