construction method help!

[Packman]

Hi everyone

 

I'm stuck with my construction method and insulation... Please can you help me with the pros and cons? I'm getting a lot of mixed reports from the things I'm reading and the people I'm speaking to!

 

Background:  I have just bought a 1950s house in Hertfordshire, cavity wall construction, completely uninsulated. It's in pretty good condition structurally, but hasn't been touched since it was built.

 

Project: I will be completely renovating and modernising the property, taking it back to brick, moving a lot of internal walls, new plumbing, wiring, windows, doors, floors, ceilings, etc. Oh, and getting rid of the asbestos! I will also be adding a 4x10m double storey extension on the back.  The back of the house is South-facing. Externally, I want to finish the house in a combination of brick slips and render. (I don't like the colour of the current brick that is used). I will also be retiling the roof, and adding a couple of gables over the rear extension. 

 

I want to take a fabric first approach to the build to make it as energy efficient as possible (within reason and budget constraints). I am not trying to take it to PassiveHaus or EnerPHTT standards at all, but I would like to try and achieve the best I can afford to do that makes sense and isn't over engineering it at large costs for the sake of small incremental gains.  I also really want to install MVHR. 

 

Where I'm at: My plans are still awaiting approval, but I want to crack on with structural calcs and BC drawings. However, unless I tell them otherwise, the extension will be specified to built in cavity wall construction, and the specifications & details will just be to satisfy normal building regs.  My thoughts were that I would full fill the existing cavities in the existing house, then wrap the whole house in EWI. Insulation material for both the cavity fill and EWI is TBC.

 

Questions:  What should I build the rear extension in, and how should I insulate?

 

Brick & Block with cavity? I assume this would be easier to tie in with the original house, as it is the same construction method as the original, and would be easier to find tradesman for this as it is the most common.  If I do this, I assume I would then continue the same full-fill of the cavity and EWI as per what I plan on the original house?

 

Single Block with EWI? Given that I want to use EWI on the existing house, perhaps it doens't make sense to build the extension in cavity brick and block, and instead I could use a single blockwork and continue the EWI on that?

 

A different monolithic single block construction method? E.g. the JUWO evolved SmartWall? (https://evolvedsupplies.co.uk/juwo-evolved-smartwall).  The sales materials sound convincing (obviously!) as it seems it would be cheaper and quicker to build the brick and block, provides a slimmer finished wall, has better thermal properties and means I wouldn't need to use EWI on the extension as well. But don't know if that is just all hot air (pardon the pun) and also don't know if that might give me an issue in the future with lenders and insurers?

 

Timber Frame? I've been told that this could be cheaper and quicker to build, and could be more thermally efficient? But it could be difficult to tie in, and I'm unclear if just insulating the frame is enough or if you would also wrap EWI over the outer leaf before finishing with the brick slips and render?

 

I'm also getting confused re thermal mass, and whether it is good or not to have!! My original thinking was that a solid wall from e.g. bricks, blocks, concrete, etc has a great thermal mass and therefore will store the heat generated inside and outside and release it slowly during the day, keeping it warm in winter and cool in summer, and creating a cosier environment etc. i.e. construction using materials with a higher thermal mass is a good thing.  However, when researching timber frame construction, I'm reading that their lower thermal mass is a good thing, and is more likely to create a better living environment than with a solid-wall construction method! Argh!!

 

Regarding insulation, I'm also getting mixed reports re products, so any advice there is appreciated!

 

Looking forward to hearing your views!

 

Thanks in advance

[Bitpipe]

We bought a similar house back in 2011 (1950s brick & brick, no insulation). 

 

After considering all options to refurb & extend, we decided to demolish and start from scratch.

 

1) as a new build you save 20% on VAT. Another way to look at this is to take your build budget and remove 20%.

2) the new house is airtight and insulated to near passive standards so MVHR etc will work as designed.

3) we were able to re-orientate on site and take advantage of the aspect.

4) Were able to put a basement under the house to maximise the footprint (ok, this last one is not for everyone )

 

Have you considered (and priced) this option? Took us a while to get our heads around it but very glad we did. Got exactly what we wanted with no compromises and at a controlled budget (no surprises).

 

I have met many people who have done what you are planning to do and without exception, they all wish they had demolished as the costs kept rising, the surprises kept appearing and the compromises kept stacking up. Plus no 20% VAT discount.

