Insulating flat/low slope roof

Old 08-23-17, 08:12 AM
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Insulating flat/low slope roof

i am posting this as a reply as I have the same issue- please let me know if it requires a new thread.
I have exactly the same type of Florida room - mine is facing south . It is also tied into the roof and there are baffle boards at the point where the low part of the roof ties in to the attic which has blown insulation done earlier this year.
My room has no drywall on walls or ceiling - the floor is 1" tongue and groove pine and part of the room has a 8x10 space with 2x6 ceiling joists and the rest of the low ceiling has 2x4s that I am thinking of notching 2x6's to so I can get some insulation in there that will be across the whole of the low ceiling.

I have had an AC guy run two air feeds one coming down from attic and through a wall that I will box in and another smaller one coming through one of the baffle board spaces

There are small windows facing east in the room on the side of house and west into an unscreened porch with a concrete slab.
The south side of the room that faces the porch ( the porch shares the low roof too) also has a row of windows and a door with steps down to the slab

I would like to insulate this Florida room and have had many different opinions from blowing insulation on the back of the ceiling ply sheeting and on joists to using foam panels with foil facing in or out. What is the best option here ?

my other problem is both of the the 6" and 4" joists are not evenly spaced on 16 or 24" centers - so my idea was to use the foil backed foam so I can cut it to a tight fit size, and maybe double it up if I use 2" R13 - is this ok to do?

Also the enclosed south facing wall has no soffit or breathing space on the south side of the room - how much of an issue is this and should I open up areas between the 6" joists where the roof terminates?

In the walls i intend to use r13 paper backed fiberglass

on on the floor I'm planning 10mil laminate over the existing pine . I can post pictures if needed.
Thanks in advance


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Last edited by Shadeladie; 08-23-17 at 01:45 PM. Reason: Add link
Old 08-23-17, 08:33 AM
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Paul, welcome!

Yes, starting your own thread is always preferable but to answer your question...

In places where the rafters limit the ventilation, builders are finding that it is often preferable to completely fill the space available with closed cell insulation. So, I would suggest you look into having your ceiling professionally spray foamed, with no ventilation above, closing off any existing ventilation. This is called a spray foam hot roof, if you wanted to do any research on it. Anywhere your rafters are less than 2x6 you could add onto the bottom, by nailing a 2x2 (or a full 2" rip) onto the bottom of the 2x4 rafter to add more r-value.

You can attempt to diy with foil faced XPS but compared to professional spray foam, you will be creating a patchwork quilt... air sealing all the edges of every rafter is the critical component of this method, and that is where professional spray foam is vastly superior, since it seals everything 100%.
Old 08-23-17, 12:43 PM
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Thanks for your reply
i was just about to buy 2x6 to notch and nail to the sides existing 2x4s as I thought strength would be better but if I can extend the 2x4s by nailing 2x2 that sounds easier

As regards insulation blowing, I get your point. I looked into renting a blower but it looks like a first timer like me might find it complicated to do and get right without making a big mess.

One more ore question if I get insulation blown : can I run wiring for overhead lighting in between the extended Joists and should that be done before blowing or after? I assume if before an insulation company would know how to safely cover it?

Thanks again for your reply
would you like me to create a new thread and add a link here to that ?


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