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Underfloor insulation choice between concrete and PIR with costs

Yellowmac

Would really appreciate the opinions of the experts on this forum.

If you were doing a 70-80k renovation on a (cold) 1960s house (new kitchen, new rear windows, updating gunbarrel plumbing and wiring throughout) and you are taking up all downstairs floors to add insulation would you go for

Option A. 100mm PIR insulation between joists, fit 18mm osb 3 sheets to existing joists 12,200 euro.

Option B. Insulated concrete floor slab, radon barrier 21,500 euro.

Floors are suspended 300mm. Main goal is warmer/less drafty. Not going for underfloor heating. Builder advises both are similar jobs in terms of time/labour and mess but a better (warmer) outcome from the concrete.

Struggling to decide whether it's worth the 9k extra. Thanks for any thoughts.

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Yellowmac

https://www.boards.ie/discussion/comment/123111593#Comment_123111593

OK thanks that's very helpful. Will invest in a dehumidifier for sure

GreeBo

https://www.boards.ie/discussion/comment/123111365#Comment_123111365

The problem with doing it partially is that the warm, damp air that currently puts a little condensation on all the walls, will now put the same total condensation, but on fewer walls, making it more of a problem.

e.g 1L of water spread over every external wall is hardly noticeable, 1L on 1 wall is a potential problem.

If you are double/triple glazing downstairs but not upstairs its going to be the same problem, but magnified by dripped windows and wet window sills.

I'd buy a dehumidifier and stick it on the landing, to try to cut down on condensation problems, for a couple of hundred quid thats a no brainer for me.

hesker

A lot of people stating the importance of air-tightness and controlled ventilation which is of course true. However it’s not easy in the current setup where nearly all OSS flatly refuse to retrofit air-tightness measures and SEAI offer no grants for it or state any requirements for it.

Yellowmac

Sorry I have one last question, everyone here is very helpful and knowledgeable!

If we do drylining on all the downstairs external walls as part of this renovation, but don't touch upstairs. Are we opening ourselves up to any potential issues with cold bridges/damp areas. Where the insulated downstairs meets the uninsulated upstairs?

External bedroom walls seem to already have some amount of IWI done by previous owner (we suspect from knocking on them!) but it probably isn't the thickness that we'll be putting in om ground floors.

Thanks

GreeBo

https://www.boards.ie/discussion/comment/123102900#Comment_123102900

3-5 has a nasty habit of turning into 5-10, even if you just bought some loft roll and lashed in on top yourself it would be worth it in my opinion.

Doing the drylining, floor and topping up the attic will make it a feel like a different house. I just did the same on a 1930s detached house, house is 21C all day everyday without worrying about draughts or open doors. (We did get MVHR, HP and underfloors done though :) )

Yellowmac

https://www.boards.ie/discussion/comment/123103057#Comment_123103057

Haha sorry I misread that from the quote for the other type of insulation, not the concrete. That will give you an idea how much I know about it all. Will get the specs and get some input on them, cheers

Busman Paddy Lasty

https://www.boards.ie/discussion/comment/123103005#Comment_123103005

The floor joists are going in a skip shortly, so the 100mm insulation between them isn't an option anymore 😁

There are a few pros here that would surely help further if you throw up the builders spec for what they have priced.

Yellowmac

https://www.boards.ie/discussion/comment/123096137#Comment_123096137

Thank you, it's great to hear from someone who was in our position and is happy with the choice they made. I was also convinced by the fact that concrete would mean less vermin!! The floors are so drafty (find myself sitting on them often with small kids).

We are doing rewire and gun barrel too and it's also a late 60s semi d. Glad it worked out for you. We're hoping after this we'll never have to touch the house again!

Yellowmac

https://www.boards.ie/discussion/comment/123102967#Comment_123102967

The only info I have is 100mm PIR between joists, he didn't say the screed, I'll check with him. Thanks for the info

Busman Paddy Lasty

You won't regret it. Did the builder give you a spec for the floor materials?

We did 80mm screed. 150mm PIR insulation. Engineer was super strict on using an S.R. 21 aggregate underneath instead of any old rubble or other fill.

Yellowmac

Thanks for all the replies, I really appreciate everyone's input and advice. We are going ahead with the concrete underfloor insulation throughout downstairs and drylining all downstairs external walls. Not getting UFH for other reasons.

It's less about saving on heating or return on investment of the cost of works, more about comfort level of the house.

The attic is currently partially insulated its probably a source of heat loss but we dont want to put much into insulating it more as as we'll be converting it in the next 3-5 yrs. I think between the floors drylining and attic, the house will be much warmer in the end.

The total reno is coming in at 68k of which 21k is under floor concrete- eek.

Thanks all

GreeBo

https://www.boards.ie/discussion/comment/123095425#Comment_123095425

It wont make the house miserable and uncomfortable, the house will remain miserable and uncomfortable at this time of year.

