Advice on spec for new build

bargaret

Hi folks,
I haven't posted before but have been an avid reader for the past year or so! Anyway I'm currently getting construction drawings done on a 2600sqft dormer to go out to tender and looking for some advice/thoughts on wall and roof spec.
I searched and appreciate that a lot of this has been asked before but would like my own thread for brutally honest feedback..

The spec we have at the moment is as follows...
Walls 150mm cavity with 110 kingspan k8 to give .16u as per kingspan spec

Pitched Roof 150mm kingspan board between rafters and 100mm insulated board on top of rafters, looking to bring attic into usable space.
There is also 200mm rock wool specified on attic floor and 5min insulation on flat ceilings.

Slab 150mm kingspan board in floor. Looking to put one row of quin lite block on inner leaf to insulate walls from slab
Hollow core first floor.
Triple glazed windows throughout.

We're hoping to get <1 air changes through taping and sealing at junctions and plastering. Should this be reasonably achievable?

Will be putting in mrhv. Still debating heating system but thinking 9kw thermia a2w heat pump and underfloor heating (though cost is making us rethink this 😳).

House is currently coming in as a2 on provisional ber.

Any thoughts/ observations / advice / critique would be greatly appreciated!

sydthebeat

Sounds like a decent enough specification. Personally I don't think kingspans kool therm product is worth the money when compared to similar products (did you get kingspan themselves to spec it for you?)

A2w heat pump might find it difficult to meet renewables alone, wood stove may be needed, depends on size of house.

You need a ber assessor to crunch the numbers and advise on value for money.

Calahonda52

Re
We're hoping to get <1 air changes through taping and sealing at junctions and plastering. Should this be reasonably achievable?

This is a big ask based on taping and sealing at junctions and plastering.
If you are going the MHRV and intend using the attic, you need careful consideration of getting the ducting right from an insulation perspective.

If the goal is <1, then I would price the cost of getting to the passive house standard,not getting certified but using it as a frame of reference, using the high install cost of the UFH and the higher lifetime costs of the heatpump as the initial costs to be saved.
Perhaps also look at insulated footings.
Good Luck

bargaret

sydthebeat wrote: »

Personally I don't think kingspans kool therm product is worth the money when compared to similar products (did you get kingspan themselves to spec it for you?)

What similar products would you recommend as per above?

Yep kingspan gave us the various options and spec and u values

bargaret

Calahonda52 wrote: »

Re
We're hoping to get <1 air changes through taping and sealing at junctions and plastering. Should this be reasonably achievable?

This is a big ask based on taping and sealing at junctions and plastering.
If you are going the MHRV and intend using the attic, you need careful consideration of getting the ducting right from an insulation perspective.

If the goal is <1, then I would price the cost of getting to the passive house standard,not getting certified but using it as a frame of reference, using the high install cost of the UFH and the higher lifetime costs of the heatpump as the initial costs to be saved.
Perhaps also look at insulated footings.
Good Luck

Thanks calahonda.
Interesting point on trying to get it to passive level and cutting the ufh and heat pump. How do you spec for passive assuming it's going to be built cavity wall by a builder as part of a tender?

Also any advice welcome on what to make sure is included in spec / construction drawing detailing to try and get 1 air change on a cavity build out of a builder?

sydthebeat

Kooltherm is a phenolic foam product

Polyurethane (PU) and polyisocyranuate (PIR) are similar products which offer slightly smaller insulation values but for a more competitive price. Both kingspan and Xtratherm provide these products, also Ballytherm, and i think quinntherm may also supply. i just checked and PIR cavity board will give you the same u value at those levels.

also, 150 cavity pumped with ploystyrene beads will give you a u value of 0.17, which would be a lot cheaper than the phenolic.

Calahonda52

Plenty here on PH to get you started
http://www.seai.ie/Publications/Your_Building_Publications_/Passive_House/

You need to work with your designer and the tender docs must make it clear what is being asked for.

You will also need AT tests done as soon as the building is supposed to be airtight and all the membranes are in place.

