Cavity Trays and blown in insolation

Katz83

We are thinking of putting cavity trays in our new build. We are thinking of just putting them at the side of the house where the rain will hit most as they are very expensive. We are also pumping a 6 inch cavity with bead. Do both these work well together i was talking to one or two people who said as far as they know there was a few problems with led trays and pumped cavity

Poor Uncle Tom

A cavity tray can be formed from wide DPC. I haven't seen lead used for cavity trays for years.

Imo, a cavity tray should be used over any opening in a cavity and above any change from outer to inner in the same leaf. It is a fundamental mistake to leave them out and amounts to bad building pracitce through incomplete detailing. Cavity tray's are not expensive.

Getting a ful fill cavity with pumped bead is a problem, imo, and basically depends on how thorough the workmen are during installation. Afterwards, how do you prove that every cubic centimetre behind every cavity tray is filled? or every pocket beneath a window cill? Thermal imaging can only show so much. But one thing is certain, if you are going to have problems filling behind a cavity tray, the material that tray is made of is not going to affect the problem.

Big Lar

Now, others here may have different opinions but the theory behind dampness and bonded bead insulations is that the water should never get across the cavity.

When a droplet of water hits bead No1 50% goes left and 50% goes right, Then that water hits bead No2 and it halves again and 25% of the original drop goes left and right, when it hits bead No3 its 12.5% goes left and right and so on.

So you should have no problem with your cavity trays, where people most likely get problems with bonded bead and water coming across cavity trays is when the cavity trays are dirty IE. Blocklayers dropping mortar onto them and not cleaning them. cos that's what draws the dampness across.

Poor Uncle Tom

Big Lar wrote: »

When a droplet of water hits bead No1 50% goes left and 50% goes right, Then that water hits bead No2 and it halves again and 25% of the original drop goes left and right, when it hits bead No3 its 12.5% goes left and right and so on.

Your theory is far too simplistic and is flawed, imo.

If we accept what you say at face value and take your bead No.1 at the top of a two storey gable end, we should see evidence of dampness showing on the inside of the first floor walls.

The fact that other beads are all around each other (however tightly) provides a barrier for the water to just filter in as you suggest.

Big Lar

Poor Uncle Tom wrote: »

Your theory is far too simplistic and is flawed, imo.

But I had a disclaimer

Big Lar wrote: »

Now, others here may have different opinions but......................

Poor Uncle Tom wrote: »

If we accept what you say at face value and take your bead No.1 at the top of a two storey gable end, we should see evidence of dampness showing on the inside of the first floor walls.

Well that depends on how much water is coming through the wall "Quality of workmanship", There are also other factors to consider: Cold air on the outside will draw the moisture towards the outer wall much like you see condensation on the inside of a window.

Jonny Harris

Hi Have you heard of <SNIP> It is really good but needs to be built in as the work progresses

Poor Uncle Tom

Jonny Harris wrote: »

Hi Have you heard of <SNIP> It is really good but needs to be built in as the work progresses

The thread is about cavity trays and blown insulation not any other type of insulation.

Please stay on-topic.

Jonny Harris

Sorry Poor Uncle Tom. Didn't realise . I was just trying to enlighten others.

mickdw

Be sure to use them where ever required. You will not seal these areas later without the tray built in.
If you have stepped trays going into gable over a conservatory roof for example, get preformed plastic cavity trays. These can also be bought with a triangle of lead flashing attached for a perfect seal.
Wide DPC is obviously ideal if the tray is just a horizontal run.

The stepped trays with lead attached are approx €20 each (1 per block course), expensive but a must, particularily in exposed areas.
My personal opinion on stepped trays & pumped bead insulation is that if the trays can be kept 100% clear of the insulation, then do that.

soldsold

I left wide dpc hanging down in my build so it wouldn't catch mortar snots from the blocklayers, dpc started one block above lintols/ rsj. Then I marked holes for the pumped insulation guys when they went to blow the beads after the window reveal plasterboard was fitted. Holes were marked half way up the block, about 600mm apart.

