Retrofit Air-Tightness / Drylinging

nailer8

We are just about to commence a major renovation of a 70's house including energy upgrade to A Rating (Hopefully), Air tight with MHRV.

The construction is brick external, 100mm cavity with 50mm polystyrene, block internal, batons/25mm polystyrene and plaster board.
We are not willing to cover the brick with external insulation so to achieve a U value of 0.15 we have opted to fill the cavity with beads, rip off the existing plasterboard and install an insulated slab internally.

My question is re the Air Tight layer.

Is it best to use the inner surface of the block (assuming it is coated with sand/cement) or try to use the insulated slab as the air tight layer?

The foil backed slab's claim to be air tight but how does one seal the joints then and is there risk of mould behind the foil?

MicktheMan

nailer8 wrote: »

The construction is brick external, 100mm cavity with 50mm polystyrene, block internal, batons/25mm polystyrene and plaster board.
We are not willing to cover the brick with external insulation so to achieve a U value of 0.15 we have opted to fill the cavity with beads, rip off the existing plasterboard and install an insulated slab internally.

What thickness of insulated slab to achieve .15 uvalue?

sydthebeat

As you are upgrading the cavity insulation you must be careful not to create a situation where vapor will condense on your inner leaf

Calahonda52

Am trying to be helpful here:
This sounds like an ill thought out project.
Please describe the construction of the house in some detail.
Please detail the existing and proposed services and how they pass through the A/T layer.
Once you do that we can tell if the foil backed pb will work.
My sense is no.

Pumping beads into a 70's built cavity with 50mm poly already, in pursuit of an A rating is one of the more foolish ideas I have come across in some time.
The deap software and the BER might say A, but it won't be in reality.

In addition, there is a very high risk that the process will increase the moisture transfer across the cavity.

Retro fitting an A/T layer in a 70'S house is well nigh impossible to a standard that will not make the MVHR a heat waster.

I would scratch the A/T idea and give serious consideration to EWI.
You can get a faux brick finish.
EWI in most cases can be designed to bring the dewpoint across the cavity and into the outer layer.
Maxing out on Internal insulation is doing the opposite, compounded by the fact that the cavity insulation is at best useless.
It just wasn't done right back then.
If u are doing new windows, they can hang on outside of brick layer before the EWI, making a good job all round

MicktheMan

Calahonda52 wrote: »

Retro fitting an A/T layer in a 70'S house is well nigh impossible to a standard that will not make the MVHR a heat waster.

Don't agree. With proper detailing, there is no reason why you wouldn't be able to achieve very good ATT levels. I'm thinking ENERPHIT standard here.
Btw, at what ATT level would you rule out mvhr?

MicktheMan

Calahonda52 wrote: »

I would scratch the A/T idea and give serious consideration to EWI.

There is little point in investing in EWI if ATT levels are poor (especially in our mild but windy climate)

nailer8

@Calanhonda52
I appreciate the advice.

Firstly i am not overly concerned of achieving an A rating specifically but i want to do the best i can with the house we have. We knew a 70's house wasn't going to be great when we bought it.

I am aware we will struggle to get a decent air tightness result and the HRV may not save any money but the primary reason we are putting it in is for air quality and ventilation. I don't want drafts and uncontrolled wall vents.

At this stage EWI is not on the table. I know it is a good option but we are not doing it on this job. Its too late to change the plans.

I was never convinced re filling the cavity, that is what is specified by the architect at present.
What i was told was with the cavity pumped and 62.5mm Kingspan we would get 0.17 and without it pumped approx 0.2 (apologies 0.15 was a mistake)

My problem then is if EWI is out surely the only place to go is insulated plasterboard?

Not sure what other constructions details i can give.
The wall is currently 100mm brick outer, 50mm air gap, 50mm polystyrene, 100mm block, 25mm polystyrene, 12.5mm plasterboard. (Which for 1977 isnt bad!)

Floor is currently suspended timber over ventilated cavity, we are digging this out and putting down a concrete base, 150mm insulation and a 75mm screed.
All ceilings being replaced, plan on putting a AT membrane over 1st floor ceiling.

All windows and doors being changed also.

Dardania

In lieu of MHRV, maybe consider DCV -snip-

I used it - find it great. Just be clever with vent locations. And it will work better in a retrofit situation...

BryanF

Calahonda52 wrote: »

Pumping beads into a 70's built cavity with 50mm poly already, in pursuit of an A rating is one of the more foolish ideas I have come across in some time.

why? Assuming a remaining 50mm cavity and good external weathering achieved with Ewi, then I see nothing wrong with pumping.

