Green Building Forum - calculating u values for garden room... are these correct?

calculating u values for garden room... are these correct?

thebeacon — Tue, 03 Jan 2023 08:17:25 +0000

Hello,

Iâ€™m currently working out the U-value for the garden room I plan to build.

I see that the UK building regs stipulate that the values below should not be exceeded:

â€¢ External walls: 0.30
â€¢ Roofs: 0.20
â€¢ Floors: 0.25

My buildâ€™s internal floor space is 14sm, which means I donâ€™t have to abide by building regs values, that said I would like to try and at least meet them.

I have been using the Ubakus (https://www.ubakus.com/en/r-value-calculator/?) to help work out my values. My values read as:

Walls: 0.310
Roof: 0.242
Floor: 0.270

These are close to building regs, but they do seem slightly high. I thought using 90 and 100mm pir insulation board would be adequate, particularly because it has such good values.

Have I made a mistake with my calculations? Or would you consider these about, right? I feel like Iâ€™m already adding more insulation than what most off the shelf garden rooms have in them.

Below are how my walls, roof and floor are built up.

Wall:
INSIDE
12mm ply
1mm Vapour control membrane
100x47mm stud (400mm centres) with 90mm PIR board insulation
11mm osb3
1mm tyvec firecurb membrane (or similar)
25x50mm timber batten (vertical) 400mm apart
25x50mm timber batten (horizontal) 400mm apart
20mm timber cladding
OUTSIDE

Warm roof:
INSIDE
170x45 roof joist
18mm osb3
1mm Vapour control membrane
100mm PIR board insulation
11mm osb3
3,2mm EDPM
OUTSIDE

Floor:
INSIDE
18mm plywood
1mm Vapour control membrane
150x47mm timber joist with 100mm PIR board insulation between
Osb3 board

calculating u values for garden room... are these correct?

Peter_in_Hungary — Tue, 03 Jan 2023 10:19:35 +0000

What you have entered looks about right although the timber studs for the wall look to be at 460 centres.

Try playing with the layers. E.G. change the wall 90mm PIR to 100mm ESP, or increase the wall thickness to 120mm by addition of a batten or larger section timber and use 120 EPS and then do cost calcs to see where the money to insulation value works out.

Edit to ask have you spaced your studs to fit the widths of the chosen insulation boards - for ease of build and reduction of waste?

calculating u values for garden room... are these correct?

philedge — Tue, 03 Jan 2023 11:48:31 +0000

How far you go with insulation really depends on how your going to use the room. If its occasional evening use spring-autumn then what youre doing is probably fine. If its(or could become) a year round heated home office then I'd want loads of insulation

calculating u values for garden room... are these correct?

WillInAberdeen — Tue, 03 Jan 2023 12:53:25 +0000

Hi Beacon,
Unfortunately those are not the current U values for the (2022 updated version) English building regs for new "dwellings". After many years of complaints that they were too weak, they've finally been tightened! Its a bit complex so here goes:

1) you start off with these values:
Walls 0.18 W/mÂ²K
Roof 0.11 W/mÂ²K
Floor 0.13 W/mÂ²K
Windows and glazed doors 1.2 W/mÂ²K
Airtightness 5mÂ³/mÂ²h
Gas boiler 89.5% efficiency
PV on roof

2) you can stop there. Or, you could make any of those specs worse, so long as you make another of the specs better to compensate.
There's a backstop limit to how much worse you are allowed to go.

Unfortunately, any direct electric heating is treated as being 20% worse Primary Energy consumption than a gas boiler, so if you are heating electrically then you need 20% better insulation than the values above. If you had a little air-air heatpump, then you would be allowed worse insulation because the heatpump consumes much less Primary Energy. At those insulation levels, it isn't going to need a lot of heating anyway.

As Phil suggested, if your building is an increment to your indoor "dwelling" space , then worth getting the build quality right (cheaper to do that during the build than later on!). If its just used to store the deckchairs in, then you may be much less bothered! When we sold our last place we were asked to produce building certificates for a new outbuilding that adjoined close to the property boundary.

You might be able to use 145mm joists and so fit thicker insulation on the roof?

