drylining or Not???

JD6910

hi folks,

im building a new build house with a 200MM full fill cavity.

any advantage in dry lining the external walls taking into account cost of doing so Versus the improvement in u -value.

also im installing a geothermal system with high efficient rads so i think i may need my block walls to act as the thermal store.

DOCARCH

Preferable to keep your insulation all in one place, and, as you say/allude to, your inner leaf will give you some thermal mass which will be lost by dry-lining.

If you want to improve your u-value would/could you up your cavity wall insulation spec or widen your cavity to 250mm?

BryanF

JD6910 wrote: »

hi folks,

im building a new build house with a 200MM full fill cavity.

any advantage in dry lining the external walls taking into account cost of doing so Versus the improvement in u -value.

imo no

if you can widen the cavity, otherwise for get it.

(is the builder suggesting the drylining to avoid plastering and chasing? as this is common place)

MicktheMan

BryanF wrote: »

(is the builder suggesting the drylining to avoid plastering and chasing? as this is common place)

... and a disaster for airtightness

BryanF

MicktheMan wrote: »

... and a disaster for airtightness

agreed - ive yet to be convinced

JD6910

thanks guys.

the big item is the cavity closer block and the cold bridge it is creating. that particular block is within the fascia and soffit so maybe the cold bridge isnt a huge concern and worth dry lining because of that??????????

sinnerboy

This is what you should be aiming for - no thermal bridge. Make sure you have appointed a competent structural engineer however. I would perfer a thermal bridge to a collapsed structure.

And - Moderators Reminder to all
NO structural advice is to be sought here or offered.

JD6910

sinnerboy wrote: »

This is what you should be aiming for - no thermal bridge. Make sure you have appointed a competent structural engineer however. I would perfer a thermal bridge to a collapsed structure.

And - Moderators Reminder to all
NO structural advice is to be sought here or offered.

ya we looked at that detail and we were not comfortable with it.
we used the standard closer block detail. that closer block is now within the fascia and soffit so i wonder it is a major cold bridge area???

MicktheMan

JD6910 wrote: »

that closer block is now within the fascia and soffit so i wonder it is a major cold bridge area???

Why wouldn't it be?

heavydawson

MicktheMan wrote: »

Why wouldn't it be?

True, it's protected from direct rain ingress, but it's still got an exposed external face in terms of temperature, and capillary action from the blocks below will still probably allow water to get up to it (no scientific proof of this BTW, just what I'd imagine to be the case)

JD6910

MicktheMan wrote: »

Why wouldn't it be?

because it is concealed within the fascia and soffit and therefore may not be a "major" cold bridge.

ultimate question is: does is warrant the cost of drylining the walls inside just to guard against that somewhat minor cold bridge???????

heavydawson

JD6910 wrote: »

because it is concealed within the fascia and soffit and therefore may not be a "major" cold bridge.

ultimate question is: does is warrant the cost of drylining the walls inside just to guard against that somewhat minor cold bridge???????

IMHO no. You're going to lose heat because of it, so I'd just accept that. What I'd be concerned about it preventing moisture and mould building up on it, so a good ventilation strategy should cover you here.
Remember that dry-lining is just covering the problem. You'll only get mould growth behind the slab then (look up interstitial condensation), and all the issues that come with it.

sinnerboy

It will be a major cold bridge I'm afraid and the risk of localised condensation is great. On what basis did you decide you were "not comfortable" without a closer block ?

JD6910

heavydawson wrote: »

IMHO no. You're going to lose heat because of it, so I'd just accept that. What I'd be concerned about it preventing moisture and mould building up on it, so a good ventilation strategy should cover you here.
Remember that dry-lining is just covering the problem. You'll only get mould growth behind the slab then (look up interstitial condensation), and all the issues that come with it.

MHRV system going in so thats ventilation sorted.

MicktheMan

JD6910 wrote: »

because it is concealed within the fascia and soffit and therefore may not be a "major" cold bridge.

ultimate question is: does is warrant the cost of drylining the walls inside just to guard against that somewhat minor cold bridge???????

It will be major with elevated risk of localised condensation forming / mould growth.
Dry lining may well make the situation worse.

JD6910

sinnerboy wrote: »

It will be a major cold bridge I'm afraid and the risk of localised condensation is great. On what basis did you decide you were "not comfortable" without a closer block ?

why a "major" cold bridge? the block is concealed within the fascia and soffit and has insulation pushed out to the back of the fascia thereby covering the block somewhat.

not comfortable without cavity closer because 2 wallplates would have caused majority of weight to be applied on the inner leaf and with the 200MM cavity the wall would be dodgy. the cavity closer is solid and ties the leafs together and spreads the roof weight on both leafs.

heavydawson

JD6910 wrote: »

MHRV system going in so thats ventilation sorted.

Be sure and get the house air-tightness tested before the HRV goes in. If it's not properly air-tight, you'll have a lot of "natural" ventilation, and the HRV will be wasted.

MicktheMan

JD6910 wrote: »

MHRV system going in so thats ventilation sorted.

Notwithstanding the ventilation system (assuming it is designed and installed properly) you will still have a higher risk of condensation/mould compared to not having the thermal bridge in the first place.

MicktheMan

JD6910 wrote: »

why a "major" cold bridge? the block is concealed within the fascia and soffit and has insulation pushed out to the back of the fascia thereby covering the block somewhat.

Forget about concealment...this has nothing to do with cold bridging.

Even if top of the cavity closer block is insulated, there is still a heat conduction path from (warm) inner leaf, via closed block to (cold) outer leaf.