Am I right in thinking that with a partial fill cavity wall it doesn't matter what the external leaf is made up off?

[Gimp]

So thinking here that as the cavity is always vented with air (in partial fill) then it would be pointless being concerned over having low u-value materials for the outer leaf since the air in the cavity will remove any insulation properties of the outer leaf. Hence, I might as well have a 100mm medium or dense block(external), air gap, insulation then 100mm aerated blocks(internal). Rather than 100mm aerated block(external), air gap, insulation and 100mm aerated blocks (internal). Hence there would be no real difference in U-value. Looking at the Kingspan U-value calculator they seem to back up this assumption in that they only bother asking you for the inner leaf not the outer, celotex only see a 0.01 difference but they don't even ask whether partial or full fill. So just seeing what forum members thoughts are on this?

[Construction Channel]

thats pretty much my grasp on it, once you get to a ventilated area there is no point trying to keep it warm.

 

[ADLIan]

The cavity in standard masonry cavity construction is not ventilated i.e. it is not provided with gaps/vents to deliberately vent the cavity. Benefit of low emisivity facings on cavity resistance and aircrete blocks externally can therefore be assumed. Correct in that any elements external to a ventiliated cavity are ignored (see BS EN ISO 6946 on U-value calcs). Insulation manufacturers tend to assume brick/dense block outer so contact them for alternative construction U-values.

 

Ian

[Gimp]

I tended to thought you had to vent the cavity to stop stale air, condensation issues? Obviously weep holes may let air through (as you state not deliberately) and the cavity is not likely air tight.

[ADLIan]

Nothing to do with stale air or condensation - residual cavity is more linked to preventing driving rain penetration but also gives useful thermal performance, especially if insulation has a bright, foil face. Weep holes not classed as providing vent in this instance.

 

Ian

[joe90]

My last house had a vented cavity, there were vents from the outside into the underfloor void and also vents into the cavity alone at ground level. The top of the cavity was open to the cold roof space.

[Brickie]

Unless you live in an exposed location & are planning on doing the insulation yourself,I'd give serious consideration to using full fill insulation. Trust me-the finished job you'll get if the builder fits the insulation will give you a lower insulation performance than on paper. The care & attention to detail needed when fitting is beyond 99.9% of bricklayers,honestly. With fulfl fill,you're more likely to get what expect. 

[Gimp]

Thanks Brickie, I kind of agree, a full fill would be less hassle, unfortunately though the location is in Wales, and in the area deemed under building regs to be at risk of driving rain, hence their requirement for a cavity gap, i.e partial fill. I will be undertaking the work myself so can take the time & care (hopefully) to fit the insulation tight.

 

joe90, yeah I'm pretty sure the present house I am in (don't own it) is similar with a vented cavity. ADLIan I'm pretty sure I've heard the case for not venting cavity as well, some say its just left as an inert air space, others to avoid supplying a fire with oxygen & creating a greater funnel effect, cavity barriers would be used of course in either case but adds to the hassle of part fill cavity. Given that it will be in an area prone to driving rain then perhaps it makes sense to have it ventilated to dry out any rain that gets through more quickly perhaps.

[Declan52]

Can fully fill the cavity and use a water proofer in the scratch coat and the top coat of your render on the outside. In reality even with the full fill boards there will still be a 10mm gap between the boards and the outside skin. Plus with the way the tongues and grooves work the rain would never get through either. 

[ADLIan]

Check product installation instructions and BBA certificates - due to this being a critical application ALL of the mainstream products, full and partial fill, will be certified. Walls will dry OK without a vented cavity.

 

Due to fire regs the cavity must be sealed around openings and at the top of the cavity.

 

Joe90 - was this a standard masonry cavity wall? type of insulation? I have never heard of cavity wall being vented in this instance. Sub floor vent should be ducted thru cavity. Should fire get into the cavity it is hidden and can spread, particularly dangerous is spread of smoke/fire into the roof space. Again check BBA cert for your cavity insulation.

 

Full fill systems, in conjunction with render finish, can be used in severe exposure aeas if BBA certified.

 

 

[ProDave]

In Scotland, the cavity has to also be closed between first and second floor levels and therefore two more rows of vents installed above and below the fire stop.

 

I decided early in the design that I didn't want to pay for a lot of blockwork that added almost nothing to the thermal performance of the house. That is why I went for the wood fibre cladding and then render, so that every bit of the wall make up adds to the insulation.  It also has the benefit of a clean render finish with no "warts" (weep vent slot things) to spoil it.
 

[Gimp]

3 hours ago, ProDave said:

In Scotland, the cavity has to also be closed between first and second floor levels and therefore two more rows of vents installed above and below the fire stop.

 

I decided early in the design that I didn't want to pay for a lot of blockwork that added almost nothing to the thermal performance of the house. That is why I went for the wood fibre cladding and then render, so that every bit of the wall make up adds to the insulation.  It also has the benefit of a clean render finish with no "warts" (weep vent slot things) to spoil it.
 

If your venting the cavity as you seem to say you are, doesn't this render your external wood fibre cladding useless? since the air behind from the vent would mean no air tightness.

[ProDave]

Just now, Gimp said:

If your venting the cavity as you seem to say you are, doesn't this render your external wood fibre cladding useless? since the air behind from the vent would mean no air tightness.

I should have made it clear mine is timber frame. The wood fibre cladding fixes direct to the timber frame with no cavity and the render goes onto the wood fibre board.
 

[ADLIan]

ProDave - is that an accepted detail? TRADA always show a ventilated cavity behind the render system - I believe this is to avoid moisture transmission through the render (& insulation) back into the timber frame.