The wall and floor timbers are bridging through the insulation layers (timbers entered as 40mm thk - incorrect?). Could you get a continuous skin of insulation over/under those timbers?
Particularly at the wall-floor and wall-roof junctions, it's important for the insulation to join up without cold bridges, otherwise there can be cold damp spots in corners!

calculating u values for garden room... are these correct?

thebeacon — Tue, 03 Jan 2023 16:33:30 +0000

Thanks for all the informative replies.

Posted By: philedgeHow far you go with insulation really depends on how your going to use the room. If its occasional evening use spring-autumn then what youre doing is probably fine. If its(or could become) a year round heated home office then I'd want loads of insulation

The room will be a studio, to make artwork in and some woodwork. I plan to use it maybe 2 days a week. Ideally, it will be kept at a minimum of 13 degrees throughout the year. As it's down the garden, there wonâ€™t be gas. But I plan on installing a combi heat pump and air conditioning unit. https://www.airconcentre.co.uk/products/powrmatic-vision-3-1dw-h-air-conditioning-and-heat-pump-vis3-1dw-h

Itâ€™s worth mentioning that Iâ€™m on the Southeast England coast. Generally, we donâ€™t have as cold a winter as further North.

Posted By: Peter_in_HungaryEdit to ask have you spaced your studs to fit the widths of the chosen insulation boards - for ease of build and reduction of waste?

Ah, yes I see that I had the joists slightly wrong, they will be spaced at 400 centres. They are done like that for the wood sheathing and interior cladding to reduce waste. The joists will be 45x95mm (47x100).

With the floor I can put 120mm of PIR board in between the joists. Thatâ€™s the maximum I can fit between the joists. It now comes in at 0.246.

Floor:
INSIDE
18mm plywood
1mm Vapour control membrane
150x45mm timber joist with 120mm PIR board insulation between
9 Osb3 board
10mm batton to support osb board

Iâ€™m unsure how to cover the floor joists to stop the bridging, as I have no space under the build. The room will be 50-100mm off the ground, sat on ground screws.

I could potentially squeeze to putting 140mm pir board on the roof. Which would bring the U value to 0.172.

Warm roof:
INSIDE
170x45 roof joist
18mm osb3
1mm Vapour control membrane
140mm PIR board insulation
11mm osb3
3,2mm EDPM
OUTSIDE

Posted By: WillInAberdeenThe wall and floor timbers are bridging through the insulation layers (timbers entered as 40mm thk - incorrect?). Could you get a continuous skin of insulation over/under those timbers?
Particularly at the wall-floor and wall-roof junctions, it's important for the insulation to join up without cold bridges, otherwise, there can be cold, damp spots in corners!

For the wall I donâ€™t really want to eat away at the interior space if it can be helped. Is it sensible to attach PIR on the outside of the osb sheathing, for example, 30mm? My worry with this is the weight of the battens and cladding on the pir. Doing this would stop the thermal bridging on the outside walls, though.

Using 30mm external wall insulation only decreases the value to 0.222. I donâ€™t think I can get as low as the new building regs without compromising on internal space.

Wall:
INSIDE
12mm ply
1mm Vapour control membrane
95 x45mm stud (400mm centres) with 90mm PIR board insulation
11mm osb3
30mm PIR board insulation
1mm tyvec firecurb membrane (or similar)
25x50mm timber batten (vertical) 400mm apart
25x50mm timber batten (horizontal) 400mm apart
20mm timber cladding
OUTSIDE

Thanks for the advice so far! It really is very much appreciated.

calculating u values for garden room... are these correct?

Peter_in_Hungary — Tue, 03 Jan 2023 20:29:40 +0000

Posted By: thebeaconThe room will be 50-100mm off the ground, sat on ground screws.

Hmm - Enough space for mice and alike to make a cosy home but not enough for the local cats to take care of the problem. Can you provide a deeper space or full fill to avoid the problem. Full fill would be warmer!

calculating u values for garden room... are these correct?

thebeacon — Thu, 05 Jan 2023 08:07:04 +0000

Posted By: Peter_in_HungaryHmm - Enough space for mice and alike to make a cosy home but not enough for the local cats to take care of the problem. Can you provide a deeper space or full fill to avoid the problem. Full fill would be warmer!

