Attic Insulation - keeping rafters exposed.

14 Replies

Hello everyone! We have a 100 y/o single family buy&hold in Missouri and would like to turn the attic into usable space. ideally we could keep the rafters exposed in keeping with the character of the home, so the standard 6 inch fiberglass would not work. 

Is there a DIY compact type of insulation that would work? 

You could use a radiant barrier between the rafters.  Would imagine you would have to put up panels over them but they are fairly thin and work well.

Originally posted by @Melinda Miller :

Hello everyone! We have a 100 y/o single family buy&hold in Missouri and would like to turn the attic into usable space. ideally we could keep the rafters exposed in keeping with the character of the home, so the standard 6 inch fiberglass would not work. 

Is there a DIY compact type of insulation that would work? 

Where are you located?  Most municipalities areas have adopted some combination of the IRC, IBC, and IECC (energy code).  The requirements for insulation are not in inches, but rather in R-values or the reciprocal  U-values.   Again, your local buidling code will rule the day, but Missouri falls into Zones 4 and 5, which would require R-49 via the IRC.  Which, you would come nowhere near achieving with 6 inches of fiberglass batts.  The Owens Corning single batt fiberglass product is 14" thick.

I think the look you're hoping for is impractical.

  

I agree with Kurt, The big hurdle you face is the R value. If the R value comes into play you are probably not going to be able to add "livable" in the attic without filling in your rafters with insulation and then sheet rocking over it. I've seen a few people in my area insulate the floor of the attic before laying down the decking. In my area this loop hole works if the attic is only for storage. the space can't be counted as square footage or used for any sort of rental space. 

@Melinda Miller

Looks like I might have a 2-season bonus room on my hands. Oh well. Thanks anyway.

Best,

Melinda

Hi Melinda:  this quote of yours came from the other duplicate thread to this one.

As I mentioned in my post above, getting the exposed existing structure aesthetic won't be practical.  But exposed existing structure with adequate insulation is not necessarily your only path to usable space in your attic.

On the other end of the spectrum, you can always plan on a typical gyp board ceiling, as @Bryce Lee has suggested.  Which, can work out very well.  Given the age of the house, your rafters are probably 2x4s (they'd be the old school actual full size 2x4's, but still too skinny), so you would likely need to add some furring thickness to get up to code R-values.  (The other thread has some incorrect R-value recommendations.)  Given the additional thickness required, you may also need some additional structural framing -- different can of worms, but this is often very doable.  

If I were doing this, I would definitely go with an air-impermeable spray foam -- sprayed tight to the underside of the roof sheathing to the thickness required to get the code R-value.  No further need for further vapor diffusion resistance.  No need for venting.  Done.

   

To get to R-49 with foam board...an inch of it gets you to R-6. I have to agree with Kurt, not that it's a difficult problem when you understand the values involved. The only insulation that stands even a chance of getting you close is going to be spray foam, and it's going to cost the earth.

The property is in the City of St. Louis, where codes are enforced inconsistently at best. I've tried the foam board in other applications-could have used tissue paper instead. 

I believe the rafters are 2x8 oak, but I could be way off base there. Since the ceilings are peaked and 7' at the center anything of any depth cuts it too short. The floor is already mostly insulated and decked. Not sure I'm ready to give up on the aesthetic. It is a visually stunning space with great light and old school character. 

All right, Melinda, this is going to be a bit complicated, but I hope it's worth it.

The rafters are not oak. Oak was only rarely used in US housing built in the 1900s as framing lumber, for the simple reasons that most varieties of oak don't tend to grow straight, oak grows much slower than most pine species, and oak is usually extremely heavy. There are much straighter and lighter trees that grow faster available for dimensional lumber: firs, spruces, and various other pine species.

The rafters are not the biggest issue here -- a sturdy rafter structure is to be expected in a pre-truss structure, where simple solid rafters in line hold up the roof, not a complicated structure of thinner sticks taking up lots of floor space in the attic.

You need to look at the attic floor from below carefully. For many years, and continuing in many places to the present day, ceiling joists on the top floor were often only strong enough to keep the walls from pulling apart. They were not made to serve as weight-bearing floor joists for an attic converted to living space. There were exceptions made to this all over the country, but not all that many and mostly in more expensive, brick-veneer balloon-framed homes, where the beefed-up structure helped keep the heavier walls in place and also supported the stress of the rafters holding up a heavier slate roof. Here in the Burgh, for instance, I own a very large balloon-framed brick-veneer duplex built in the 1920s for an heiress. It has 2x8 ceiling joists on the second floor. I also own a tract-built, platform-framed brick-veneer townhouse condo built cheaply in the 1970s for ignorant trash with pretensions, and it has 2x4 ceiling joists and a truss-filled attic I wouldn't dare try to turn into living space.

The rafters are definitely 2x8, 14-16 o.c. I was guessing about the oak, so I am sure you are correct about that. This was built in the late 1920s, is brick and plaster, 1 story (+ basement & attic) on a stone foundation.

Hi Melinda,

I thought the duplicate thread mentioned 2x4 rafters -- so 2x8s, which as other have mentioned would be more typical.

OK.  Well, a full 8" depth (and the older lumber might very well be) could pretty much get you there for R-value.  However, and still as before, the exposed existing structure aesthetic would be gone.  I'm assuming you liked seeing not only the rafters, but the sheathing boards as well, which, with any insulation solution on the underside of the roof, gets completely covered.

Some other points mentioned, but to stick with what info you are giving us -- the 7' you mention -- to be clear, is that at the high point down the center along the roof peak?  Hopefully I'm misinterpreting what you wrote, and you have higher ceilings than that.  Per code for a sloped ceiling, you need 7 feet min. over not less than 50% of the floor space.  

Sorry about the duplicate post. I didn't realize I did that and now I don't know what to do about it. Good question about the ceiling height. Thanks for making me think about this. Half the area is about 7' 6" at the peak. The other half is over 7' ish for pretty much the whole thing. So it's close but ok I think. 

I would like to keep the look of the sheathing but I would also like to be Queen of the World.

Somebody I wish I could tag him here mentioned a radiant barrier. Any experience with that?

 @Melinda Miller -- 

A radiant barrier can be a great part of a total assembly, but it doesn't take the place of insulation.

Also, the height of the ceiling directly below the peak is not quite what the code is addressing.

The code provision I've mentioned works like this:  Take the floor space you'd want to create in your attic -- maybe use  masking tape to mark the perimeter of the area.  That is the new floor space you intend to create.  Now, if you were to take a tape measure and measure vertically from the floor to the ceiling at various points within that space, some areas close to the center will approach your 7'-6" height, while other areas out toward the exterior walls will be much less.  The code is saying that at least half of the total area you've taped off must be at 7'-0" or higher. 

You could also cut a stick of wood 7'-0" long -- then move the stick around in your attic space to get a quick feel for how much floor area might have a 7'-0" ceiling clearance available.  As an example, if you found that there is 200 SF of space with at least a 7'-0" ceiling height, then the code is saying  that you could have an additional 200 SF of floor space with a ceiling height less than 7 feet, for a total of 400 SF of living space, in this example.

All that said, with the peak heights you're mentioning, I'm guessing you don't have the ceiling height available to meet code for living space.

      

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