Its either roasting as the heat is on full blast or freezing cold 15 minutes after the heat turns off.

I am ignoring the cost of heating and purely looking at the comfort levels and enjoyment of the house. People lived in mud huts with open fires and a hole in the chimney for even longer than they lived in suspended floor semi D-s.

OP, if you can in anyway afford it, go the whole hog. If you cant afford to do that, I would wait until you can. Spend what you have on attic insulation etc. But don't spend 80 on a refurb instead of spending 90, that extra 10K will be meaning less in 5 years time but the insulated, heated slab will be the gift that keeps on giving.

Paulzx

I had a similar choice a few years ago. Redo the suspended floor or concrete in the space. I did not want underfloor heating so this wasn't a factor in the equation. House is late 60's semi d. The price difference between both options was much less than your quote.

I went for the concrete floor. This included rebar, insulation and radon sump. Cost was about 9 grand.

I felt that it was the best way to rule out draughts and also problems with pests in the underfloor void. I have no regrets about the option i took.

I done a rewire and got rid of gun barrell plumbing as well

Busman Paddy Lasty

Get rid of the suspended timber floor. It's well worth the €9k extra. Not about financial payback but comfort. Having a floor that feels warm in the morning barefoot.

Getting rid of drafts does improve comfort levels. You can run your rads at a lower temperature.

Alkers

https://www.boards.ie/discussion/comment/123093999#Comment_123093999

People have lived in these kind houses for years and were never miserable and uncomfortable.

I agree fully, the spend on heating will be more than not doing it, but not significantly so. There will be heat loss through the floor and associated drafts but it won't make the house miserable and unconfortable. It makes more sense to me to spend slightly more on heating each year than to spend 20k here as discussed.

GreeBo

https://www.boards.ie/discussion/comment/123092129#Comment_123092129

It might be possible to make it draught proof (which, as pointed out before, is very hard to do unless you are doing it yourself), but it will still be cold.

If you just look at cost vs payback you could spend nothing on heating ever and save loads. Your life will be miserable and uncomfortable but you will "save" money.

Doing the 20K work will immeasurably improve your standard of living and enjoyment of the house.

Alkers

https://www.boards.ie/discussion/comment/123090779#Comment_123090779

I don't think it will be as bad as you you let on.

You can still draught-proof the floor without insulating it. Then a good underlay and carowr which you'd likely do anyway. The ewi will extend below floor level also.

I know it's far from perfect, but at 20k, even if it meant the house was free to heat, it would take 20 years to pay itself back!

monseiur

https://www.boards.ie/discussion/2058388447/underfloor-insulation-choice-between-concrete-and-pir-with-costs

Definitely go for option 'B' if your budget allows. As you are replacing the back windows, and all rads are down consider dry lining the inside of ''external'' walls with say 50mm insulated plasterboard and upgrade attic insulation to at least 300mm. Contact SEAI there are good grants available for heating upgrades, insulation upgrades etc.

GreeBo

EWI with an uninsulated, suspended timber floor is not going to leave you with a "nice" warm house feeling IMO. If the floor is cold and draughty then the house wont be comfortable, no matter what else you do on the walls, roof.

If you are gutting the house then now is the time to do it all IMO. It will be unlivable during any of this, so you wouldnt want to do it twice.

Concrete floor with underfloor heating.

Strip the walls back to blocks and add insulation + air tightness membrane, VCL.

Same for the ceilings.

Martee

Grappling with similar pickles at the mo. If looking at splitting out works from OSS keep in mind the qualification requirements to use one inc. min uplift in BER and max existing BER.

BenIrishHome

Do you mean not doing the heat pump and stay with my gas boiler? That’s probably the major cost. But I have an EV and a good roof for solar, so seems like the next step to go off the gas entirely.

The attic does need better fibre insulation and I can do air tightness. I’ve been researching MVHR and they seem quite good, also I could DIY 90% of that.

I need new doors and windows and we’re doing a small rearrangement of the layout so we might as well do the whole job while we are at it. UFH is a must have for me and would suit this house, so that gets me back full circle to needing a heat pump for that slow and steady gentle heat.

Yellow_Fern

€90k seems an emormous cost. A lot of that cost wont make your house more comfortable like a heat pump. As the other poster writes, get a good airtightness survey, lift flooring to solve airtightness and because you have a bungalow, you can potenially install mvhr to each room very easily. You could potenially make enormous improvements without even spending €15k.

MicktheMan

https://www.boards.ie/discussion/comment/123066867#Comment_123066867

You were the one who sumised that draftproofing was more important than insulation.

Not sure what your point is here.🤷‍♂️

Are you saying there are other areas where drafts/leakage is going to be worse than the vents?

Yes, likely pretty much, but every house is different to some extent. I'm not saying that the vents are not contributing but they are likely not as big a contributer as you might think. Similarly for your windows possibly. Most attic hatches are definite improvement opportunities especially the ones into the crawl space in dormers … think of these as external doors! But there are nearly always other aspects that are a complete surprise to the home owner when pointed out.

