Timber frame pumped cavity

B

breathnach1

Registered User
Messages
93
Is it possible to pump cavity of timberframe house (external brickwork to face)

If so is it a good idea or would it cause decay to the timber due to lack of ventilation
 
(Pumping the cavity)Bead insulation in between studs -cavity been the external wall in and inner plasterboard!

Have since been informed that its a bad idea in timber frame houses as the timber need air to breath, pumping the cavity may cause decay to the timber.
 
I think your advice is probably correct.

The whole thing rests on the rate of water vapour passing through from the warm side of the insulation being massively reduced by the vapour check [no nailing pictures through the plasterboard on the walls!], while the cold side of the insulation has no restrictions and is free to "breath" and the cavity penetrations are detailed to allow water to fal laway to the outer leaf.

I don't know how the full fill cavity insulation would affect this, but you could tease it out a little.

As long as the cavity fill allows moisture to pass through it and isn't affected by it, that part might be okay.

I've heard of full cavity insulation used in masonry houses apparently without ill effect.

What seems to happen in walls is that a "dew point" is reached where water vapour passing through the structure condenses into moisture at a particular temperature and relative humidity.

This is designed to occur on the face of the breather paper on the cold side of the insulation, where vapour can pass through, but condensed moisture in the form of water droplets cannot come back in and is vented away, drips down and is thrown out and away.

If the cavity was filled the dew point might occur somewhere else i.e. on the outer face of the cavity insulation fill or in any cavities within the insulation fill if it was unevenly filled.

In the absence of ventilation it will tend to follow gravity and find its own way out. This is what happens in a normal cavity on very wet days when the outer leaf is saturated and the air itself is carrying a lot of moisture.

Droplets in theory dribble down the inside face of the outer leaf until then find an open mortar joint. weephole, Cavity Tray, Damp Proof Membrane or a wall tie. The others should be designed to throw the water out towards the outer leaf, but the wall tie is horizontal and the drip will try to cross it.

That's why there is a drip detail in the middel of the wall tie, usually intended to be just before the cavity insulation, which was usually placed on the outer face of the inner leaf.

In the present case, you have a continuous breather paper facing into the cavity which should stop water getting back in.

With the breathing paper still in place, it would have difficulty finding its way back to the timber and as long as the outer brick or masonry "skin" can breath it should eventually migrate out.

But the problems may happen at opes and penetrations in a timber frame house cavity, where, unless the breather paper is sealed to them, water migrating downward may build up enough pressure to push past.

Hold a spoon against a running tap stream to see what water can do.

Believe me, we live in an older house and have just got a gas bill for over €300 for the past three months so I am interested in any and all forms of insulation.

I wonder has anyone used the cavity fill succesfully with timber frame cavity contruction?
 
breathnach1 said:
(Pumping the cavity)Bead insulation in between studs -cavity been the external wall in and inner plasterboard!

Have since been informed that its a bad idea in timber frame houses as the timber need air to breath, pumping the cavity may cause decay to the timber.
Click to expand...

timber frame lifespan depends on teh ability of teh frame to transmit moisture from internal to external.. .if you pump the cavity you remove this ability and severely compromise the structural integrity of the frame...

doing this may negate the guarantee / certification o fteh frame..
 
You seem to know your stuff sydthebeat, and I was genuinely searching for an answer on that last line of my post.

I think as long as the fill doesn't accrete and seal the cavity or form a vapour check the timber might be all right. For a start, the outside of it will be wormer than it would otherwise have been "looking at" the building paper facing onto the cavity.

Just dunno...
 
external insulation is your best option and there is a grant for it also
 
boyle.mick said:
external insulation is your best option and there is a grant for it also
Click to expand...

not for a timber framed house....

the cavity must be vented....
having a vented cavity behind an insulating layer totally negates the insulating layer.

there is a problem here of allowing condensation transmit to the external away from the frame.. whilst at the same time stopping weather driven moisture into the construction from the external. putting a vapour proof insulation stops the weather driven moisture, but also stops the condensation escaping... installing a non-vapour proof insulation can have the unwanted effect of allowing weather driven moisture from the external to the frame...

in a timber frame build the rendered block is simply a weather shield, it forms no other purpose.... the timber frame is the active element.
 
You cannot put insulation in the cavity of a TF house. It is a requirement that it have a CLEAR cavity.
 