As I sought planning permission to build the room, would I be able to lower it by the 50-100mm without going back to them, or would I need to speak to them about it? I wouldnâ€™t be making the buildingâ€™s highest point any higher, but the building would be larger than the dimensions given to PP.

I donâ€™t have rear access to the garden, because of this, Iâ€™d like to use ground screws as the foundations. Maybe a silly question, is it possible to put ground screws all the way into the ground? Would a layer of DPC (https://www.screwfix.com/p/capital-valley-plastics-ltd-damp-proof-course-black-30m-x-300mm/94278?kpid=94278&ds_kid=92700065972061625&ds_rl=1249407&ds_rl=1241687&ds_rl=1245250&ds_rl=1245250&gclid=CjwKCAiAh9qdBhAOEiwAvxIok_AjZ53m0joYlr47Zl08ScN-MBUQjnjmp5X--BV__HVJALkjiv3u9xoCVskQAvD_BwE&gclsrc=aw.ds) need to be applied under every joist in the base frame?

calculating u values for garden room... are these correct?

philedge — Thu, 05 Jan 2023 13:26:51 +0000

You dont want your timber anywhere near the ground even with a DPC. With a DPC lying on the floor youll get an accumulation of leaves, grass clippings, worm cast etc on top of the DPC that will act like a sponge to any moisture and keep your wood wet for months leading to rot.

I'm not to sure about the load bearing ability of ground screws long term but if you set the timbers above ground by 2-4 inches then put a run of stainless mesh round the perimeter to keep vermin out but let air flow through.

Make sure the roof runs into gutters and the downspouts discharge out of splashing range

calculating u values for garden room... are these correct?

GreenPaddy — Fri, 06 Jan 2023 09:35:26 +0000

A couple of thoughts, that might already have been discounted before, apologies if there's repetition:

1. Check with building control which Uvalues they will be expecting you to reach. You're not constructing a dwelling, may be considered ancillary to a dwelling, or even non-domestic. They will judge it, so ask them to confirm the rules.

2. I would build the floor upside down, same format as your walls, with the OSB face fixed to the frame, then flip it over. That will let you fully fill the joist depth (I've done it, not just a theory)

3. For external edge protection from vermin, dig down into the ground, and fit a grip/mesh, maybe rigid fibre glass, as animals will burrow to get under. So maybe 200mm below ground and the 100mm above ground to meet your subfloor.

calculating u values for garden room... are these correct?

thebeacon — Fri, 06 Jan 2023 09:54:03 +0000

Posted By: philedgeI'm not to sure about the load bearing ability of ground screws long term but if you set the timbers above ground by 2-4 inches then put a run of stainless mesh round the perimeter to keep vermin out but let air flow through.

Thanks, yes this seems achievable.

I have been looking for examples of attaching exterior wall insulation, and have not found much.

Would I attach the 30mm pir board into the timber studs with fixings like these: https://www.ecomerchant.co.uk/walls/insulation/insulation-fixings/ejot-str-h.html. How often should they be fixed?

My big concern is the weight of the cladding and battens and having to screw through the 30mm pir board. Would there be any sagging?

I plan to place the vertical and horizontal battens at 400 centres, would 1 screw through the crossover of both battens and through the pir board into the timber stud be enough, or would you recommend more?

With a 130mm screw, 80mm would be taken up on the batten and insulation with 50mm landing in the stud:
25mm timber batten
25mm timber batten
30mm pir board EWI
95mm timber stud

calculating u values for garden room... are these correct?

Dominic Cooney — Sat, 07 Jan 2023 08:57:55 +0000

As I sought planning permission to build the room, would I be able to lower it by the 50-100mm without going back to them, or would I need to speak to them about it? I wouldnâ€™t be making the buildingâ€™s highest point any higher, but the building would be larger than the dimensions given to PP.

Increasing the floor depth/thickness has no bearing on planning permission, itâ€™s the external dimensions that matter.

(Chartered Town Planner)

calculating u values for garden room... are these correct?

Dominic Cooney — Sat, 07 Jan 2023 09:00:35 +0000

Also fixing through 30mm PIR isnâ€™t much of an ask, I am currently fixing through 100mm PIR in my bathroom ceiling overhaul with 150mm screws and 25mm battens