New Member - SIPS Extension

[JakeF]

Hi all:

 

First of all, great forum. I have been a reviewer of ebuild for a while and only found this live forum recently. I am just about to embark on an extension to a 1960's bungalow. See my DIY Sketch-up plan for a bit of context. After spending lots of money for not much in the past I do have an architect on board but basically in the capacity of a CAD technician, and basic advice, I am providing details. Being someone who suffers from an uncontrollable Attention To Detail i'm sure I will find myself on here alot over the next year or so.

 

One thing that I have asked him to draw up is the attached foundation detail, I know this has previously been a source of much debate but I think I have cracked it, what do you think? I just wanted to make a few points/raise a few questions:

 

1. Is it OK to cast the base slab with traditional formwork, then attach the EPS to the outside after it is cured/struck? (doing away with L shape pieces and solving external cold bridging/condensation issue?)
2. Does anyone have details of a structural/foundation engineer who can do my foundation design? I have local names but think it will be best to go with someone who has specific EPS/insulated slab experience?
3. Should I forget number 2 and go with a packaged design and supply foundation (have had some budget quotes from AFT who seem OK)
4. Should I forget 1-3 and go with something much simpler and "run of the mill" for the foundation. I am not going for Passive standard but will want minimal heat loss as we will be having UFH in the extension

 

Any feedback would be much appreciated as I have been going round in circles with this for weeks and weeks

 

Cheers

 

 

Edited August 4, 2017 by JakeF
Order of Pics

[MikeSharp01]

Welcome. Looks interesting, can't see a problem putting the upstand on afterwards but you could just move the formwork out 100mm and put the upstand inside the formwork, might be simpler! Also you have about 300mm of insulation under the floor and only 150mm in the walls so your overall U value will be relatively low with the floor performing better than the walls. Might be worth thinking about thickening up the walls, with some PU, on the outside of the SIP as you have room over the ali flashing, will cut your bills in the long run.

Edited August 4, 2017 by MikeSharp01
Added 'with some PU

[Stones]

A warm welcome. I really like the concept of clearly defined new extension and link.

[JakeF]

31 minutes ago, MikeSharp01 said:

Welcome. Looks interesting, can't see a problem putting the upstand on afterwards but you could just move the formwork out 100mm and put the upstand inside the formwork, might be simpler! Also you have about 300mm of insulation under the floor and only 150mm in the walls so your overall U value will be relatively low with the floor performing better than the walls. Might be worth thinking about thickening up the walls, with some PU, on the outside of the SIP as you have room over the ali flashing, will cut your bills in the long run.

 

Thanks for the reply. I see what you are saying, I have really well insulated shoes but only thin socks around my ankles? I haven't done a specific calculation but understood that it can be a diminishing return after 150mm thick and a 0.17 U value. I might have to look at doing a proper calculation, but as I have the space on the outside as you say, I don't have to make the decision now - although is there issues with attaching external insulation and then cladding onto base SIPS?  Also was going to look into approriate foam "fill" instead of EPS 300mm deep.....

 

27 minutes ago, Stones said:

A warm welcome. I really like the concept of clearly defined new extension and link.

 

Thank you, The original house is a 60's yellow brick and I wanted the extension to be obviously different, I can't say that the neighbours are happy! Also, I have obtained some very scary prices on the glass link so it has gone from being a frameless box (circa £15k) to an aluminium framed box (circa £9k) to a modern looking lean-to conservatory style, hopefully sub £5k

Edited August 4, 2017 by JakeF

[Jeremy Harris]

Welcome.

 

Worth remembering that heat loss through the floor will always be lower in winter than heat loss through the walls and roof, for the same level of insulation.  The ground under the house will stay at around 8 deg C, the outside air temperature could well be well below this in winter, and the rate of heat loss depends on the difference in temperature either side of the wall/floor/roof and the U value.

 

Therefore, it makes sense to have the best U value for the roof, the next best for the walls and the poorest for the floor, at least for most of the UK.

 

A U value of 0.17 W/m².K for the walls is barely better that our pretty crap building regs minimum, and I'd suggest that a target of at least 0.14, ideally around 0.12, would be better.  Try and get the roof down closer to 0.1, as heat tends to rise, so that the temperature difference across the roof will be a bit higher.

 

As a guide, out floor U value is around 0.1 W/m².K, our walls are about 0.12 W/m².K and our roof is slightly under 0.1 W/m².K, and that's a reasonable compromise in terms of balancing fabric heat loss with ventilation heat loss.

[Ferdinand]

Welcome.

 

I would say model your whole house including energy requires fr the next 10-20 years, as you may find some investment in your existing structure will be a good idea.

[JakeF]

Thanks for all the replies guys. I'm happy that the foundation looks OK as that was my main headache for the moment

 

I will look into options for additional wall insulation, options?

 

1. External 

2. Increase size of SIPS panel

3. Internal

 

The main house has cavity wall insulation and about 300mm loft insulation. Nothing for the suspended timber floor so I am looking at options for that as a minimum