 

To some of your points above

 

10 minutes ago, Packman said:

I want to take a fabric first approach to the build to make it as energy efficient as possible (within reason and budget constraints). I am not trying to take it to PassiveHaus or EnerPHTT standards at all, but I would like to try and achieve the best I can afford to do that makes sense and isn't over engineering it at large costs for the sake of small incremental gains.  I also really want to install MVHR. 

 

It will be very difficult to make the existing part of the house airtight to a meaningful level. Most of the leakage will occur at the floor / wall interface where joists go into the wall. In our build, there is an airtight fabric that runs up the inside wall, comes out over the floor structure and back up the inside wall of the upper floor. In addition to the airtightness layer inside, every joint, penetration etc is sealed and taped.

 

 So MVHR will still help your air quality and ventilation but you may struggle to get meaningful heat exchange efficiency as that really only comes into play when your have airtightness below 2.0 (passive standard is 0.6) ACH. You can get a blower test when you're done to see how close you are.

 

10 minutes ago, Packman said:

I'm also getting confused re thermal mass, and whether it is good or not to have!! My original thinking was that a solid wall from e.g. bricks, blocks, concrete, etc has a great thermal mass and therefore will store the heat generated inside and outside and release it slowly during the day, keeping it warm in winter and cool in summer, and creating a cosier environment etc. i.e. construction using materials with a higher thermal mass is a good thing.  However, when researching timber frame construction, I'm reading that their lower thermal mass is a good thing, and is more likely to create a better living environment than with a solid-wall construction method! Argh!!

 

 'Thermal mass' is often bandied about in the likes of Grand Designs - do a few searches on this site for good explanations why it's not that meaningful. A better consideration is high decrement delay - i.e. slow to let external heat or cold penetrate the wall.

[Packman]

Thank you very much for your input Bitpipe!

 

35 minutes ago, Bitpipe said:

1) as a new build you save 20% on VAT. Another way to look at this is to take your build budget and remove 20%.

 

 

As the property has been vacant for more than 2 years, we can get a 5% VAT rating.... whilst not as good as zero-rating, it's still much more significant than having to pay full 20% vat! 

 

37 minutes ago, Bitpipe said:

Have you considered (and priced) this option? Took us a while to get our heads around it but very glad we did. Got exactly what we wanted with no compromises and at a controlled budget (no surprises).

 

 

We have considered it, but the building is in pretty good condition, is already pretty large, and we wouldn't be able to change the orientation due to it's location on the street (and in any case we wouldn't want to change the orientation), so we thought it would be a waste to demolish, and whilst we wouldn't have to make certain compromises, we think it will still work out more expensive to demolish and rebuild rather than to renovate, remodel and extend.

 

Out of interest, who/what did you use for your new build? Did you use a package build, or still traditional construction just from scratch?

 

 

43 minutes ago, Bitpipe said:

'Thermal mass' is often bandied about in the likes of Grand Designs - do a few searches on this site for good explanations why it's not that meaningful. A better consideration is high decrement delay - i.e. slow to let external heat or cold penetrate the wall.

 

Thank you for this, that is very useful and I hadn't heard of that before.  So if i've understood correctly, high density = high decrement delay,  but low decrement delay is better?  And bricks / block / concrete etc is high density and therefore high decrement delay, whereas timber frame is low density and therefore low decrement delay and therefore better?! Or am I over simplifying?!

 

 

 

[Bitpipe]

7 minutes ago, Packman said:

Thank you very much for your input Bitpipe!

 

 

As the property has been vacant for more than 2 years, we can get a 5% VAT rating.... whilst not as good as zero-rating, it's still much more significant than having to pay full 20% vat! 

 

Ok, that's a big advantage.

 

7 minutes ago, Packman said:

 

We have considered it, but the building is in pretty good condition, is already pretty large, and we wouldn't be able to change the orientation due to it's location on the street (and in any case we wouldn't want to change the orientation), so we thought it would be a waste to demolish, and whilst we wouldn't have to make certain compromises, we think it will still work out more expensive to demolish and rebuild rather than to renovate, remodel and extend.

 

Out of interest, who/what did you use for your new build? Did you use a package build, or still traditional construction just from scratch?

 

 

We used a timber frame package build as it guaranteed high performance (meeting passive standards) and was fairly fool proof for novices like us.