Let's just consider this with all the "ifs" and "buts".

Vapour checks arrest the passage of water vapour through a material, e.g. insulation.

Cavity construction allows moisture laden air to be vented more quickly from the cold side of the insulation + building paper so that the vapour-checked air passing through the insulation cannot build it up in the construction.

Its a bit like a bath with a large outlet and a dripping tap - it'll never fill.

A vapour proof insulation on the cold side of a timber frame could trap moisture in the frame - accepted.

That's like putting in a reasonably well-fitting plug in the bath - it'll tend to fill, given time.

However it is difficult to conceive of a granular fill insulation that can compact sufficiently to be fully water proof, never mind vapour proof.

Let us assume that the cavity fill insulation isn't 100% vapour-proof.

As long as the rate it passes vapour exceeds that at which the vapour-checked insulation admits it, that would be like putting a square plug in the round outlet.

The taking away of moisture might be at a reduced rate, but unless that rate falls below the rate of admission through the vapour-check, no build up can occur.

My concern is that the component "bits" of the cavity fill might provide surfaces for the moisture laden air to condense on, and that these droplets could flow down through any cavities and could find their way back into the dwelling unless the ope and frame penetrations were adequately detailed.

I'm sort of being the devils advocate here and all this is theoretical posturing on my part.

It would be useful to know if anyone had tried it and whether the results supported my view or sydthebeat's

:)
 
onq said:
As long as the rate it passes vapour exceeds that at which the vapour-checked insulation admits it...
Click to expand...

thats the crunch point right there....

the vented cavity allows the frame to transmit moisture at a certain rate...
if you fill the cavity you are vastly reducing this rate, therefore the fill material would need to allow moisture transmittance to a rate at least equivalent of a vented cavity.... thats a tall order.

the only material that i think could be suitable is sheeps wool, which claims not to loose insulative properties even when a certain rate of saturation is met. Although you will have the add on problem of sagging due to saturation.

Im not sure you will get many engineers / architects willing to sign off this construction though...
 
sydthebeat said:
"... the fill material would need to allow moisture transmittance to a rate at least equivalent of a vented cavity....".
Click to expand...

Surely it would only need to transmit moisture at a rate greater than the vapour checked composite construction was letting it get past the building paper?

But you could be right. It might not be a "vapour diffusion rate" thing, it might be a "blow hard enough to evaporate any moisture on the outside of the breather paper" thing!
 
onq said:
Surely it would only need to transmit moisture at a rate greater than the vapour checked composite construction was letting it get past the building paper?

But you could be right. It might not be a "vapour diffusion rate" thing, it might be a "blow hard enough to evaporate any moisture on the outside of the breather paper" thing!
Click to expand...

i think its the 'blow hard' option.... (cant believe i just said that :D )

thats the reason the cavity must be vented. if it was the other option, a non-vented cavity would suffice.

To be honest, if you are thinking about increasing the insulation levels of a timber frame build, be it proposed or existing, i think you need to look inward and use breathable insulative materials which should be designed to remove as much thermal bridges as possible. Some timber frames (generally the cheaper ones) have very significant thermal bridging factors, which may not be included in u value calculations.
 
<nods>

Yes I see that, and you mentioned insulation on the other side of a vented cavity not being very useful, but surely it all adds up to keeping the house warmer than it otherwise would be?

For example, say the cavity in the timber frame was preserved, but the weathering outer leaf in block also had external insulation applied - would that not add to the overall insulation, even if the ventilation of the cavity was maintained?
 
onq said:
<nods>

Yes I see that, and you mentioned insulation on the other side of a vented cavity not being very useful, but surely it all adds up to keeping the house warmer than it otherwise would be?

For example, say the cavity in the timber frame was preserved, but the weathering outer leaf in block also had external insulation applied - would that not add to the overall insulation, even if the ventilation of the cavity was maintained?
Click to expand...

no... definitely not...

any insulation on the external side of a vented cavity is totally negated...
picture it.. the heat is reflected back into the cavity only to be vented away on air currents. Its a major problem with ill-fitted cavity board insulation... a phenomenon called 'thermal looping' occurs...

[broken link removed]
(page 51)
 
Thanks very much for that sydthebeat.

I am always happy to get advice from one who seems to know his stuff

:)
 
You must log in or register to reply here.
Back
Top