Wrapping hollowcore slabs

68508224

We are installing hollowcore slabs soon on our first floor in a 2 storey cavity wall built newbuild.

I have seen some posts on here by sas explaining it from a few years ago. Just want to check that Solitex roofing felt is still a good option for this with mortar placed on top to protect from ripping? Any advice greatly appreciated.

Metric Tensor

You need to use an air tightness membrane - most modern roofing felts are designed to be "breathable" so not ideal.

68508224

Metric Tensor wrote: »

You need to use an air tightness membrane - most modern roofing felts are designed to be "breathable" so not ideal.

Ok thanks Metric Tensor. Any advice on the corners? Cut and overlap at 90 degrees taping up the joints or try to bring around the corner without cutting?

Metric Tensor

I think either way would be fine provided you tape the joints well where they do occur. Bear in mind that whatever you leave after you has to be folded down the wall and plastered over so it needs to be neat.

Lumen

Solitex is windproof not airtight. Totally different.

You want something more like Intello, although the breathability seems a bit overkill for masonry.

Obvs plaster won't adhere to membrane so mesh will be needed.

rampantbunny

I used solitex. I didn't add a mortar bed (even though I was present on site at time of hollowcore install), as I thought it would be a pain in the ass. Solitex is a tough material, so I just put a layer of DPC on top of it (glued with mastic) to add some protection.
My AT test results were not passive but only marginally above.

I'm not suggesting you should ignore previous advice in this thread. I've definitely enjoyed/benifted from the advice of both posters in the past. All I'm saying is that solitex could well still be good enough for this task. It's a pig to fold at corners (AT membrane def. be easier), but it's a lot tougher IMO. Unless you are on the ball, block-layers will walk all over the membrane which lies on top of your slab once they start back at the upper-floor blockwork. Not sure AT membrane would survive this.

Also, and you probably know this; if wrapping the slabs, make sure you continue your AT efforts throughout the build or this good work will be less effective. Whatever material you use, best of luck with the project.

Yellow_Fern

Excuse my ignorance, but is it worth the OPs while to seal the ends of the cores with expanding foam? Incase you damage in the wrapping etc although I am aware the wrapping is not really for sealing the cores as such.

Lumen

I'm not claiming any special expertise, but Solitex doesn't even have a value for airtightness on the data sheet.

As I understand it, the problem with permeability of cast concrete is that micro fractures develop over time, so whilst it might pass the initial AT test, that won't last the life of the structure.

It's hard enough getting concrete waterproof, let alone air tight.

Yellow_Fern

Lumen wrote: »

I'm not claiming any special expertise, but Solitex doesn't even have a value for airtightness on the data sheet.

As I understand it, the problem with permeability of cast concrete is that micro fractures develop over time, so whilst it might pass the initial AT test, that won't last the life of the structure.

It's hard enough getting concrete waterproof, let alone air tight.

I didnt know this at all. I presume these cracks would be very small though?

68508224

Thanks for the above everyone. Learning a huge amount. Next on my list to tackle is the wall-floor (subfloor) junction. We have 120mm insulation and 55mm liquid screed to go in at a later stage so from what I've read I should seal this junction before insulation and screed. Any recommendations on suitable tape or width of tape needed for this? I'm guessing 250mm to have 50mm stuck on floor and 200mm going up the wall but am completely new to this so would appreciate any advice. ��

Lumen

I'm in a bit of a rush so these experiments aren't perfect. I can do them more thoroughly if anyone is interested.

I cut holes in two identical bicycle inner tubes and patched them with Intello and Solitex Fronta membranes, using paper air tight tape.

Then pumped with a bicycle pump and submerged underwater to check for leaks.

You can see that the Intello leaks at the edge where I've been sloppy with taping, whereas the Solitex leaks through the membrane.

Setup pics: https://imgur.com/a/fqEx3W4

Videos:

https://www.youtube.com/watch?v=zWd96dVL_xw

https://www.youtube.com/watch?v=dar3qvnW9kQ

Lumen

Yellow_Fern wrote: »

I didnt know this at all. I presume these cracks would be very small though?

Yes, but over a large area.

mfceiling

Read the title and came here to tell the OP to check out SAS's posts and pics on this forum but you've done that already OP!!

Saw the detail he went into up close and i have to say I've seen very few, if any houses since that the detail was done so well.

rampantbunny

mfceiling wrote: »

Read the title and came here to tell the OP to check out SAS's posts and pics on this forum but you've done that already OP!!

Saw the detail he went into up close and i have to say I've seen very few, if any houses since that the detail was done so well.

He's not going to be happy to hear that solitex wasn't the optimum choice. I thought I went OCD on my own build, with the solitex, the extra-special airtight plaster, every tape and flexible glue proclima sold, even taping over staples in my AT membrane..only to now learn those ba*tard microcracks were waiting to undo my good work.

Lumen

Look, I don't want to go upsetting anyone, I'm just a curious person.

I just don't understand the logic of trying to achieve air tightness of concrete with a windproofing product.

