undersized framing ... how to strengthen?

[Hal Nash(Guest)]

Hi.  Back again.  I am attempting to rehabilitate a 16x30 shell that has undersized framing, well, undersized according to today's stick-built structures.  It is about 70 years old and constructed of cedar....that's why it's still standing.

The ceiling joists are 2x4s that span about 15'3".  I am thinking about installing blocking, and possibly sistering an additional 2x4 along side each of them to stiffen the ceiling.  The attic space above the ceiling will not be supporting anything like storage, but just the weight of the ceiling finish and fiberglass insulation above.

Would it be possible to add 2x4s to the roof rafters and "custom" build trusses to support the ceiling?

Thanks for your input.

Hal

[bartholomew]

You could turn the rafters and joists into trusses but, assuming you could get the 16 footers into the attic without too much difficulty, I'd just go with sistering the joists. Cutting and installing all the truss members to make true trusses wouldn't be worth the effort. I'd probably also use 2x6's just so I had the option of using the attic for light storage. If the roof also needs to be beefed up then adding a single brace at the end of each rafter/joist, a few feet in from the wall, would do a lot to stiffen the rafters.

[Amanda_931]

Might also check on joist and rafter tables using the kind and grade of lumber you think they are to see if they really are undersized.  

(also a brace half-way up between the rafters--that would not help with load per se, but would with the tendency of the walls to pull apart)

[glenn-k]

I built a truss a piece at a time to take out a hall way wall that was unnecessary except that it was bearing part of the roof - otherwise in the way -- worked perfectly with no movement as the wall sections were removed.

Gave us room to widen the living room and add a bedroom.

Also added an archway to the kitchen.

[hnash53]

Bartholomew,

You said "If the roof also needs to be beefed up then adding a single brace at the end of each rafter/joist, a few feet in from the wall, would do a lot to stiffen the rafters."

Are you saying to nail a vertical brace connecting a roof rafter with a ceiling joist?  That wouldn't stress the roof rafters would it?  I've installed collar beams (ties) on all the rafters also.

You also said sistering would work, especially with 2x6s.  Since the ceiling joists are 16" OC, the insulation would be a bit tight, wouldn't it?

Thanks for your input.

[manhattan42]

You really have no other option but to contact a structural engineer for the solution to this problem.

Span tables do not include cedar woods. The deflection will need to be calculated and measures to strengthen the joists stamped with an architect or engineer's seal before you can be issued a building permit in this case.

You will also need an engineer to stamp any design for trusses. Trusses are an engineered wood product and cannot be self-designed by the builder or owner. Permits cannot be issued for trusses without stamped and sealed designs.

There is normally no need for additional "collar ties" between the opposing rafters in the attic as Amanda_931 suggests. The ceiling joists tied horizontally on the top of the bearing walls is all that is required to keep the rafter from spreading. Collar ties won't hurt, but they won't help much either in most cases.

"Sistering" the joists might work but you would need 16' continuous joists to stiffen the existing ones to be most effective. You won't be able to get full-length joists into the attic without cutting holes in the roof.

Bottom line, you need the engineer's assessment of what you can do here.

Designing for structural integrity is not something at which someone can 'guess'.

[jonseyhay]

Or you could use a hanging beam

 

Hanging beams are usually at right angles to ceiling joists and are located directly above them. The joists are attached with joist hangers to the beam. The ends of the beams need to rest on the wall plate directly above a stud and should be held vertical.

[bartholomew]

Are you saying to nail a vertical brace connecting a roof rafter with a ceiling joist?  That wouldn't stress the roof rafters would it?  I've installed collar beams (ties) on all the rafters also.

You got it, vertical or leaning somewhat towards the wall. Where now the rafters might sag or flex a bit under load, instead some of that extra load gets transfered to the beefed up joist. You wouldn't want to do that if the joists are marginal. And if you are in an area with heavy snow loads, then yeah an Engineer should be consulted.

You also said sistering would work, especially with 2x6s.  Since the ceiling joists are 16" OC, the insulation would be a bit tight, wouldn't it?

My understanding (which I read online once but don't know where anymore), is that the insulating value per inch (R-value) of fibreglass actually increases as it is compressed (up to a point, you don't want to compress all the air out of it). You can get both regular and high-density fibreglass batts.  From Owens-Corning's site, ""High density" means more fibers per square inch than standard insulation products. That means you get a higher R-value per inch." So I don't think you have to worry about compressing the batts a bit to make them fit. Of course you will have more thermal bridging between the batts, but you can cover the joists with a second layer of batts (running perpendicular to the joists) and in most places you'd want more than 6" of fibreglass anyway.

[manhattan42]

"My understanding (which I read online once but don't know where anymore), is that the insulating value per inch (R-value) of fibreglass actually increases as it is compressed (up to a point, you don't want to compress all the air out of it). "

Compressing fiberglass insulation decreases it's ability to insulate.

Batts should be cut to to fit between joists that are less than 16" on center.