Retro fitting an A/T layer in a 70'S house is well nigh impossible to a standard that will not make the MVHR a heat waster.

not impossible, as in sure you'll appreciate it comes down to the spec & install of air-tightness etc. I have specified, supervised and achieved passive retrofit standards in air-tightness in a 50's build, as have several other regular posters in this forum.

I would scratch the A/T idea and give serious consideration to EWI.
You can get a faux brick finish.
EWI in most cases can be designed to bring the dewpoint across the cavity and into the outer layer.
Maxing out on Internal insulation is doing the opposite,compounded by the fact that the cavity insulation is at best useless.
It just wasn't done right back then.
If u are doing new windows, they can hang on outside of brick layer before the EWI, making a good job all round

Agreed. Assuming Detailing of all junctions is carried out. Edit: Existing cavity insulation can be improved by pumping and is a must with ewi to reduce thermal looping, thermal bypasses would also need to be addressed.

nailer8

EWI is not on the table here.

I suppose my questions at this stage are:

- Fill the remainder of the cavity or not?
- How to achieve a vapour barrier layer and Air Tight layer and where it should be when using internal insulation?

I am pretty sure now it shouldn't be on the surface of the block as that would put the AT layer on the cold side of the insulation.
Can it be done on the outside any way?

john_eire

What about spray foaming to fill the cavity will increase u value and airtightness
Search walltite,

nailer8

Having met with the contractor and architect i am a little wiser as to what they plan to do but still not happy.

Their plan is to use the built in vapour barrier in the insulated slabs. Seal all joints including ceiling and floor. Seal to windows etc.
Claims to have achieved good AT results on recent projects.

Is it even possible to get those joints sealed adequately?
How will services be handled going through it. Sockets etc.

Would a better build up be to put in an insulation board then a VB/AT membrane and put the plaster board on over it.

Dardania

From looking at page 261 of this (only chosing them as they are the first that came to mind): http://www.kingspaninsulation.co.uk/getattachment/841ad8d6-79b3-4c3b-ac99-ff609575b639/Design-Manual-(Large-File---17Mb).aspx?disposition=attachment they seems to have the vapour resistance on the rear (adacent the batons)

Maybe forget about the cavity insulation altogether, and just focus on the internal insulation?

For penetrations of services, run them in the baton space, and present them in some sort of sealed ope through the plasterboard? You can also get air tight sockets, switches etc...


Just as a general comment, I echo the other posters in their advocation of EWI - my take is that is presents great thermal mass, so a good consistent heat (well suited to the more efficient heat sources e.g. condensing boilers working at low temps, heat pumps etc...). What are your floors like - can you use them for thermal mass? Otherwise you REALLY have to get the airtightness right so as not to have suddenly hot/suddenly cold house (low inertia)

Edit: Excerpt from the design guide regarding condensation:

Water Vapour Control / Condensation
Consideration should be given to the risk of condensation,
when designing thermal elements.
When internally lining a construction with insulation,
condensation can be controlled by ensuring there is a layer of
high vapour resistance on the internal surface of the
construction. Kingspan Kooltherm® K18 Insulated
Plasterboard contains an integral vapour control layer and,
when installed correctly, with appropriate detailing at joints
between sheets, penetrations and wall perimeters, can provide
the necessary vapour resistance. If required, the vapour
resistance of the wall lining can be increased by the application
of two coats of Gyproc Drywall Sealer.
A condensation risk analysis should be carried out following
the procedures set out in BS 5250: 2002 (Code of practice for
the control of condensation in buildings). The Kingspan
Insulation Technical Service Department (see rear cover) can
provide this service.

nailer8

I know EWI is great. I know it is the best option in most cases. The exterior of this house an unusual granite brick, which is a lot of the reason we bought the house in the first place. We are NOT putting EWI over it. End of Story. As i have said EWI is not on the table for this job.

With that in mind, i am trying to do the best i can with the options available to me. We have a reasonable budget and i am open to most suggestions.
I want the house well insulated and as air tight (and healthy) as possible.

The insulation and AT/VB layer will have to be internal, what i would like is to get the best working procedure for installing it.
I have reviewed BS5250 but it is lacking install details.
Anyone aware of other standards.
Cant believe 10'000's of houses have been dry lined (grant aid) in ireland without a proper standards.

Re Thermal Mass i know it is desirable to have as much as possible. Most of the internal walls are block (cold bridges!), couple of large chimney breasts, and the new floor is concrete so there will be some.

I dont have anyone appointed to do a final BER & Air Tightness test, i know you cant name names on the forum. If anyone wants to PM me suggestions that would be great. Or if some of you guys do that for a living send me your details.

Dardania

You'll have to discount the walls & chimney block as thermal mass, so it sounds positive you have concrete floor - will help with bufferring heat.

Have a look at the link I sent ( there are some details on install etc.) Also that vendor does a design service - maybe check with your design/build team what details they're working to?