Like, why not radon barrier or some other impermeable material? Where does the recommendation come from?

rampantbunny

68508224 wrote: »

Thanks for the above everyone. Learning a huge amount. Next on my list to tackle is the wall-floor (subfloor) junction. We have 120mm insulation and 55mm liquid screed to go in at a later stage so from what I've read I should seal this junction before insulation and screed. Any recommendations on suitable tape or width of tape needed for this? I'm guessing 250mm to have 50mm stuck on floor and 200mm going up the wall but am completely new to this so would appreciate any advice. ��

The advice I got from engineer specialising in AT builds was to just apply plaster at the slab and wall junction before installation of insulation. You have to bevel the insulation to fit against this.
Later, I taped from poured screed to plastered wall. Skirting boards covered the tape in most places. TBH, the tape from screed to wall would prob have been enough provided you ensure penetrations in your screed are looked after.
I also plastered the first course or two of blockwork when I was adding mortar to the slab/wall junction..otherwise it would have been exposed blockwork abutting my insulation boards i.e.. air infiltration.

250mm tape sounds good (but expensive?) to cover the slab junction and also the sides of the plasterboard but I'd prob still tape junction at screed /wall myself.

You can go OTT on this OP. I'm going to get a headache soon thinking back over the obsessive details.

rampantbunny

Lumen wrote: »

Look, I don't want to go upsetting anyone, I'm just a curious person.

I just don't understand the logic of trying to achieve air tightness of concrete with a windproofing product.

Like, why not radon barrier or some other impermeable material? Where does the recommendation come from?

I for one am not getting upset, but ditto here, my posts aren't meant to offend. I'm using my phone and no idea how to do smileys in posts .. doesn't help.

It's completely logical that something airtight be used to achieve airtightness. Maybe solitex was thought of a few years back as better than most materials, not as good as specialist products. Can't recall prices but I think solitex is cheaper, def tougher and AT membrane torn is potentially much worse than the solitex. Is the potential for air loss in the cavity pretty neglible considering surface area here vs rest of walls, floors, roof - maybe.

Back then (and were not talking years and years ago) plenty of builders/experts were active on the forum so there was opportunity then for knowledgeable people to rail against the use of solitex but they didn't. That's why I would have used it in my build.. good enough for smarter people than me so off I went.
I deliberately waited 2+ years after build finished and house settling somewhat before I got AT test done. I didn't believe that a test conducted early would be a true reflection of AT performance. A good result vindicated my hard work only let down by a few eager trades that damaged some of the fabric.
Maybe solitex use was reason I fell short of getting passive result.. but I do know of lads that achieved passive while using it. IMO, good enough back then could still be good enough now.

68508224

rampantbunny wrote: »

The advice I got from engineer specialising in AT builds was to just apply plaster at the slab and wall junction before installation of insulation. You have to bevel the insulation to fit against this.
Later, I taped from poured screed to plastered wall. Skirting boards covered the tape in most places. TBH, the tape from screed to wall would prob have been enough provided you ensure penetrations in your screed are looked after.
I also plastered the first course or two of blockwork when I was adding mortar to the slab/wall junction..otherwise it would have been exposed blockwork abutting my insulation boards i.e.. air infiltration.

250mm tape sounds good (but expensive?) to cover the slab junction and also the sides of the plasterboard but I'd prob still tape junction at screed /wall myself.

You can go OTT on this OP. I'm going to get a headache soon thinking back over the obsessive details.

Thanks rampantbunny. I have just checked the wall-floor junction and noticed the radon barrier coming up from under the concrete slab and passing out over the next course of blocks of internal leaf. I've attached a picture as I'm probably not explaining it well!

Can you or anyone advise on this? Should I tape/paint the top of the radon barrier meeting the block above it and the bottom also where it is coming up? Or the entire barrier from the slab edge to above the barrier? Thanks for your help and everybody else too 👍

Metric Tensor

Yellow_Fern wrote: »

Excuse my ignorance, but is it worth the OPs while to seal the ends of the cores with expanding foam? Incase you damage in the wrapping etc although I am aware the wrapping is not really for sealing the cores as such.

You can get the hollowcore with solid ends from many manufacturers if you specify it when ordering. I think it's a better job but the downside is you need to drill into the cores when the slab is placed in order to make sure any water inside drains out.


Expanding foam is not airtight so would be fairly useless for pumping into the cores. You could fill them with mortar or something airtight if you wanted.

Metric Tensor

rampantbunny wrote: »

Maybe solitex use was reason I feel short of getting passive result.. but I do know of lads that achieved passive while using it. IMO, good enough back then could still be good enough now.

Whilst I don't agree that it was the right thing to use at any point and SAS's posts were before my time I don't think it realistically made much of a difference to your results. My guess would be that although it's not the "right" product for the job the difference over the minor area concerned at the pressures we're talking about would be pretty small and possibly not even measurable.



(P.S. when you say "passive" what actual air-tightness value do you mean? I know the "passivehaus" brand of certification requires < 0.6 m3/h/m2 but I'm not sure if that's what you are referring to.)