It took a fair bit of insulation to fill each hole and I'd hate to think of the cold bridges if this wasn't done. You could also fit shaped pieces of solid PIR/ EPS boards but it would be a load of work and really hard to get a good fit

sydthebeat

soldsold wrote: »

...... but it would be a load of work and really hard to get a good fit

Its not really....

When partial fill cavity was our standard spec we always specified full fil insulation at every ope... jams heads etc. It was simply a case of wedging 2 pieces of 50 or 60 x 100mm board in lengths to suit the opening.
All too often ive seen a jamb being returned in a broken block, which is completely counter productive. only once in 12 years have i seen proper return blocks used....

On the cavity trays, with the above spec, you can easily wedge solid insulation into, say, heads to ensure a proper fill prior to pumping.

Poor Uncle Tom

sydthebeat wrote: »

..you can easily wedge solid insulation into, say, heads to ensure a proper fill prior to pumping.

Exactly, that is the proper way to do it.

Unfortunately, any building sites I see where the insulation is going to be pumped there will not be any other insulation to be seen. "The insulation is being pumped, I didn't price for insulation boss" is all you'll ever get.

Katz83

sydthebeat wrote: »

Its not really....

When partial fill cavity was our standard spec we always specified full fil insulation at every ope... jams heads etc. It was simply a case of wedging 2 pieces of 50 or 60 x 100mm board in lengths to suit the opening.
All too often ive seen a jamb being returned in a broken block, which is completely counter productive. only once in 12 years have i seen proper return blocks used....

On the cavity trays, with the above spec, you can easily wedge solid insulation into, say, heads to ensure a proper fill prior to pumping.

Could you explain this to me a bit more. I dont quiet understand. we going with pumped cavity and led trays and your saying this is ok. We just fill around the lead trays with board insolation is it. Sorry if this is a stupid question. Also we are only going to put led trays at the gable that is exposed as we cannot aford to do both as it is a 1000 euro to do both so we said at least if we do the one that most the weather will hit that would be ok. what do you think

sydthebeat

lead trays are usually used where a roof meets a wall, ie where stepped dpcs are required, as they usually require apron pieces. Otherwise plastic dpcs will suffice.

where and why are you using lead? is it say where a single storey meets a two storey?

the locations im referring to above is at ope heads where a dpc is stepped from the inner down to usually above the outer leaf head.

Katz83

sydthebeat wrote: »

lead trays are usually used where a roof meets a wall, ie where stepped dpcs are required, as they usually require apron pieces. Otherwise plastic dpcs will suffice.

where and why are you using lead? is it say where a single storey meets a two storey?

the locations im referring to above is at ope heads where a dpc is stepped from the inner down to usually above the outer leaf head.

Our blocklyaer suggested cavity trays dont know much about this stuff. it is a bungalow and have samll roof off it which contains a very small conservatory.

sydthebeat

Katz83 wrote: »

Our blocklyaer suggested cavity trays dont know much about this stuff. it is a bungalow and have samll roof off it which contains a very small conservatory.

makes sense....

assuming the ceiling heights are the same, i dont see where there is anything off standard with this detail. the lead flashings will occur over the eaves level so pumping will be as standard....

Katz83

sydthebeat wrote: »

makes sense....

assuming the ceiling heights are the same, i dont see where there is anything off standard with this detail. the lead flashings will occur over the eaves level so pumping will be as standard....

thats great thanks for clarifying things for me

mickdw

Katz83 wrote: »

Our blocklyaer suggested cavity trays dont know much about this stuff. it is a bungalow and have samll roof off it which contains a very small conservatory.

If that small roof is running into a gable wall (i.e. not tying straight into another roof, you will need the stepped trays as per my previous post. Preformed trays with lead attached are the job.

very quick sketch done on mspaint. First thing to understand is that the outer block in allowed to be wet. The trays are to cut off this downward movement of water/moisture through the blocks or down the cavity on the surface of the wet outer block.


I much prefer to have these trays free of any insulation if your design can achieve the insulation requirements some other way