Sub 1 ACH is not something every builder can achieve, especially the sub-trades.
With MVHR the AT needs to be good else its wasted $$$$$$$$$$$

Again, rather than be relying on cavity insulation which is v difficult to get right and even harder to supervise and sign off on, what about EWI

Also why 3 G on south facing aspects of house? Maybe the marginal cost of 3 over 2 is not worth worrying about.

Rather than the Kingspan stuff have you considered natural wood fibre insulation and pumped cellulose

Curious Geroge

Nearing completion I would change mine from 150 to a 200 cavity; even 180 if you fear bigger costs.
Ensure you wrap the slab; airtight membrane in the roof; taping windows and floors..
Be careful not to leave the triple glazed windows open to any vendor as you have a large range of design and standards; same applies to the HRV and A2W units. There are plenty of options and plenty of information available in the threads here.

Ensure your spec includes a garage if your doing it; include the boundary walls/pillars; plastering of boundary walls; paving or prep for paving as part of the groundworks. If your adding drops to the ceiling, kitchen island drop, if you want a cassette fireplace or standard; wet rooms or trays; wall hung wc or standard; concrete stairs or wooden.. the list goes on and on

Calahonda52

Curious Geroge wrote: »

Nearing completion I would change mine from 150 to a 200 cavity; even 180 if you fear bigger costs.

Is there not an issue with the spec for wall ties beyond 150?

Curious Geroge

Calahonda52 wrote: »

Is there not an issue with the spec for wall ties beyond 150?

That was my deterrent when we started but I reckon that was because everyone was talking about TeploTie which are non-thermal bridging wall ties (which is great but cost outways this). You'd assume there are plenty of standard metal ties available at a reasonable cost.

Calahonda52

OP: meant to ask: as they down our way: Whats with the Hollow-core first floor slab?
Is the inner cavity wall leaf thicker than the outer leaf?
What is the detail like
1: for passing the cavity insulation up past the end of the hollowcore
2: ensuring the integrity of the airtight membrane.

I know HC is quick in terms of getting the floor down but there are additional costs, not restricted to the cost of drilling through for services.
Then there is the cost of the suspended ceiling to hide the MHVR, as well as the extra height required.

Careful planning for services, including MHVR, can be done within a timber floor, it is done all the time in TF houses, it just requires thought.

Naochanaoi

On the hrv another option is ductless. There are at least two companies I know of, supposed to be more efficient.

sydthebeat

Naochanaoi wrote: »

On the hrv another option is ductless. There are at least two companies I know of, supposed to be more efficient.

Any of them tested and certified?

Calahonda52

Naochanaoi wrote: »

On the hrv another option is ductless. There are at least two companies I know of, supposed to be more efficient.

Are these not just one room jobbies and you leave the doors open and hope the air, as opposed to the earth, moves?:)

joeirish

We've just completed the first stage of our build (foundations, walls, roof, windows, doors, external block skin, plastering, roof slates). Veyy good spec with about 0.6 ACH. After two years of research we went for timber frame. Price is very competitive, build is very fast and quality is absolutely exceptional (and that is from the follow on trades also). PM me if you want any further information. I've posted on the costs thread with some photos of what we have done so far http://www.boards.ie/vbulletin/showthread.php?p=93972494#post93972494. This is our first self build and it was a steep learning curve but really pleased with the whole thing so far. Good luck on yours.

joeirish

Naochanaoi wrote: »

On the hrv another option is ductless. There are at least two companies I know of, supposed to be more efficient.

I researched these and they do work out a bit more expensive and I had some concerns over effectiveness. So I went for ducted and seems fine.

Naochanaoi

Got a quote from one, cost about €300 more than a ducted system.
Would go for it if it works.
What turned you off them bar the cost?

joeirish

They all have to be installed on external walls and seemed to be more appropriate for smaller accommodations such as apartments. Potential for draughts seemed an possible issue. Unsure about efficiency also. And in the end no reason not to have the ducting.

Calahonda52

joeirish wrote: »

We've just completed the first stage of our build (foundations, walls, roof, windows, doors, external block skin, plastering, roof slates). Veyy good spec with about 0.6 ACH. After two years of research we went for timber frame. Price is very competitive, build is very fast and quality is absolutely exceptional (and that is from the follow on trades also). PM me if you want any further information. I've posted on the costs thread with some photos of what we have done so far http://www.boards.ie/vbulletin/showthread.php?p=93972494#post93972494. This is our first self build and it was a steep learning curve but really pleased with the whole thing so far. Good luck on yours.