[glenn-k]

I heard the same as Manhattan mentioned - where we compress fiberglass pinched between the purlins and roofing on steel buildings, the insulation value is very low.  This has been an industry practice for years and years on utility and farm buildings.

[Amanda_931]

Pinched between purlins and roofing would not only almost certainly be compressed beyond the high density stuff and in any case not predictable.  And I'd think it might get a fair amount of moisture from condensation.  But that's not quite the same as saying that controlled higher density wouldn't be better.  

If  there's no insulation there now, the dense-pack cellulose between the ceiling joists might work.  I think I've read that while the real dense-pack^tm must be installer-installed  , you may be able to approximate it yourself.  I'll need to look that up one of these days.

[bartholomew]

Yes, Amanda, you've got it.

We're talking about two different things here so I'll try to be clearer. You can increase the density of a fiberglass batt in two ways:

A. Reduce it's volume while keeping the amount of fiberglass the same. This is what happens when installers squeeze batts behind pipes, purlins and electrical boxes.

B. Increase the amount of fiberglass while keeping the volume the same. This is what manufacturers of high-density batts do.

I cannot remember who did the study I referenced but it was by a reputable agency. It found that moderate increases in density led to a higher R-value per inch. For example (using made-up numbers that are easy to work with), a 33% increase in density might lead to a 25% increase in R-value per inch.

So let's say you had a 12-inch thick batt rated at R-4 per inch. Total R-value is R-48. Using method A, if you compress it down to 9 inches it will be R-5 per inch, for a total of R-45. Greater R-value per inch, slightly less total R-value.

On the other hand, using method B, you could take a 16 inch R-4 batt and compress it down to 12 inches resulting in a total R-value of R-60. Compared to the standard 12 inch batt it would have a 25% increase in R-value per inch and also a 25% increase in total R-value.

Those increases in R-value per inch only occurred up to a certain point. By the time the density was doubled, the R-value per inch was back down and about equal to that for standard batts. Beyond that, further increases in density resulted in lower R-value per inch. What I took away from the study was that there would be some gains by using one size thicker batts than normal, eg. 10.25" in place of 8.25".

With Hal Nash's scenario, we are talking about method B, taking a batt intended for a 14.5" bay and squeezing it into a 13" bay. That's only a 12% increase in density. I would only trim it if I couldn't get it to lay flat.

[manhattan42]

Again, compressing fiberglass reduces R-value...all the time...every time.

Compressing fiberglass insulation should not be and is being confused with increasing density.

Increasing fiberglass density will increase the R-value. This is done by increasing the amount and often the TYPE of fibers in the material.

All you ever get by compressing fiberglass insulation alone is increased energy loss.

Comparing compression and density is like comparing apples and orangutans.

[bartholomew]

Again, compressing fiberglass reduces R-value...all the time...every time.

Well, Owens Corning respectfully disagrees with you. They provide data on their website showing higher R-values per inch from compressing fiberglass batts (in Excel format): http://www.owenscorning.com/around/insulation/CompressionChart.xls

I've imported the data into NumSum ( http://numsum.com/spreadsheet/show/21111 ), highlighted some cells and reproduced it below.

For a 2x10 cavity, insulating values from lowest to highest are obtained by using (from the orange cells):

a standard 9.5" batt for R-30
a standard 12" batt compressed for R-32
a cathedral ceiling (i.e. high density) 10.25" batt compressed slightly for R-35

For a 2x8 cavity, insulating values from lowest to highest are obtained by using (blue cells):

a standard 8" batt for R-24
a standard 9.5" batt compressed for R-25
a standard 12" batt compressed a lot for R-27
a cathedral ceiling batt 8.25" compressed slightly also for R-27
a cathedral ceiling 10.25" batt compressed for R-30

For a 2x4 wall (green cells), high density 3.5" batts and compressed 5.5" batts both give R-15.

Of course in most cases you can just buy a high-density batt in the size you need. But if you've got some thicker batts left over from another job or you can get a good deal on a different size, there's no reason to worry that using it will result in lower total R-value than the "right" size. You do have to be careful that compressing the batt into the cavity doesn't collapse any insulation baffles.

[glenn kangiser]

So, if I'm reading this right, Bart, it seems, according to Owens-Corning that you don't get the full value of the insulation if it was in its full original designed space, but you still gain over standard insulation in the smaller space -- like making your own high density insulation per their chart then.

Another note on the method of installing insulation on steel buildings -- where the insulatin is pinched at the purlin there s very little R value but it flares out almost immediately and is full value between purlins.  There is a special insulation made for this called UL Vinyl that we used to get.  I have never had any problem with moisture condensing above it - the plastic vapor barrier on the inside is continuous.  It was used as a fairly cheap way to insall insulation on shops, etc.  Usually it is only 2 to 3 " thick but greatly improves the conditions in the shop.  Do I like to do it?  No way - it's a real PITA  but is pretty nice to work in the shop later.