[ProDave]

Yes it is an approved system. It was designed and detailed on the construction drawings by a local Eco hose builder who use this system a lot and approved by the local Building control before we started.   The render system is by Baumit.com.

 

The wall make up is shown here (along with many other variations) http://www.ecomerchant.co.uk/walls/lime-plasters-renders-mortars-ancillaries/insulating-systems-ewi-iwi/steico-protect-system-wood-fibre-board-lime-render.html  Click on the Technical info tab, and download the file. It shows options of timber frame, CLT and masonry walls.  The only difference is we used Pavatex wood fibre board rather than Steico, and I didn't buy my stuff from Ecomerchant, instead I sourced it via the local firm who gave me a better price.
 

[Stones]

1 hour ago, ADLIan said:

ProDave - is that an accepted detail? TRADA always show a ventilated cavity behind the render system - I believe this is to avoid moisture transmission through the render (& insulation) back into the timber frame.

 

I think NHBC also insist on a minimum 20mm if using EWI.  They of course don't have any interest in thermal efficiency, merely mitigating their risk as a warranty provider. 

[ProDave]

2 minutes ago, Stones said:

 

I think NHBC also insist on a minimum 20mm if using EWI.  They of course don't have any interest in thermal efficiency, merely mitigating their risk as a warranty provider.

And I'll bet you don't have a cavity anywhere.

 

Good job I decided NHBC was too much for too little.
 

[Gimp]

3 hours ago, ProDave said:

I should have made it clear mine is timber frame. The wood fibre cladding fixes direct to the timber frame with no cavity and the render goes onto the wood fibre board.
 

So yours is ventilated as timber frame, correct? As you need to keep dry, remove moisture, condensation, etc. Does the act of ventilating not also reduce the thermal efficiency though?

 

So what is being said I gather from ADLIan is that if timber frame ventilate, but if masonry unventilated as less susceptible.

[ProDave]

Normal wisdom is a conventional timber frame house with a brick or blockwork outer skin, needs to have a ventilated cavity. Indeed filling the cavity between a timber frame and the brick outer skin is frowned upon.

 

But the wood fibre cladding option does not need a ventilated cavity.

 

Part of the design process is a vapour analysis (someone will be along to tell you the correct name for that). What that does is to ensure there is no condensation risk within the frame. that is achieved by making sure each layer as you move outwards is more vapor permeable than the inner layers.
 

[Stones]

3 hours ago, ProDave said:

And I'll bet you don't have a cavity anywhere.

 

Good job I decided NHBC was too much for too little.
 

 

The only cavity we have is a between timber cladding and outer face of our ICF, but I assume the same will apply with wood fibre as well? Render system applied direct to EWI, no cavity.

 

 

[Crofter]

12 hours ago, ProDave said:

Normal wisdom is a conventional timber frame house with a brick or blockwork outer skin, needs to have a ventilated cavity. Indeed filling the cavity between a timber frame and the brick outer skin is frowned upon.

 

But the wood fibre cladding option does not need a ventilated cavity.

 

Part of the design process is a vapour analysis (someone will be along to tell you the correct name for that). What that does is to ensure there is no condensation risk within the frame. that is achieved by making sure each layer as you move outwards is more vapor permeable than the inner layers.
 

 

And is that why your sheathing went on the inside of the frame?

[ProDave]

13 minutes ago, Crofter said:

 

And is that why your sheathing went on the inside of the frame?

Yes.  I lost count of the number of builders that told me I had put the frame up "inside out".  The OSB racking layer is on the inside and only the wood fibre cladding on the outside. It has it's own challenges of course, the big one being the insulation was fitted from the outside just before cladding which meant I only fitted the insulation as I went and if I could not get a section covered in the day I had to fix some plastic sheeting to keep the rain out if it were to rain before I could continue.

 

Another consequence of not having the blockwork shell, is on some walls the structural engineer specified two staggered layers of OSB racking panels on the inside.
 

[jack]

21 hours ago, Brickie said:

Trust me-the finished job you'll get if the builder fits the insulation will give you a lower insulation performance than on paper. The care & attention to detail needed when fitting is beyond 99.9% of bricklayers,honestly. With fulfl fill,you're more likely to get what expect. 

 

Even then, perfection isn't assured.  From a friend's extension going up at the moment:

 

 

He mentioned that their current house (loft bedroom) is quite hot at the moment so they're getting air conditioning upstairs.  I asked about increasing the insulation depth instead or as well, but he said they don't have the ridge height.

 

 

[ProDave]

That picture is quite shocking Jack. It shows the builders regard fitting insulation as a "box ticking" exercise, rather than actually giving any thought to making it well insulated.
 

[ADLIan]

Sorry been awol on trans atlantic flight!

 

Nice pic Jack. I've seen similar with polyurethane and gaps of several cm between the slabs!

 

Back to the main topic. Generally no need to vent the cavity on standard masonry cavity or TF with brick/block cladding (though weep holes will allow some vent this is not the primary aim of these gizmos). Extra insulation can be applied external to the TF studs but the render should not be applied directly to this. With render finish on timber frame there should be a vented and drained cavity behind the render system (and carrier board or mesh) but in front of the extra insulation. The performance of the insulation will not be affected in this instance.

 

Sorry ProDave but I do not see any meaningfull approval, i.e. BBA or similar, on the link and your detail is not supported by TRADA (see Timber Frame Construction, 5th Ed), NHBC Standards or Building Regs. The fact that BC accepted it may simply mean a lack of undertsanding here. It is not a condensation issue but an issue with rain/moisture penetrating the render system and then having ready access to timber and wood based materials - the drained/vented cavity avoids this problem and is shown by the mainstream insulation manufacturers in TF systems.

 

Ian