Nice looking house, thanks for sharing:
Curious as to why flat roof, especially at upper floor level?
Also where was the sound issue: are urs the next U2?

bargaret

Calahonda52 wrote: »

You will also need AT tests done as soon as the building is supposed to be airtight and all the membranes are in place.

Sub 1 ACH is not something every builder can achieve, especially the sub-trades.
With MVHR the AT needs to be good else its wasted $$$$$$$$$$$

Plan is to get 2 air tests done, first after first fit and then after house is plastered and completed. Still not sure how to define the tender with regards retention etc for air tightness results.. I've been told builders don't want the 1 air change and build it into price . Would penalties for >2 and bonus for <1 be a better approach?

Calahonda52 wrote: »

Again, rather than be relying on cavity insulation which is v difficult to get right and even harder to supervise and sign off on, what about EWI

Also why 3 G on south facing aspects of house? Maybe the marginal cost of 3 over 2 is not worth worrying about.

Rather than the Kingspan stuff have you considered natural wood fibre insulation and pumped cellulose

Prices seem competitive on 3G vs 2G based on quotes we've gotten so far

Haven't considered EWI, assumed that was for retrofits on older houses and don't want a wide wall if can help it.
Haven't looked into cellulose, if anyone has done it or can recommend a supplier to get a quote off would appreciate a pm or whatever.

Thanks everyone for all the feedback so far, really appreciate it!

bargaret

Calahonda52 wrote: »

OP: meant to ask: as they down our way: Whats with the Hollow-core first floor slab?
Is the inner cavity wall leaf thicker than the outer leaf?
What is the detail like
1: for passing the cavity insulation up past the end of the hollowcore
2: ensuring the integrity of the airtight membrane.

I know HC is quick in terms of getting the floor down but there are additional costs, not restricted to the cost of drilling through for services.
Then there is the cost of the suspended ceiling to hide the MHVR, as well as the extra height required.

Careful planning for services, including MHVR, can be done within a timber floor, it is done all the time in TF houses, it just requires thought.

Reason for hollow core is we like the solidness quietness of it over time, living in a timber frame at moment and you hear every creak.

Need to follow up with designer on the detailing you mentioned. Had assumed we would be including the standard recommendations in tender drawings (pulled from guidelines), is there specific detailing you would recommend over and above this?

joeirish

Calahonda52 wrote: »

Nice looking house, thanks for sharing:
Curious as to why flat roof, especially at upper floor level?
Also where was the sound issue: are urs the next U2?

we wanted the extra height for the upstairs windows as there's a really nice view over the trees to Lough Derg. That room is an upstairs sitting room.
The noise issue was because it's a TF house and these can be noisy. So all walls sound insulated and between ceiling joists. And it does make a big difference. I think I should be ok playing my banjo in one room while the missus is elsewhere in the house.

Calahonda52

joeirish wrote: »

we wanted the extra height for the upstairs windows as there's a really nice view over the trees to Lough Derg. That room is an upstairs sitting room.
The noise issue was because it's a TF house and these can be noisy. So all walls sound insulated and between ceiling joists. And it does make a big difference. I think I should be ok playing my banjo in one room while the missus is elsewhere in the house.

Much obliged for the answers, its always nice to know what drove the decisions, not just for me but for others who will pass this way.
Its good that, based on the shadows, the view matched the southern/western aspect

bargaret

Curious Geroge wrote: »

Nearing completion I would change mine from 150 to a 200 cavity; even 180 if you fear bigger costs.
Ensure you wrap the slab; airtight membrane in the roof; taping windows and floors..

hi just around "wrapping the slab" and airtightness membranes. I assume we should we be including this in our spec / bill of quantities.

what is the standard approach for detailing / installing airtightness membranes?

Is it typical to have a membrane inside the inner leaf of the cavity before the insulation board?
are some membranes better than others or a standard grade of plastic used?

need to do some homework!