[bartholomew]

Exactly, and it is important to distinguish between total R-value versus R-value per inch. For a given batt, as you compress it more the R-value per inch goes up and total R-value for the batt goes down (method A; scanning down the columns of the spreadsheet). For a given cavity depth, as you stuff in thicker batts both the R-value per inch and the total R-value for the cavity increase (method B; scanning the rows from right to left but taking into account the high-density batts).

[PEG688]

Bart Remember argueing with a building inspector is like wrestling witha pig , ya both get dirty and the pig likes it.  Especially if you live and work inside thier jurisdiction , on the internet YMMV

 PEG

[JRR]

Interesting chart, Bart,  ... I'm not sure I'm reading it correctly.  

It seems Product R-38c is the best choice for 2x12 studs, .... the best choice for 2x10 studs, ... the best choice for 2x8 studs.  Why do they even manufacture Product R-38 or R-30?  Cost difference?

It also seems each product does lose "R" as it is compressed.

[glenn kangiser]

Took me a bit to figure out too, JRR but Owens-Corning is saying that while it wont give as high a value as if it had the optimum space, it will still give more value in the smaller space than the regular insulation, if it is compressed a bit into that space.  In other words - you are not getting as much bang for your buck but are still gaining by compressing it per their specs into the smaller space/.  I hope that makes it a little more clear.  

It is true that the value drops from the amount it would be if you had the optimum space.

[PEG688]

Oh ya then like the kit house we did that has radient heat panels in the ceiling we have to bore 3 ea. 5" holes in the walls to put fresh air vents in the house. LOL Anyone ever hear of a window or door ?

Yes I know it's code , if it was my house before the dust settled as the building thug inspector   departed I'd pull out the vent screen and put a hand full of highly compressed insulation into the hole  

 Sorry manhatten  

[glenn kangiser]

Makes a lot of sense to seal a house up so tight the chemicals from the manufactured products try to kill you, so you have to drill holes in the house to let fresh air in.

My wife definitely does not like to be locked in the same house with me without a few air changes. :-/

[Billy Bob]

http://www.wclib.org/pdfs/SimpSpanTbls.pdf

There's the link to simplified span tables, which includes listings for western cedar species.

Hal, you mentioned that you are rehabilitating a "shell".  Will it be a residence, or a shop? Is it going to be an open plan, or is there a chance you will have load bearing interior walls?  Alternately, is there room for a beam and posts?  That would depend on what's under the main floor, of course.  Jonesy's hanging beam looks cool, too, even if you had to "beef up" the studs in the end walls a bit.  It might be tough to get such a critter in place, though, vs. one supporting from below.

Regarding the idea of trusses, I expect Manhattan is correct if you're subject to local building codes, especially if the building is intended for residential use.  Otherwise there are a number of plans/ideas around for roof trusses for agricultural buildings, etc. Here's an example from the Canadian Agricultural Service:

http://www.agf.gov.bc.ca/resmgmt/publist/Leaflets/BldgEng/305-32.pdf

Good luck!

[glenn kangiser]

Here are some joist and truss designs from another thread.  Some good typical information.

Posted By NELSELGNE

     
Re: Homemade trusses

5 pages of Rafter and Truss plans:
http://www.woodworkersworkshop.com/resources/index.php?cat=426

[bartholomew]

JRR, I'd bet that your cost explanation is correct. The raw materials for making glass are cheap but melting them down takes a lot of energy. You probably noticed that 3.5" batts were listed three times with three different R-values, for standard-, medium- and high-density. A document by the Partnership for Advanced Housing Technology has this to say:

Each type of insulation has a density at which its R-value per inch is greatest, but reaching this density is not cost-effective. For 3 1/2-inch-thick fiberglass batts, an R-13 batt contains 40 percent more material, and an R-15 batt 180 percent more material, than an R-11 batt (Fig. 1). Achieving the maximum possible R-value for a 3 1/2-inch fiberglass batt requires packing in the equivalent of eight R-11 batts.

The source is here...(81 page pdf)... http://www.pathnet.org/si.asp?id=482

So a 3.5" high-density batt actually has the fiberglass content of a 10" standard batt.

Here is the Fig. 1 from the document...

[hnash53]

The place that I am attempting to rehabilitate into a "mother-in-law cabin" is rural with very little concerns about code, at least in regards to framing, spans, etc.  The structure has stood for 70 years and is well constructed, albeit with undersized framing.

Here's what I've done.  I installed collar beams (ties) to all the roof rafters.  Then constructed "custom" braces tying together the roof rafters with the ceiling joists.  What I've got now are lots of rigid triangles.  The roof is much more rigid now.  But because the roof rafters are 24" oc and the ceiling joists are 16" oc, not every ceiling joist is tied in.  Every third ceiling joist is tied in as part of my "custom" trusses.  I also installed blocking between each of the ceiling joists.  There won't be anything in the attic except insulation.

I am thinking about using some lighter material for the ceiling finish...when I get to that point.  Lighter, that is, than sheetrock (I can't believe they've not come up with something lighter than sheetrock!".

I'm considering using 3/4" shiplap or t&g for the interior finish on the ceiling.

Any other ideas for lightweight ceiling finish?