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Insulating a Converted Attached Garage

ScottC7 | Posted in GBA Pro Help on

Hi All,



My 1960’s home in Iowa, climate zone 5, has an attached 2 car garage (20’ wide by 26’ deep) that was finished and brought inside the conditioned space by a previous owner.  It is now a bedroom with a walk-in closet and a mudroom with an exterior walk door on the back wall.  I have always struggled to keep this room warm during winter.  It is usually below 60 degrees F.  I am looking for advice on how to best solve two insulation issues that I think will have a large impact.


The first issue is the ceiling insulation.  The gable roof structure was modified so that the front half is now an insulated cathedral ceiling.  2×8’s are sistered to the original 2×6 rafters.  Fluffy insulation with a kraft paper face is poorly installed between the rafters.  There are no ventilation baffles at the wall and no continuous chutes to prevent wind washing.  There are also 4 leaky can lights installed in the ceiling.  I am limited to solutions from the inside because the asphalt shingle roof is only 2 years old.  Also, if the ceiling drops more than 3 inches it will interfere with a walk door opening.  There are probably ways to modify the door though.  What is the best approach?


Should I create an unvented roof assembly?  This would involve maintaining the existing rafter depth and using high density spray foam.  For this situation, can I also add a continuous layer of rigid foam under the rafters to prevent thermal bridging?  If so, what kind of rigid foam and what thickness should be used?   This would be followed by 1x strapping and drywall.


Alternatively, should I create a vented roof assembly?  This would involve adding depth to the rafters.  I would add a continuous vent channel from the soffit to the ridge vent, material to be determined.  Then I would add fluffy insulation down to the bottom of the rafters.  Then a continuous layer of rigid foam under the rafters would be added.  What type of foam and what thickness makes sense here?  This would be followed by 1x strapping and drywall.


The second issue is the concrete slab floor insulation.  This does not have any insulation or vapor barrier.  The slab feels leaky around the entire perimeter and the slab edge at the front wall, where the 16’ overhead door used to be, runs under the new wall exposing it to the outside.  Is it a good idea to install a continuous layer of XPS or EPS on top of the concrete followed by a plywood subfloor and vinyl plank flooring?  How thick should the rigid foam be and is there a need for a separate vapor barrier?  Any suggestions on using sleepers vs. a floating subfloor?  Also, raising the floor in the mudroom is not possible due to the exterior walk door.  Is it worth it to insulate everywhere except the mudroom?  The mudroom is about 25% of the total floor area.  Additionally, it is exposed to the outside on only half of the back wall.  There are 3 exterior walls, and this is roughly 15% of the total linear feet.


I would appreciate any suggestions and advice.  If you have any comments about critical details outside of my specific questions, feel free to share.  Thanks! 

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  1. user-2310254 | | #1


    You might be able to use the flash and batt method to address the cathedral ceiling. (See for more.) But it also sounds like you have a leaky room that needs air sealing. The recess lights are particularly problematic. (See for how to tackle this issue.)

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