P.S. I know air-tightness contractors who get very values well below 1 without wrapping the slabs at all - so there are more than one ways to skin this hollowcore cat. I still prefer to see them wrapped though.

rampantbunny

Metric Tensor wrote: »

Whilst I don't agree that it was the right thing to use at any point and SAS's posts were before my time I don't think it realistically made much of a difference to your results. My guess would be that although it's not the "right" product for the job the difference over the minor area concerned at the pressures we're talking about would be pretty small and possibly not even measurable.

(P.S. when you say "passive" what actual air-tightness value do you mean? I know the "passivehaus" brand of certification requires < 0.6 m3/h/m2 but I'm not sure if that's what you are referring to.)

P.S. I know air-tightness contractors who get very values well below 1 without wrapping the slabs at all - so there are more than one ways to skin this hollowcore cat. I still prefer to see them wrapped though.

From memory, n50 result = 0.67

I used wide-slab instead of hollowcore. As you prob know, wideslab is a solid concrete slab, wider than the hollowcore alternative. Less joints in the cavity, so yes, wrapping vs no wrapping - might not have made much of a difference in my build.
I knew of two definite penetrations to the AT membrane in roof that I was told were mended by trade. Couldn't verify at time as whole area was plasterboarded over. Leakage was detected in the vicinity of these areas during test. I'm putting the reason for falling short of n50 < 0.6 down to those leaks - and that my work was flawless - I could be wrong though..I have form.

Not to derail the OPs thread; what's your opinion on testing house on Day 1 vs testing house years later. Would you expect results to change over time? I'd be surprised if the results were the same. Every house grunts and groans, tapes at junctions are elastic to a certain extent but cracks appear in plaster, timber can twist and bend, rubber seals get stiff.

rampantbunny

Metric Tensor wrote: »

You can get the hollowcore with solid ends from many manufacturers if you specify it when ordering. I think it's a better job but the downside is you need to drill into the cores when the slab is placed in order to make sure any water inside drains out.


Expanding foam is not airtight so would be fairly useless for pumping into the cores. You could fill them with mortar or something airtight if you wanted.

I used a foam from one manufacturer that was supposedly more airtight than standard. I've no affiliation with the company so I'll risk naming the product here, Geocell Airblock (but I'm sure there are others). Probably easier than trying to apply mortar unless the hollow-core slabs are onsite for a few days before installation.

Metric Tensor

rampantbunny wrote: »

From memory, n50 result = 0.67

That's very good. Congrats.

rampantbunny wrote: »

Not to derail the OPs thread; what's your opinion on testing house on Day 1 vs testing house years later. Would you expect results to change over time? I'd be surprised if the results were the same. Every house grunts and groans, tapes at junctions are elastic to a certain extent but cracks appear in plaster, timber can twist and bend, rubber seals get stiff.

I would 110% expect the results to get worse as the years go by. Building materials change shape and size with different temperatures and different humidities and this expansion and contraction is bound to have "some" effect on air tightness. Also - as the years pass the gluing compounds on the tapes and mastics will reduce in effectiveness - regardless of what the manufacturers tell you. Seals on windows and doors will get squeezed, nicked, broken, etc. Add that all together and I'd say the effect would definitely be noticeable over a timespan of years. Of course I have no proof of this because I have never seen a house tested on Day 1 and then again on say Day 2000.


How long after you moved in did you test yours for air-tightness?

mfceiling

Metric Tensor wrote: »

You can get the hollowcore with solid ends from many manufacturers if you specify it when ordering. I think it's a better job but the downside is you need to drill into the cores when the slab is placed in order to make sure any water inside drains out.

Or when the ceiling is being installed the guy who is drilling into it for his hangers gets drowned with every hole he drills!!!!

sas

Happened across this on the landing page so I wanted to explain why solitex.

The same company supplies both solitex and intello here. It was their guidance to use solitex at that joint. The explanation I was given was that it's down to the durability of both materials. Given typical site conditions, solitex just offered a better chance of remaining intact.
I even suggested using both (intello with solitex over it to protect it) but they said it wasn't necessary and that I could achieve passiv standard using solitex alone.

Our airtightness result was n50 0.32.

It's now 8 years since we moved in. The house continues to perform as hoped/planned.

Hope you're keeping well mfceiling.

sydthebeat

There a blast from the past!.

rampantbunny

Metric Tensor wrote: »

That's very good. Congrats.
:
How long after you moved in did you test yours for air-tightness?

Thanks. Would have been nice to get passive but when you're not onsite to keep an eye on things it's difficult to ensure everyone is being as careful as you. My approach was to do everything to the best of my ability to hopefully off-set others carelessness. I accepted there would probably be tears and moved on.

Test was conducted about 2.5yrs after house was finished. Not long in terms of house life-span but long enough for a few hairline cracks to form in the plaster.

Sorry for going off-topic a bit OP.

Calahonda52

sas wrote: »

Our airtightness result was n50 0.32.

I can still see the white smoke wafting out into the still, crisp morning air, due to the back of the line of sockets in that wall not being A/T.

8 years...my my