Please critique my wall/ floor structure plans

soldsold

Hi,

I posted this thread on the constructireland forum today but would also really value any of your opinions.

We are just about to start on a 200m2 extension to 100m2 bungalow aiming to get to passive level, or very close to passive. Not overly concerned with actually achieving passive house levels, but it would be a nice bonus. Main considerations are:

1) Maximum DIY input to keep costs down, and aiming to use materials and techniques that lend themselves to DIY input. This rules out timberframe, external insulation, and many others.

2) Minimum cost by using cheap, standard building materials where possible – eg concrete blocks and blocklaying are currently an extremely cheap building method, DIY parge coat of plaster is very low cost compared to air tightness barrier at 12 euro/m2, etc.

3) Maximum building expected lifespan at high energy performance (eg leave no chance of insulation sagging, no vapour barriers tearing, no air tightness tapes coming loose after 20 years, etc). ie reduce the risk of failure over the years as much as possible, we intend this to be 'home' for a very long time.

4) Wall, floor and roof u values of about .11 to .14, window u values 0.8 to 1.1 - trying to hit the 'sweet spot' of u value vs. cost/ length of payback time.

5) Minimise thermal looping, cold bridging, and maximise air tightness by very good planning and detailing.

6) Use tested/ certified MHRV to ventilate and recover at least 85% of heat (MHRV in the extension only), a small solid fuel stove with balanced flue for cold spells, and use the current boiler as backup in case we dont hit passive standard. We will plumb for a couple of small radiators in case they are needed.

7) Extension will be the main living area – sitting room, dining room, kitchen, playroom, toilet and guest bedroom. Current bungalow will become bedrooms mainly, so will have a spring loaded insulated door where it joins the extension so it can be kept as a separate heating zone to the “passive” extension. Current bungalow to be made airtight as much as is practical and have cavities pumped (currently 50mm aeroboard in 100mm cavity).

So my plan is:

Foundations – Will use standard strip foundation due to cost – my brother digs foundations, so we can DIY site excavations, foundations and floor insulation/ subfloor. We will detail to avoid cold bridging as much as possible as outlined below, and for cost reasons I am not planning to use insulated raft foundations, foamed glass blocks, etc.

Floors - 300mm Polystyrene below floor. Maximising insulation here as there is no restriction to how much you can use unlike in walls and roof space which give significant extra cost (eg very deep joists etc).

Walls – Ideally would use external insulation but at minimum of 100 euro/m2, a well detailed and insulated cavity wall with dry lining seems to be much more cost effective when using some DIY input.
So I plan to get a blocklayer to build a 150mm Quinnlite inner leaf with 150mm wall ties attached. Blocklayer to clean the mortar off while building and I will later DIY attach 100mm partial fill Kingspan PU boards and tape all joints, possibly use external insulation type mushroom fixings to secure the boards tight. Then build 100mm concrete block as outer leaf, leaving a 50mm air cavity (i.e. total cavity width 150mm) and later get this 50mm cavity pumped with 50mm of pumped bead insulation.

Q. Why use Quinnlite blocks? Better u value, reasonable extra cost per m2 compared to using dense concrete blocks, allows inner leaf to be built on its own without getting blown over by wind (would need dense blocks on the flat if not using quinnlites and wanting the inner leaf to be built separately), and good for decrement delay to regulate interior temperature (according to vikinghouse website you get an 11 hour decrement delay for 150mm quinnlite blocks and 150mm external PS insulation.)

Q. Why use PU sheets AND pumped insulation? PU to be used as it will give a better u value within the restriction of the allowed 150mm standard cavity, and I can DIY the fitting of the boards. Pumped in insulation will be used to keep kingspan PU sheets pressed against the wall and stop thermal looping in the cavity and help keep taped joints intact. I will be pumping the current bungalow cavity walls in any case so it will not be a big deal to also do the 50mm cavity in the extension while the scaffolding is up and the pump is on site, and will help bring the u value down, so it would seem silly not to pump the cavity.

