Air tightness & securing posi joists floor?

[ruggers]

I was unsure if this post should fall under airtightness or build construction.

Looking for advice from anyone who's used this depth of joist and considered good airtightness.

 

The joist companies have recommended 253mm deep posi joists for my masonry self build with cavity wall which is an odd size for block work courses being 225mm. How do builders get around this? After speaking to people on here regarding air tightness around joists for a build that will have MVHR, I'm looking for advice on the best way to fix the first floor joists to the inner leaf so I can discuss this with the person doing my building regs plans.

 

There seems to be 3 different methods but I'm not sure which is the best and the pro's and cons. Cost vs labour and airtightness.

1.Joists supported sat on the inner leaf. (Optional Tony tray method)

2.Joists hung off masonry hangers that fit over the wall.

3. A timber ledger board fixed to the face of the inner leaf and secured to the wall by mechanical fixings, then the joists attached via timber joist hangers. Would this method be best for airtightness and make levelling floors easier between inner leaf and inner supporting walls?

 

 

[joe90]

My builder and I went with no2, hangers into the wall, parged over before fitting, we had a good AT result.

[ruggers]

Were you on standard joists to match your brick work. No issues with springing or squeeks?

The 253mm high makes a lot of labour for a brickie, notching around 50 joist ends sounds messy and costly. Manthorpe do a nice joist seal but it's wasted if the notched blocks around it aren't airtight.

 

I liked the idea of no.3 for air tightness & being able to level the floors easily for any block work irregularities, but it requires all of the ledger plates to be nice & straight and it makes it hard to pass services/cables between floors close to the outside walls having to work around the ledger board.

[joe90]

2 hours ago, ruggers said:

Were you on standard joists to match your brick work. No issues with springing or squeeks?

No, I joists calculated at 600mm centres but installed at 400mm centres cus I hate bouncy floors! My blockwork was dead accurate so not a problem with levels but even if mid block a lot simpler than building around joists! I like no3 in theory but watching the work done with brick/block hangers and I joists made me realise how easy it was.

[JohnMo]

Isn't it defined in your structural calculations what you should be doing?

[joe90]

27 minutes ago, JohnMo said:

Isn't it defined in your structural calculations what you should be doing?

We had no structural calcs, the builder just did it and the size of the I joist was given by the architect in the drawings.

[JohnMo]

Joy of England, we can't get the warrant until structural engineering is completed.

[joe90]

3 minutes ago, JohnMo said:

Joy of England, we can't get the warrant until structural engineering is completed.

What else needs an SE fir the build, just get this included !!!

[ruggers]

On 02/03/2022 at 08:09, JohnMo said:

Isn't it defined in your structural calculations what you should be doing?

At this stage I'm able to discuss these options and they can take it to the SE if required. Somethings require one others don't. I think the ledger plate would require some SE input. Adding the membrane around the outers of the joists Tony tray method seems a good way to it when using 253mm joists. Pending how the block work is fitted around that size of joist ends it could end up with notched blocks and more potential for air leakage.

 

Using hangers hung from 2.7m down would leave 2447mm ceiling height, then ceiling board and wall board would leave around 30mm between bottom of plasterboard and ground floor. Slight extra cost for the masonry hangers.

 

The joist company said 253 is standard, they'd have to look to see if 225 or 300mm can work for my spans and the cost.

 

The two things I keep reading but getting no solid answer on are:

1.Masonry hangers can cause a lot of potential air leakage issues in future with partial fill cavity. I thought they'd be a great seal?

2.Masonry hangers can induce a lot more floor spring than wall bedded method. 

[joe90]

1 hour ago, ruggers said:

1.Masonry hangers can cause a lot of potential air leakage issues in future with partial fill cavity. I thought they'd be a great seal?

I don’t see why, only a small “hole” in the inner skin filled with the hanger, steel unlike timber won’t shrink with age , the hangers don’t even go right through the blockwork 🤷‍♂️.

 

1 hour ago, ruggers said:

2.Masonry hangers can induce a lot more floor spring than wall bedded method. 

Again I don’t see why, springy floors are caused by flex with the joist.

[Rishard]

I’ve often fitted ledger boards using resin anchors to the masonry. You can set them in to a lazer for perfect flatness. I then flush up joist tops with the ledger then attach the hangers. Even I joists seem to have some variation in height. Good practice to Parge the wall behind the ledger for airtightness. I’ve not done this part before but I’ve not been air testing my projects. New to passive but fitted plenty of joists/floors. You’ll need restraint straps for structural approval. These often used to be notched in joist tops and sailed into the cavity to be built in with the block. This method is obviously no good for thermal bridging : air tightness or your joist height not matching your block courses but the could be mechanically fix to the inner skin. 400mm spacing is better, 22mm flooring, glue and screw it down and it’ll never squeak, or come up again. I’ve done some at 300mm centres on shallow joists to be sure they won’t flex. Get plenty of noggins in and you’ll have a good stiff structural floor. 

[ruggers]

Thanks Joe, The only way I can think that masonry hangers could let in in would be at a later date after the floors been walked on jumped on etc and over time little cracks appear. Adding some soundproof paint around each hanger might help with this if it has some flexibility in it. 

