Extension Walls Insulation Query

bamboozle

Hi, i'm getting the following done for the walls of my extension, am just wondering if the insulation is sufficient or if i should go for insulated plaster board on the interior wall also...

'100 thick solid block outer leaf, 100 cavity and ties. 60mm kingspan insulation k8 and 100 thick solid block inner leaf. plaster externally, gypsum plaster finish internally'

sydthebeat

widen the cavity by 20mm and put 80mm in place of the 60mm....

bamboozle

unfortunately the walls are now 90% completed with 60mm insulation.

as such i was thinking of asking them to put maybe 20mm insulated plaster board on....is this a runner (i seem to remember reading somewhere here it causes problems with condensation)

thanks!


(never thought i'd see the day where builders are way ahead of schedule!)

RKQ

20mm thick foil faced kingspan or similar should be fine.
Ensure you minimise "cold bridging" particularly at ceiling / wall junction as this will also help.

Can you install "airtight" membrane? Check out ProClima, Moy or Siga.

Maximise attic insulation also.

bamboozle

can you advise exactly cold bridging is......i'm guessing its to ensure insulation goes right up to the junction where wall meets ceiling?

what is airtight membrane? never heard of this before

thanks v. much

quentingargan

You can download a free u value calculator from SEI here and work out the benefits of adding a further insulated plasterboard inside your walls. If you are on a fixed budget (aren't we all...) you may find that spending an extra few quid on better double glazing gives a better result. You can do your own BER assessment on your house using the energy rating calculator, which is a spreadsheet that SEI produced. I can't find it on their site but if you pm me I can send it to you.

Cold bridging is where you have joined the insulated surface on the inside to the uninsulated surface at the outside. This becomes a focal point for leaking heat, but it also becomes an area where condensation might concentrate and cause damp. There is always a certain amount of cold bridging at windows, lintils etc., but there are recommendations in the DEAP manual from SEI on how to avoid this.

RKQ

A "cold bridge" is an uninsulated or poorly insulated area, especially at heads of reveals of external openings. Well installed installation at these areas will prevent cold bridges. See Homebond manual.
Critical or Robust details are used to solve this problem.

Note a highly insulated house will loose heat quicker at a cold bridge - the fast flowing heat creates condensation on the cold surface - heat loss is similar to a leak in a tyre! The higher the pressure or insulation level the faster the leak.

If I were you I'd also dry-line the walls provided it didn't make a huge difference to floor area / room width (extension)

Look at Siga, ProClima and Moy for details of air tightness in domestic structures.

bamboozle

thanks all, we have decided to dry line the walls with a thermal liner of 37.5mm (25mm Insulation and 12.5mm plaster)
i'll have to get on the builders case regarding cold bridges next, as there will be a lot of glass in this room i fear cold bridging could be a problem.

shazzyb

Hi, We're doing a self build at the moment and the windows have just gone in. What should I be looking for to ensure I minimise cold bridging around the doors / windows. What is the best method of ensuring this does not become an issue. Thanks.

sinnerboy

there is a lot of good guidance for dealing with avoiding cold bridging with internal insulation here

http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,18753,en.pdf

rule of thumb - keep internal insulation to at most 50% thickness of cavity insulation

tip - don't use composite plasterboards. use foil faced insulation boards , say 25mm thick . fix them first and foil tape all joints and then separately fix plasterboard over . vapor barrier continuity and air tightness improve

picorette

bamboozle wrote: »

thanks all, we have decided to dry line the walls with a thermal liner of 37.5mm (25mm Insulation and 12.5mm plaster)
i'll have to get on the builders case regarding cold bridges next, as there will be a lot of glass in this room i fear cold bridging could be a problem.

Checked with Kingspan. Your original construction would have had a u-value of 0.24 (Part L requires a maximum of 0.27). The extra dry-lining reduces it to 0.20, as would Syd's suggestion of widening the cavity and making it 80mm of K8.

Would anybody be aware of any problems in widening the cavity, with wall ties or cavity trays?