I will then DIY a parge plaster coat to inner leaf to make airtight and finish inner leaf with 37.5mm PU insulated plasterboards to lessen the effect of wall-tie cold bridge. Low cost as I can DIY this part. I cannot imagine any risk of condensation with this wall structure. I am not concerned about wall thickness, I actually like the deep internal window reveals it gives.

All internal walls (where possible) to be DIY timberframe, non supporting walls – to avoid needing foundations which would cause more cold bridging

Windows – Insulated frames with U value 0.8 to 1.1 (total of glass plus frame) depending on cost. Aluclad windows for long life, possibly triple glazed to prevent internal condensation on frames and extend life further, again depending on cost.

Cold bridging/ Airtightening:
1) Normal airtightening detailing around service pipes, etc.
2) Windows to be positioned so they are built into the insulation layer in the walls to avoid cold bridges.
3) 50mm Floor edge insulation around perimeter walls to help avoid cold bridges – 50mm ok due to perimeter walls being drylined 50mm internally.
4) Separate window lintels over inner leaf and outer leaf to avoid cold bridge.
5) Apply a parge coat plaster finish to inside of exterior walls to make airtight before drylining.
6) Wrap a breathable windproof membrane around timber ceiling joists to avoid air gaps as the joists shrink (as per www.tonyshouse.info/tonytray.pdf). This looks like a good technique and avoids the cost of joist hangers.

A couple of questions I am still considering:
1) Anyone guess at the u values I will get from the above floor (my guess is about .11) and especially the wall structure (I guess .13)?
2) I have heard someone mention that quinnlites can be used below dpm level if another dpm is used to protect them. Anyone have details of how to do this to stop the quinnlites from soaking up water below the dpc?
3) Still undecided on roof type – I would hope to get a dormer style sloping warm roof that has u value of about .12 to .14, made from low cost materials to avoid the cost of SIP’s, etc, has a very good decrement delay without the cost of wood fibre boards etc, and had no risk of having insulation sagging over the coming years. Any thoughts or is this asking too much?

Would appreciate any of your comments!

Thanks,

Steve

Builderfromhell

I only glanced over your spec. (too tired to read all) but you would be wise to avoid internal dry lining on a cavity wall unless you get good advice on detailing. Condensation and mould growth is a seriouse problem here.
Externally insulate a block on flat structure would be my recommendation.
Good luck

sinnerboy

From here

http://www.sei.ie/Renewables/Renewable_Energy_for_the_Homeowner/SEI_Passive_House_A4.pdf

Measure/Solution Passivhaus Standard for the Prototype House
in the Irish Climate

Insulation Walls U < 0.175 W/m2K
Insulation Roof U < 0.15 W/m2K
Insulation Floor U < 0.15 W/m2K
Window Frames, Doors U < 0.8 W/m2K
Window Glazing U < 0.8 W/m2K
Thermal Bridges Linear heat Coefficient Ψ < 0.01 W/mK
Structural Air Tightness n50 < 0.6/ air changes per hour

If you overshoot the runway - i.e. chase u values endlessly - you will

1. spend more than you need to
2. over complicate the build - the more you increase elemental u values - the more important the junction details become
3. you will over heat in summer . your reduced winter heating bills will be replaced by summertime cooling bills . Tip - include several motorized veluxes and/or other high level glazing elements into your design for free natural cooling

So simplify .

Before you firm up on u values at all , never mind specifications , run your design through DEAP and/or PHPP . Play around with the u values and see how the energy rating is affected . ( not talking about BER cert now - forget about that for the moment ) You may well be surprised - maybe you don't need triple glazing . Or PH levels of air tightness .


The "150 cavity rule" is actually "the 110 cavity rule" .

The Tony Tray places the joist end into a damp cavity and closer to the external temp . Don't do that .

No need to guess u values - download this free software

http://www.xtratherm.com/software/software.php

.

soldsold

Thanks for the feedback!