 

7 minutes ago, Packman said:

 

Thank you for this, that is very useful and I hadn't heard of that before.  So if i've understood correctly, high density = high decrement delay,  but low decrement delay is better?  And bricks / block / concrete etc is high density and therefore high decrement delay, whereas timber frame is low density and therefore low decrement delay and therefore better?! Or am I over simplifying?!

 

Sort of. Our wall construction has a very long decrement delay - it takes a long time for heat or cold to penetrate from outside. However it is a lightweight construction, essentially an MDF/OSB sandwich pumped full of warmcell insulation (which is treated powdered newspaper). You need to view the wall as a system and figure it out from there. The relative density  of the material is misleading - wood has a higher specific heat capacity than brick for example. https://www.engineeringtoolbox.com/specific-heat-solids-d_154.html

 

Essentially I think you want to avoid an extension that performs very differently (ie. heats up much more quickly or slowly, internally or externally ) to the rest of the house, unless it's going to exist as a fairly standalone unit (i.e. not all open plan into the existing). How it's actually built is moot but you're probably more likely to match it if you use similar methods (i.e. blocks with, insulated cavity).

 

Whats your space heating strategy?

 

 

[Dave Jones]

do the math to see if its worth dropping.

 

sale price now, + cost of extension = new sale price. is this a positive number. If it is may be cheaper to drop the whole thing.

[Iceverge]

Knock and rebuild it. You're only saving brick walls by the sounds of things and in the overall scheme of a house they're actually pretty cheap. You'll save a lot of money on the demolition if it can be done by one guy in the seat of an excavator vs many  workers manually hauling everything out piece by piece. 

 

Otherwise build your extension from dense concrete blocks and pump your current cavity with EPS blown beads. Then add 200mm+ of graphite EPS over everything and cover with a synthetic render. 

 

Decrement Delay = Phase Shift  = How long heat takes to soak through a surface . It's related mainly to overheating, roofs especially. When the exterior surface of the structure gets very hot in the sun you want to delay the heat transfer until it's nighttime. Its's a combination of specific heat capacity and mass of the structure and the sequence of layers within it. Timber frame walls with PIR = very poor. Timber frame walls with woodfiber = excellent decrement delay. 

 

Thermal capacity or a wall/roof is the ability of the inner few cm of a wall to absorb and release heat later without warming too much themselves. This slightly slows the rate a structure heats up and also cools down. It does not cure overheating or underheating but helps buffer the extremes. You don't need 300mm concrete walls for this, a few mm of plasterboard is plenty in a well designed house. Remember the thermal capacity of the structure is added to everything else in the house (furniture,inner walls, flooring books etc)  too to help buffer heat.  

 

"Thermal Mass" isn't a measurable commodity but rater a general concept encompassing these above misunderstood principles. Whenever a "professional" mentions it your bulls**t alarm should go off as it's almost guaranteed they have no idea how to measure or design any of the above. 

 

Good luck with the project anyway. 

Do you have any plans to show us? 

 

 

 

 

 

 

[Dave Jones]

get the VAT back on a demo as well.

[Packman]

1 hour ago, Dave Jones said:

do the math to see if its worth dropping.

 

sale price now, + cost of extension = new sale price. is this a positive number. If it is may be cheaper to drop the whole thing.

 

Thanks Dave. The cost of the reno and extension should be around £250k if i used a main contractor.  I would then end up with a 250sqm house.

The future value should be at least the same (but most likely a little bit more) than the current value + the renovation & extension cost.

 

But if I demolished and built from scratch, to create a 250sqm new build house, working on an avg of £1500 /sqm, the cost would be £375k.

In this scenario, the current value + the new build cost would end up being more than the future value. 

 

So based on this, I don't think it's affordable or feasible to demolish and rebuild. Plus, the elevations and layout of the house we would rebuild would end up being extremely similar to the plans we have for the renovation and extension.

 

Unless I'm missing something?

Edited January 18, 2021 by Packman

[Packman]

2 hours ago, Bitpipe said:

Whats your space heating strategy?

 

 

We were planning for UFH throughout, powered by a gas boiler with weather compensator.

 

Would potentially "upgrade" to ASHP powered by Solar PV in the future, but not for now. 

 

Edited January 18, 2021 by Packman

[Packman]

1 hour ago, Iceverge said:

Knock and rebuild it. You're only saving brick walls by the sounds of things and in the overall scheme of a house they're actually pretty cheap. You'll save a lot of money on the demolition if it can be done by one guy in the seat of an excavator vs many  workers manually hauling everything out piece by piece. 

