Best cavity block insulation

srdb20

Lads I'm pulling my hair out here, ifi build a block house here, whats the best and I mean the best performing cavity wall insulation? And how is it layed out, I.e. Block - cavity - block - internal slab etc like measurements for each

also I've been hearing about building on the flat and using external insulation, what's the advantages here? And is it more effective and efficient?

Oh andis it much more costly but more importantly is it worth it?

Cheers lads

soldsold

Best performing cavity wall: 300mm cavity filled with insulation, with 150mm internal drylining. Add 200mm external insulation just to be sure

Best "realistic" cavity wall for a passive house? 300mm cavity full filled with beads of rockwool batts or EPS beads and using basalt wallties which now have BBA certification, or alternatively a 200mm cavity with internal drylining of 50mm insulated plasterboard. Both have been built successfully in the last 12 months or so. The 300mm cavities i know of are both in UK - google Bill Butcher's Denby Dale house for a very comprehensive weekly passive house blog, or also google tonyshouse.info for another example. The 200mm cavity wall with internal drylining was built in wexford, I visited the house and it looks like a really good approach. No heating system needed in any of these houses

Most cost effective approach? Kingspan or similar cavity boards to meet building regs, but the performance will be poor for various reasons - gaps between boards, thermal looping in the cavity, reduced insulating gas levels over time (although Im not convinced this is such an issue)

Most logical approach? Either...

1) 200MM cavity full filled with EPS beads - but the wallties will form a significant cold bridge (its what ive just built but havent insulated yet)

2) 200mm cavity with insulated dry lining internally of 50mm or so. Reduces thermal mass, but internal block walls, chimneys, tiled floors etc should provide enough thermal mass (Im planning to use insulated slabs in some of the rooms ie the ones that have fewer windows to get the effect of the slabs without much messing about. Also allows thicker edge insulation for floors and this reduces cold bridging.

3) Reduce the cavity size so you dont need massive wallties (my wallties are 17mm wide x2.5mm thick and will probably suck a significant amount of heat out). Not sure what the max cavity is that can still use thinner wire ties rather than straps, probably 150 to 170mm. Then combine this with an insulated slab to minimise thermal bridging.

What would I do second time around?

Probably a 300mm cavity using basalt wallties if I was confident they would be structurally ok over the life of the building. The basalt wallties used now have BBA certification since Oct 2009 so i'd be more confident using them. Id probably get an infraction if I mentioned the name of the wall ties so I wont, just will say that they rhyme with Zeplo and begin with T

Would be interested in others opinions of using basalt wall ties by the way, its too late for me as my cavity walls are built but I'd be interested in opinions.

Re: external insulation - its a good system and will reduce thermal bridges very well but is still pricey at 80-110 euro per m2.

Where are you building?


Steve

srdb20

Building in Tipp, i was all set on the cavity wall build but after hearing about several problems im leaning towards building on the flat and using external insulation...

Where ya building yourself?

soldsold

Building an extension in Birr, basically adding open plan living quarters onto a small bungalow where the bungalow then becomes bedrooms.

What sort of cavity issues are you concerned about?

beyondpassive

If your set on block, Sold's 200-220 cavity with EPS bead and cosyboard on North and East walls is an optimal approach. Their are a few 'low hanging fruit' options to optimise the performance of the build. Insulation and airtighness are a no brainer, reducing heating costs, eliminating condensation damage risk and future proofing the building if you need to sell later. Designing a heating system for an 8 to 10 kW's boiler rather than 25Kw 115 BTU's becomes easy.

1. Most of your heat loss will be through the roof. consider 60 Gutex on a deep rafter or twin rafters, cellulose filled with OSB board as racking and vapour control airtightness. keep all servises inside the OSB. When you heat a house air becomes boyant and moisture laden and forces its way out through the top half of the building, which in turn draws cold air in at low level. Good roof and eaves insulation combined with a rigerous airtightness regime will drastically improve comfort levels and reduce the heat demand.

2. Cold bridging. Use AAC block on the warm side of the rising wall and the 2 starter inner leaf blocks to reduce the heat loss through the rising wall. 50 mm perimeter insulation and pour the slab on 300 EPS insulation on grade. Equally instead of closng the cavity with a wallplate block, detail the eaves to enure suffucient continuity of insulation between the wall and roof elements.

3. Heat recovery ventilation and room sealed stove. Hole in the wall vents and chimneys give you very little control over ventilation and drafts. MHRV filters air, recovers and redistributes heat and reduces moisture. In Ireland particularly on the south and west coast, humidity is the big cause of defects in buildings. Combined with underfloor to tiled areas, the ventilation system will distribute heat to the other rooms. Stable 20 degree comfort with <60% humidity is the objective here rather than ramping up the heat twice a day with rads.

dfader

hey steve,

Do you know if a 200mm cavity is structurally sound for a two story?

you might be interested in a similar topic here;

http://www.boards.ie/vbulletin/showthread.php?t=2055810212

soldsold

I wouldnt be concerned about structural stability, but your engineer might ask for some inner walls to be built on the flat to make sure. Thats what I did. 300mm cavities have been built two stories high so should be no issue for 2 stories. My walltie manufacturer said the ties need to be at 450 centres vertically and horizontally, and at every course around windows and doors. They are very stiff ties compared to normal wire ties.

So the shorter answer is dont worry about the two story being an issue, but get an engineer that is able to do structural calculations or face the cost of a structural engineer being called in

moan 77

I am building too using 100mm standard block 100mm pir insulation 50mm cavity 100mm standard block with render finish. My biggest worrie is the edge insulation most say 25mm sound too light for me, as i am using 100mm pir in floor why not bigger on the edge say like 50mm.

beyondpassive

Hi Moan,

edge insulation should be 37 to 50mm insulation with a bevel cut into the corner to allow the floor screed run up to the skirting.

This does not deal with the problem, that the inner leaf of block is a massive cold bridge, because it is built off the uninsulated rising wall. using quinnlite on the foundations can help with quinnlite on the first 2 course of inner leaf. However using quinnlite on the outside of the rising wall, isn't ideal. The quinnlite absorbs ground moisture worsening its conductivity and can be at risk from frost heave. So you need to detail it carefully.

Consider using 300 EPS instead of 100PIR under the floor. You then dont need a structural slab, just pour 100mm slab over the EPS. Consult your engineer of course and explain to him that you don't want half your heating warming the footpath and the area outside the house.