I’ve been on holidays so just replying now.

Sinnerboy, thanks for the info/ tips, a couple of questions:


1)Why would the house overheat in summer if you use more insulation? I had understood it would stop the heat from getting in (assuming doors and windows are kept closed as needed) and that the decrement delay effect of more insulation would balance temperature fluctuations?

2) Is there an alternative to the “tony tray”, or a modification that will stop the issue you described? I would have thought that a breathable membrane would remove the risk of dampness here? I understand that joist hangers are an improvement (for airtightness) to joists bedded into the wall, but that they can also leak. Am I better to go with concrete first floors?

Thanks!

Steve

sinnerboy

A well insulated house in summer time , with the heating system turned off will still have to "cope" with heat generate by the use of the building . Body heat , cooking , tv's , pc,s dishwasher , showers , etc all generate heat . The house can overheat . A simple coping strategy is high and low level ventilation .

Just use joist hangers and take care that blockwork mortar joints are all well pointed . Then plaster to all wall surfaces including in between the joists . The plaster coat is no longer simply aesthetic .

.

MacTheKnife1

sinnerboy wrote: »

Tip - include several motorized veluxes and/or other high level glazing elements into your design for free natural cooling

Good point. I have high velux/fakkro windows - can you get a telescopic pole to open these windows? I am sure I have seen that in another house many years ago.

Carlow52

MacTheKnife1 wrote: »

Good point. I have high velux/fakkro windows - can you get a telescopic pole to open these windows? I am sure I have seen that in another house many years ago.

velux do one, about 70 euro
else fit string open /close from velux
else buy some 15mm dowel, 8 foot and fit a hook:)

Graaaaa

Just getting a little puzzled about the whole floor area/perimeter thing...
Before the current regs on minimum element u-values, it was often considered sufficient in large buildings to just lay a 1m wide strip with insulation under the perimeter of the ground floor slab, the reasoning being that the soil itself is an insulating material and that the travel distance for heat loss from the internal floor area to the colder soil outside the building was too great to worry about.

Now the regs say that the whole floor has a minimum u-value, fair enough, I won't argue, especially makes sense with suspended ground floors.

However when people get carried away throwing 300mm under the WHOLE floor, isn't that just wasting their cash? Surely this should only be a 1-1.2m internal strip from the perimeter that gets the zealous treatment?

Can anybody give the definitive calculated answer based on the residual ground temperature etc. using BS EN ISO 13370-2007?

I just can't stand to see good intentions going to waste.

sinnerboy

Good luck with that .

You must insulate the whole floor .

Graaaaa

Yes I accept that you must insulate the whole floor.

My point is in relation to the OP's intention to throw in 300mm under floor for good measure.

Say the house is 5m x 10m.

1. Assume 100mm insulation is sufficient for u-value: = 50 sq.m x 100mm boards.
2. Providing 300mm throughout: = 150 sq.m x 100mm boards
3. Providing an extra 200mm ONLY to the perimeter: = 102 sq.m x 100mm boards.

By ignoring the insulation properties of the soil, will the OP be wasting his good intentions by almost 50% hard cash?

sinnerboy

I see . I doubt few would be able to calculate that. I could not .

Graaaaa

Hmm, I see a pet-project on the horizon.

Will see what I can work out and maybe a user-friendly spreadsheet will emerge some day...

If anybody with the technical knowledge or experience would like to be involved or available for consultation please get in touch.

sinnerboy

2 fundamentals

1. heat loss - not many will have done that particular calc . Will require soil charcteristics testing to make the exercise worthwhile in terms of accuracy

2. structural . the slab design will have to take account of the differing load bearing components - i.e. differing insulation thickness's below . the slab may require reinforcement to cope with the differing stresses

My judgement call - put the insulation in . You will spend more on specialist advice and on site hassles than you spend on insulation . My bet - not an exercise worth undertaking for houses . Possibly , maybe , worthwhile on a large non domestic project .

Put the Excell spreadsheet away Graaaa