 

 

Yeh, fair point, although the labour time of putting the walls up is pretty big...?

 

1 hour ago, Iceverge said:

Otherwise build your extension from dense concrete blocks and pump your current cavity with EPS blown beads. Then add 200mm+ of graphite EPS over everything and cover with a synthetic render. 

 

 

Thanks. So just single skin concrete blocks for the extension, wrapped in EWI? And full-fill the existing cavity and then wrap in the same EWI as the extension? 

 Graphite EPS for the EWI is what I was thinking, but 200mm seems quite a lot?! Most people have suggested 100mm to me??

 

What's your view on things like woodfibre for EWI and mineral wool or fibre glass for cavity fill?

 

2 hours ago, Iceverge said:

Whenever a "professional" mentions it your bulls**t alarm should go off as it's almost guaranteed they have no idea how to measure or design any of the above. 

 

 

Noted, thanks! Have added to my BS meter for when speaking to "pros" !

 

 

 

[Packman]

2 hours ago, Iceverge said:

Good luck with the project anyway. 

Do you have any plans to show us? 

 

Thanks!

 

[Iceverge]

12 minutes ago, Packman said:

 

Yeh, fair point, although the labour time of putting the walls up is pretty big...?

You'll have to do the sums but the brickwork on a newbuild is often half the cost of a fitted kitchen. You wouldn't consider compromising a house remodel to work around an old kitchen.  

 

The increase in cost of thicker EWI is only the material cost of the extra insulation. It can't be upgraded later cheaply . Unlike other technological advances there is an end game with reducing a house's energy demand. The consensus seems to be about passivhaus levels.

 

In computing for instance there will always be demand for more power as we develop more and more demanding software. For domestic housing you can build to match your heat need to that produced internally by showering cooking, living lighting etc doing away with the need for a central heating system. You'll only install a small backup heater for weather extremes and short periods of low occupancy.  The extra cost on a new build is marginal, often negative.On our house I estimate an extra €325 floor insulation, €2000 wall insulation, €1500 roof insulation, €2000 (MVHR in lieu of passive ventilation (DIY), €1000 airtightness (DIY), €1000 thermal bridge mitigation and €800 upgrading to Tripleglazing.  Total is less to €9k but we were then able to remove all central heating from our house and use an immersion in lieu of a heat pump which saved a similar figure. We ended up about cost neutral for a warmer, draft free house with plenty of fresh air. 

 

A word of warning, this only works if you go the whole hog and don't stop before the finish line but the takeaway point is that there is a finishing line. 

 

 Ask those suggesting 100mm to you what they are basing it on and again I suspect it'll be "rules of thumb" and "we always do it this way". Retrofits are often limited by the soffit depth but as I understand it you're replacing the roof too? 

 

EPS blown beads in the cavity for me. Less issues with water wicking and sagging compared to mineral wool.

I really like woodfiber as a material but it's very expensive as and given you have a brick wall internal to it you wouldn't benefit much from its great decrement delay. Also as it's an organic material I'd be keen to see it behind a vented cavity in the damp British isles. 

[Dave Jones]

48 minutes ago, Packman said:

 

Thanks Dave. The cost of the reno and extension should be around £250k if i used a main contractor.  I would then end up with a 250sqm house.

The future value should be at least the same (but most likely a little bit more) than the current value + the renovation & extension cost.

 

But if I demolished and built from scratch, to create a 250sqm new build house, working on an avg of £1500 /sqm, the cost would be £375k.

In this scenario, the current value + the new build cost would end up being more than the future value. 

 

So based on this, I don't think it's affordable or feasible to demolish and rebuild. Plus, the elevations and layout of the house we would rebuild would end up being extremely similar to the plans we have for the renovation and extension.

 

Unless I'm missing something?

 

dont forget the VAT, you can reclaim on a new build. Big chunk.

[Iceverge]

36 minutes ago, Packman said:

 

Thanks!

 

Very Smart looking. I'll have a proper look later. 

[Mr Punter]

The design looks good but I think the budget is way out.

 

Did you get a proper quotation from a main contractor for £250k for the extension and renovation?  If so, let them chose how they build it, just tell them the required spec (u values, air tightness etc) and let them work out how they will best achieve it.  For what they are doing it seems remarkably cheap.

 

I have looked at this and I think it will be cheaper and simpler to demolish and start again as there are almost no external walls that have been retained and any that are will require considerable works.  Also need new ground and first floors and new roof structure.

