Additional Attic Insulation – Framing Question
Hey all,
I am planning on adding insulation to my currently uninsulated attic. This is an old Sears house built sometime in the forties. My current plan is to blow in either cellulose or fiberglass after sealing any penetrations or air leaks. In order to get the required R value for my climate I need to add an additional 8″ of space to fill with insulation. Since our attic is used for storage currently (has a tongue and groove floor installed sometime around the time the house was built) I would like to be able to use it for storage after.
My question is this:
Should I be concerned about additional loading of the 2×6 members (spaced 16″ o.c.) after adding 2×8 lumber with 1/2″ ply on top? Am I better off sistering up the joists with 2x12s instead of running them perpendicular? My only concern with sistering up the joists is getting the lumber up there in the lengths needed.
Could I sister the joists up with 2×8 and then do a perpendicular run of 2×6’s to nail the ply to?
I figured before I went and potentially caused structural issues to my house I should double check.
Replies
I have worked on and remodeled many Sears kit homes of your homes vintage. I have found that many structural members were not structurally adequate from inception. These were sold as complete D.I.Y. packages that most likely never saw the light of day of a plan reviewer or inspector. I have found that beams, floor joists, and ceiling joists were somewhat undersized for the loading (either from a dead and live perspective. or both) conditions they were asked to resist--or that was changed on site by the D.I.Y'er (which was not rare). Also, structural redundancy and composite action between structural members that can be achieved with modern building materials (like plywood, adhesives, screws, bolts, metal connectors) just weren't prevalent in the day of these catalogue home kits. Long, simple spans with shallow, full dimensional lumber was prevalent, weak moment connections at posts and walls were the norm, and unchecked bending moments and deflection resulted in the tell-tale creeps and sags.
All that is to say that you would be wise to consult with structural engineer before making structural changes to your attic floor/ceiling joists that are only 2x6's. Yeah, I know you posted to get free advice on insulation for your same home a few days ago... and are now looking for free structural advice here again. I hate to tell you, but you won't get it. No professional structural engineer is going post structural solutions for you on this forum. Period. It is also interesting that you went from considering blowing insulation on top or your floor, to now wishing to use your attic for (unspecified) storage. I had an inkling that was your ultimate goal. I see it all the time as a professional remodeler.
All of the ideas you have initially floated may or may not work. Some might lead to failure if loaded excessively, some may give you a far superior floor structure than you ever dreamed of--if executed correctly. That is why I recommend that you review them with a professional engineer (as welll as that energy audit consultant to boot) so that they can professionally consider all the factors of your specific home and intended use. Its not terribly expensive from a due diligence perspective. However, I realize that from a cheapskate's perspective, everything is expensive. Bottom line is that you have not given near enough information even for an experienced builder to reasonably weigh in (pun intended) on the structural feasibility of your considerations; except to present general theory/practice and express specific caution where caution is due. But if history is any indication here on F.H., I have no doubt that some will try. They'll try to convey a specific solution they did, saw, or heard about... and/or confirm the appropriateness of any of yours. Some already have.
As they continue to do so, bear in mind what I recall one long time poster of Breaktime (one of the better ones) used to say: "Value of advice given is directly proportional to price paid".
Cheers!
This is a different thread than the other one you extensively commented on.
Unclemike42 - you're 100% right. Though I extensively read the post he is referring to. I applaud you in your restraint in replies in that thread - I am definitely not as patient as you!
While this is my first post I’ve been perusing for a while - you always post helpful comments back. Thanks for being part of the community!
For the record I have one of the S.Es from the engineering firm I work at coming over to take a look. I’ll post back with his assessment and some pictures.
Thanks for your kind words. I look forward to seeing your updates.
Yes, you should be concerned. You will want to understand the span as well as spacing of the existing 2x6 joists, and have an idea of the joist ratings.
You might want to start with a sketch of the supporting walls below, and existing joist spans.
You also may want to consider the need for this storage space, and the impact of loosing almost a foot of vertical space.
I will come back and update on what I find. I quick secondary inspection (I admit I probably posted prematurely) shows a typical span length of 9-12 feet from bearing wall to bearing wall with 16" o.c. Obviously these spans wouldn't pass snuff in some cases today but I haven't found much settling or any damage to these members in my investigation so far. The existing attic is used for storage currently - it had T&G when we moved in and has a stairwell up there. I've been digging around for the original sears plans to see if this was an add later or original.
Not too concerned about the loss of headspace - from floor to top of the ridgeboard there is about 6 feet of clearance. I can crawl around to get the few boxes we store up there no problem.
I'm considering beefing up the joists by stacking a 2x6 on top of the existing (similar to this JLC article https://www.jlconline.com/how-to/interiors/beefing-up-attic-joists-for-living-space_o) after a quick discussion with my S.E regardless.
I'm more concerned about doing the work up two stories of narrow halls.
I'll update with what I finally decide. Thanks for the help - I can have the tendency to jump ahead in the process in my mind.
From an energy conservation/cost perspective, the goal is always to minimize the volume of your dwelling's thermal envelope. However, since storage has now entered your scope of concerns, you may want to re-think making the roof line the top limit of your new code-compliant thermal envelope rather than limiting it to improving it at the ceiling joist line . There are pros and cons to each approach, but if storage value is high, then I believe the benefits of the former will usually outweigh the latter.
I originally considered changing it out from a vented attic with the thermal envelope at the ceiling to a hot roof. I may bring it back into consideration based off what I am finding.