An air tightness test (as part of an overall heat loss survey) would be the best place to start to understand where the predominant issues are and how to resolve etc.

BenIrishHome

https://www.boards.ie/discussion/comment/123065869#Comment_123065869

You were the one who sumised that draftproofing was more important than insulation. Especially in a notorious dormer bungalow. Are you saying there are other areas where drafts/leakage is going to be worse than the vents? Can't just be attic hatches. tbf my 25 old double glazing is probably half the problem.

MicktheMan

https://www.boards.ie/discussion/comment/123064692#Comment_123064692

You have me starting to think i can do this myself now…

You probably could if you have both the skills (competent diyer) and the knowledge of the why, the what to do and the where. The former you either have or don't and a proper heat loss survey (note: not a BER) will supply the latter.

A significant number of my clients decide that they have (or they know someone close to them who have) the skills to do the majority of the work themselves.

But I'm walking around looking at vent after vent in every room. 1 in each bedroom, two in living room and 2 in kitchen, all the bathrooms etc. They're just open holes to the outside. I never feel drafts or wind from them but probably because all my lovely expensively warmed air is streaming out.

Perhaps try the following test yourself the next time it's anyway windy / cold outside. Temporarily seal all your wall vents for say 24 hours and see if this make a noticeable difference to you heat loss / retention.

I'm not saying the 'hole in wall' vents are good but merely questioning your thought process that they are a major issue for heat loss.

BenIrishHome

https://www.boards.ie/discussion/comment/123064151#Comment_123064151

You're probably not wrong. I have done enough projects in the past to probably PM my own retrofit, and DIY some of it too. Just with busy life and kids and stuff sometimes you want it off your hands.

There are a couple of areas it seems beneficial to use the OSS:

Windows and Doors - only OSS can get you a grant for windows and doors. They reckon about €5,600 in grants on €25k of doors. That sounds alot but its approx. 15 windows including some big bay ones, a new front door and a big bifold unit to the kitchen.
Air Pump Install - Good grants on this and they install it themselves.
Solar - likewise, these guys do their own solar installs. If they had an outside contractor I would shop around. Fairly good quote of €10k on 14 panels (6kw system)

Their 'plan' for airtightness was mostly windows and doors and "additional draughtproofing" which was only about €1500, probably just sealing up around attic hatches etc (we do have 4/5 of these including into the eaves spaces)

I agree with what you are saying. Surely its draughts letting the heat out. But I'm walking around looking at vent after vent in every room. 1 in each bedroom, two in living room and 2 in kitchen, all the bathrooms etc. They're just open holes to the outside. I never feel drafts or wind from them but probably because all my lovely expensively warmed air is streaming out. Hence me convincing them i definitely need MVHR.

You have me starting to think i can do this myself now…

MicktheMan

My big problem is - the heat just drops in the house as soon as the boiler turns off. So we’re leaky right?

So what's their plan on for air tightness then?. Am I right to conclude that the OSS came up with a €90k plan to address horrendous heat loss in the absence of an air tightness test … in a dormer. Your house is likely losing heat at a rate of knots (pun intended) because of a lack of air tightness, not a lack of insulation.

These OSS outfits exist because "Hey, there's a government backed grant for it so it must be good, right?

It's crazy what's going on and allowed to go on.

In my experience if you were to take, say, a fifth of that amount, you would have change in your pocket and a changed house if you were to ignore grants, OSS's etc and concentrate on addressing the real cause of your heat loss and improving both your comfort levels and your indoor air quality.

BenIrishHome

https://www.boards.ie/discussion/comment/123060224#Comment_123060224

no, funny enough they were recommending only manual vents which didn’t require air tightness test. I’ve asked them to add the MVHR system to the quote. Another €9k (€6k after grant). Will include a before and after airtightness test.

My big problem is - the heat just drops in the house as soon as the boiler turns off. So we’re leaky right? So surely I need to get all these always-open vents shut off, and then a system to deal with the moisture. Manual vents seem stupid.

Of course i need better attic and wall insulation. But to do all that plus UFH, why would I not do the MVHR?

of that €90k, about €25k was attic insulation and “sloped roof” insulation. Ie taking down all the attic plasterboard and putting in new thicker warmboard. Huge job, expensive. Is it worth it? Not on its own. We might be doing a couple of dormers up there so taking apart the inside roof, so might as well do the new warmboard, but I’m in two minds. Feeling this is the one area I might be getting shafted by the OSS, might see if a local attic insulation company would do me a better price.

MicktheMan

https://www.boards.ie/discussion/comment/123058697#Comment_123058697

€90k retrofit of a dormer bungalow… please say that they have tested for air tightness, put a robust plan of attack in place including a re-test when done to prove the works.