 

 

[Dave Jones]

your SE elevation looks close to the boundary,  you can only have 1m2 of windows in that entire elevation if you are 1m from the boundary. Fire regs.

[Packman]

23 minutes ago, Dave Jones said:

your SE elevation looks close to the boundary,  you can only have 1m2 of windows in that entire elevation if you are 1m from the boundary. Fire regs.

 

Thanks for the headsup... but we're over 1.8m from the boundary on the left and almost 3m from the boundary on the right.

[Packman]

2 hours ago, Mr Punter said:

Did you get a proper quotation from a main contractor for £250k for the extension and renovation?  If so, let them chose how they build it, just tell them the required spec (u values, air tightness etc) and let them work out how they will best achieve it.  For what they are doing it seems remarkably cheap.

 

 

Yes I did. Thank you, may be best as you say to set the parameters and let them build how they think is best.

 

2 hours ago, Mr Punter said:

I have looked at this and I think it will be cheaper and simpler to demolish and start again as there are almost no external walls that have been retained and any that are will require considerable works.  Also need new ground and first floors and new roof structure.

 

 

Would love to build from scratch as am concerned about the uncertainties, but think the cost of building from scratch will be more.  The entire N, E and W external walls are staying (although the W wall will become an internal wall). It's only the S external wall on the back of the house that is going. We'll also rebuild the boundary wall on the W as that is currently a garage and is just a single skin of bricks on piddly foundations. The roof is the same, we're just cutting in a gable and dormer on the front, and 2 gables on the back with a little flat roof between them to cover the extension. But the existing roof is largely still retained. Well at least that is the hope / intention!

 

[SteamyTea]

Did you say the rear of the house was south facing?

If so, your proposed design effectively stops the use of PV.

 

If you want to fit UFH, how much can you did down to fit the insulation?

[Iceverge]

Perhaps approach the contractor with a fairly easy to achieve airtightness spec of about 2 ACH50 and insist that it's verified and written into the contract.

 

My guess is that they'll tell you that you

1. Don't need it

2. They'll do a great job but there's no need to test it

3. It can't be done

4. It'll cost £100k more. 

 

You can build for 1990's money but you'll end up with a 1990's house. 

 

My other gut feeling is that the contractor is giving you a low price to get their foot in the door and as soon as you're building they'll find all sort of "unexpected problems,"  ( Not unlikely TBF) and alter the price accordingly. 

 

Ask your architect about some reputable contractors they've worked with and get a few more quotes . 

[Bitpipe]

1 hour ago, Iceverge said:

My other gut feeling is that the contractor is giving you a low price to get their foot in the door and as soon as you're building they'll find all sort of "unexpected problems,"  ( Not unlikely TBF) and alter the price accordingly. 

 

Ask your architect about some reputable contractors they've worked with and get a few more quotes . 

 

This would be my concern. Even an honest contractor will find unexpected items when they start stripping back. Floors and floor slabs seem to be a common theme, plus walls out of plumb etc.

 

Is your price an estimate or a fixed price?

 

While you can, go get some more quotes and get a few for demo and rebuild also. 

[Packman]

thanks everyone for the advice. I am getting some demo and rebuild quotes too!

 

[Packman]

On 19/01/2021 at 07:38, SteamyTea said:

Did you say the rear of the house was south facing?

If so, your proposed design effectively stops the use of PV.

 

Yes, it is S-facing.  Is that due to the gable roof design on the back?

 

On 19/01/2021 at 07:38, SteamyTea said:

If you want to fit UFH, how much can you did down to fit the insulation?

 

Unsure as don't know how deep the existing slab is. Some contractors have suggested building up, as we have enough head height to be able to loose some. It's 50-50 - some have said to def dig down, others have said it's better to insulate and build on top of the existing slab. Any views?

 

[SteamyTea]

2 hours ago, Packman said:

Is that due to the gable roof design on the back?

Yes.

 

2 hours ago, Packman said:

others have said it's better to insulate and build on top of the existing slab. Any views

Can you add 300mm of insulation, and then 100mm of concrete?

You may need more 'depth' around the edges.

[Packman]

5 minutes ago, SteamyTea said:

Can you add 300mm of insulation, and then 100mm of concrete?

Possibly, but didn’t want to do that much. We’ve got about 2.7m internal height on ground floor so doable. However 300+100mm seems a lot compared to other advice I’ve been getting.. why are you suggesting so much?