[ruggers]

30 minutes ago, Rishard said:

I’ve often fitted ledger boards using resin anchors to the masonry. You can set them in to a lazer for perfect flatness. I then flush up joist tops with the ledger then attach the hangers. Even I joists seem to have some variation in height. Good practice to Parge the wall behind the ledger for airtightness. I’ve not done this part before but I’ve not been air testing my projects. New to passive but fitted plenty of joists/floors. You’ll need restraint straps for structural approval. These often used to be notched in joist tops and sailed into the cavity to be built in with the block. This method is obviously no good for thermal bridging : air tightness or your joist height not matching your block courses but the could be mechanically fix to the inner skin. 400mm spacing is better, 22mm flooring, glue and screw it down and it’ll never squeak, or come up again. I’ve done some at 300mm centres on shallow joists to be sure they won’t flex. Get plenty of noggins in and you’ll have a good stiff structural floor. 

Some good info. there Rishard. I've used a ledger board on one side before and the other end build into the inner leaf but it was over a decade ago using solid joists and I'd never heard of airtightness back then. What are the restraint straps you refer to, are they part of the hanger or an additional brace? How have you seen services like cables and pipes pass the ledger board between ground floor and first floor?

 

I've noticed they do a green glue 3mm joist tape now that you can apply to each joist top and screw down through the 22mm board into the joist. It's supposed to offer some acoustic seal and prevent squeaks the same as glue. Looks good but not seen any reviews yet.

[Rishard]

Restraint straps are much the same as wall plate straps. 5mm thick steel straps usually with 100mm bend and come in a variety of lengths. These are often part of your floor plan spec. They sit 1/3rd the span of the joist perpendicular to the span. They need solid noggins between the joists to fix the straps to. I cut these in to the joist top and set noggins low so the strap doesn’t foul the flooring. In the past I’ve stood the ledger off the wall at the fixing points with timber to create a void for services to run. I’ve seen it done with compactfoam as an alternative to timber too. 
 

Before installing the joists I lay them all out side by side vertically supported at their end points and shuffle them around to get the tops in size order. Even PAR timber can vary by 6mm. Crown the timbers up at this point too. 

[ruggers]

Didn't realise you can add spacers behind a ledger board because it would affect the structural dynamics and add more stress to the wall anchor bolts. the most I'd need to add behind would be 15mm water pipes and cable access between floors and where the returns drop back down to the fuse board located in the attached garage. A wall chase would be too deep and wide and not help air tightness really.

 

 

[Rishard]

I use m16 threaded bar resin anchored into the blockwork every other joist spacing. A small chase out of the blockwork would work but as you need to Parge the wall you would need to have your pipes in place. I’m sure I read somewhere about people using airtightness paint in their chases before running cables ect.

 

[Rishard]

I’m sure I read on hear that one m10 has a shear of 13kn. Your putting plenty in but you could always check with your SE. 

[ruggers]

Thanks for the photo, appreciated. It certainly helps with that gap behind to run services as that was the only draw back I could think of.

I need to discuss the 3 options with my architect, I've been back & forth between wall bearing using tony tray, masonry hangers or ledger board. The joiner I'll be working with & the Architect are most familiar with wall bearing joists but not so familiar with airtightness to a high level.

 

The three options seem to be similar in cost. TT requires the membrane which I need to check which type is used & further info on installing end & corner detail. Ledger board requires cheaper hangers and resin fixings and masonry hangers are a little bit more expensive but probably the easiest install method. There doesn't seem to be a lot 

[tonyshouse]

Build them into the masonry would be my preference. Even if only the top chord went in with a bearer under or over the bottom chord. Horrid if it starts to work loose. 

[Rishard]

Yes building them in is by far the easiest method. I’ve done both ways much more than masonry hangers. @tonyshouse - is there any issue with the tray separating the masonry both below and above the joist blockwork courses? I guess it’s no different from a dpc in that it sense. 

[joe90]

48 minutes ago, Rishard said:

is there any issue with the tray separating the masonry both below and above the joist

The only thing I have heard is that this is called a “slip plane,” but as you say, no difference to a .DPC.

[tonyshouse]

Gravity is very good at holding things in place and we don’t really have earthquakes 

[ruggers]

On 08/03/2022 at 06:32, tonyshouse said:

Build them into the masonry would be my preference. Even if only the top chord went in with a bearer under or over the bottom chord. Horrid if it starts to work loose. 

I didn't think top hung was possible, I suppose with a bearer under the bottom chord as well, then it has good support, but top hung with bearer over the bottom chord, what benefit would that have? 

 

Are you still suggesting to use the membrane wrapped around the joists?

[tonyshouse]

ask manufacturer/engineer, I think top chord can be built in, if it was mine I would build in both chords 
 

Only needed for cavity walls or air leaky walls , if wall is wet it should not be , use lime pointing or render 

Edited March 10, 2022 by tonyshouse

[ruggers]

16 hours ago, tonyshouse said:

ask manufacturer/engineer, I think top chord can be built in, if it was mine I would build in both chords 

I'll make enquiries about top hun joists. Would the TT still be used for this?

 

My thoughts with both chords sat on the wall using the TT method and 253mm joists, with the wall ties being every 2 courses of block at 450mm, the spacing won't work out. The membrane would have to go up an extra course higher to avoid feeding it back in through & around notched blocks. It is possible as per a previous diagram someone posted on here which you replied to, but it would mean having the wall ties on the same course as the joists are sat on as well as the membrane under them, there would be no mortar to secure the wall ties down if they're slipped under the membrane.

 

Having them a course above or below would pierce the membrane. Unless I misunderstanding it, I can't see how to work it around wall ties.